Aeolian features and processes

Nicholas Lancaster
Division of Earth and Ecosystem Sciences, Desert Research Institute, Reno, Nevada, 89512, USA

Lancaster, N., 2009, Aeolian features and processes, in Young, R., and Norby, L., Geological Monitoring: Boulder, Colorado, Geological Society of America, p. 1-25, doi: 10.1130/2009.monitoring(01). For permission to copy, contact editing@geosociety.org. ©2009 The Geological Society of America. All rights reserved.


INTRODUCTION


Aeolian processes, involving erosion, transportation, and
deposition of sediment by the wind, occur in a variety of environments,
including the coastal zone, cold and hot deserts, and
agricultural fields. Common features of these environments are
a sparse or nonexistent vegetation cover, a supply of fine sediment
(clay, silt, and sand), and strong winds. Aeolian processes
are responsible for the emission and/or mobilization of dust and
the formation of areas of sand dunes. They largely depend on
other geologic agents, such as rivers and waves, to supply sediment
for transport.
Areas of sand dunes occur in inland and coastal settings,
where they often provide a distinctive environment that provides
habitats for endemic and rare or threatened species. In both
coastal and inland settings, dune migration and sand encroachment
may impact neighboring ecosystems and resources, as well
as infrastructure.
Transport of fine sediment by wind may cause dust storms,
events in which visibility is reduced to less than 1 km by blowing
dust. Dust storms impact air quality in their immediate vicinity
as well as in areas downwind. Deposition of dust may have a
significant effect on the composition and nature of soils in arid
regions and beyond. Far-traveled dust from distant sources may
have a significant effect on soil chemistry and nutrient status
(e.g., Farmer, 1993).

Aeolian Processes, Landforms, and Deposits

This section provides a brief introduction to aeolian processes
and landforms and their deposits. For more detailed information
and an in-depth discussion of the topic, see Lancaster
(1995) for desert dunes, and Nordstrom et al. (1990) for coastal
dunes. Goudie and Middleton (2006) provide an excellent review
of desert dust processes, while Goudie et al. (1999) provide a
good short overview of aeolian processes in general.
Figure 1
Figure 1. Modes of sediment transport by the wind (after Pyle, 1987).

Transport of Particles by Wind

Movement of particles by the wind takes place by a combination
of direct wind shear stress and atmospheric turbulence. There
are three modes of sediment transport by wind: creep or reptation;
saltation, and suspension (Fig. 1). The mode of transport
depends primarily on the ratio between particle settling velocity,
and hence particle size, and wind shear stress and turbulence
intensity. Very small particles (<20 microns) are transported in
suspension (tens of km or greater) and are kept aloft by turbulent
eddies in the wind. True suspension occurs when the particle settling
velocity is very small compared to the turbulence intensity
of the wind. Larger particles (20–70 microns) undergo short-term
suspension for distances of tens to hundreds of meters; material
of sand size (70–1000 microns) is transported mainly in a series
of short hops (saltation), in which the vertical component of wind
velocity (turbulence) has a minimal effect on particle trajectories.
Material coarser than 500 microns in diameter (coarse sand)
is transported on surface by reptation and creep. The modes of
transport are interdependent: saltating sand particles eject silt- and clay-sized
particles into the wind and impact coarse grains that
are rolled along the bed.
Figure 2
Figure 2. Transport of sediment by the wind: (A) Relation between threshold wind shear velocity and particle size (from Bagnold, 1941). (B) Mass flux of sand as a function of wind shear velocity (from Lancaster, 1995). Data from laboratory wind tunnel experiments. (C) Relations between horizontal flux of sand-sized particles and vertical flux of dust (from Nickling et al., 1999). Data from field experiments. Figure from Lancaster (2005).
Grains begin to move and sediment is entrained by the wind
when fluid forces (lift, drag, moment) exceed the effects of the
weight of the particle, and any cohesion between adjacent particles
as a result of moisture, salts, or soil crusts. The threshold
wind speed at which grains begin to move is strongly dependent
on particle size (Fig. 2A). For quartz sand, the minimum threshold
velocity is associated with fine sand (~100 microns diameter).
The mass flux or transport rate of sand has been determined by
numerous laboratory wind tunnel and field studies to be proportional
to the cube of wind shear velocity above a threshold value
(Fig. 2B). For any wind shear velocity, there is a potential rate of
sand transport or transport capacity, which is only reached when the
availability of sediment is unrestricted (e.g., most loose sand
surfaces). In these conditions, the wind is saturated with respect
to transport capacity.
Very fine grains (silt and clay size) are inherently resistant
to entrainment, yet are readily transported by the wind. Recent
studies have shown the critical role of impacting sand grains in
the mobilization of silt- and clay-size particles and demonstrated
the close relations between the horizontal flux of sand-size particles
and the vertical flux of fine particles. In these situations, the
horizontal mass transport rate is directly related to shear velocity
(Fig. 2B), so dust emissions scale to the fourth power of wind
shear velocity (Fig. 2C). Where there is a limited supply of particles
able to abrade soil clods or playa crusts, dust emissions
are limited by the supply of particles rather than the wind shear
velocity, and the vertical flux of dust is almost independent of
wind shear velocity.

Wind Erosion
Erosion by wind involves two linked processes: abrasion
(mechanical wearing of coherent materials, including playa
crusts and clods created by tillage) and deflation (removal of
loose material). Considerable attention has been devoted to the
processes and rates of wind erosion because of their impact on
agriculture, especially in semi-arid regions, and the implications
of dust emissions for air quality. Wind erosion abrades crops,
removes organic matter, nutrients and fertilizer, and changes soil
texture. The products of wind erosion (especially dust particles)
impact air quality, atmospheric radiative properties, and human
health, causing respiratory illnesses. Rates of wind erosion vary
widely and for a given wind shear velocity are dependent on soil
or sediment texture and the degree of crusting and cohesion. The
highest emission rates for fine-grained sediment are associated
with soils of loamy texture, especially those that have been disturbed
by vehicular traffic and/or animals.

Aeolian Deposits
Aeolian deposits include sand seas and dune fields, deposits
of silt (loess), and fine-grained material that forms a significant
component of desert margin and other soils.

Aeolian deposits—silt and clay size. Deposits of wind-transported,
silt-sized quartz particles, termed loess, cover as
much as 10% of Earth’s land surface. Loess deposits are widespread
in areas of northern China, southern central Asia, central
Europe, Argentina, Alaska, and the central United States. Much
of the material was thought to be derived from silt particles produced
by glacial grinding and supplied to aeolian processes by
glacial outwash (“glacial loess”), but other processes, including
frost shattering, salt weathering, reduction in size during transport
by rivers, and aeolian abrasion are important, especially in
the formation of “desert loess.”
Silt- and clay-sized material of aeolian origin is also an important
component of many desert margin soils. Deposition of silt
plays a role in the formation of many stone pavement surfaces in
desert regions (desert pavement). These surfaces are characterized
by a surface layer of gravel or larger clasts (particles) that overlie
fine-grained materials. Detailed studies of these surfaces show
that the surface layer of gravel rests on a layer of soil-modified
dust that may be a meter or more thick and mantles bedrock or
materials deposited by other processes (e.g., alluvial sediments).
The dust is trapped by the clasts and deposited between them. The
fine material is incorporated into the mantle by the shrinking and
swelling of clay minerals so that the clasts remain at the surface
as they inflate over periods of thousands of years.

Aeolian deposits—sand dunes. Aeolian dunes form part of
a hierarchical system of self-organizing bedforms which comprises:
(1) wind ripples (spacing 0.1–1 m); (2) individual simple
dunes or superimposed dunes on compound and complex dunes
(spacing 50–500 m); and (3) compound and complex (mega-)
dunes or draa (spacing more than 500 m). Dunes occur wherever
there is a sufficient supply of sand-sized sediment, winds to
transport that sediment, and conditions that promote deposition
of the transported sediment. These requirements are satisfied in
two main environments: (1) coastal areas with sandy beaches and
onshore winds; and (2) desert areas. Most dunes occur in contiguous
areas of aeolian deposits called sand seas (>100 km2) or
dune fields.
Figure 3
Figure 3. Wind ripples, Gran Desierto, Mexico. Wind direction from left to right.
Wind ripples (Fig. 3) typically have a wavelength of 0.05–
0.2 m and an amplitude of 0.005–0.010 m. They are ubiquitous on sand surfaces,
except those undergoing very rapid erosion
or deposition, and form because a flat sand surface over which sand transport by saltation and reptation occurs
is dynamically unstable.
Figure 4
Figure 4. Major dune types (after McKee, 1979b).
Aeolian dunes occur in a self-organized pattern that depends on the wind regime (especially its
directional variability) and the supply of sand.
Sand dunes occur in four main morphologic types (Fig. 4): Crescentic (transverse), linear, star, and parabolic.
The simplest dunes form in areas characterized by a narrow range of wind directions. In the absence of vegetation, crescentic dunes will be the dominant form. Isolated crescentic dunes or barchans occur in areas of limited sand supply, and coalesce
laterally to form crescentic or barchanoid ridges as sand supply
increases (Figs. 5A and 5B).
Linear dunes are characterized by their length (often more than 20 km)
sinuous crestline, parallelism, and regular spacing (Figs. 5C and 5D).
They form in areas of bimodal or wide unimodal wind regimes. Star dunes
have a pyramidal shape, with three or four sinuous sharp-crested arms
radiating from a central peak and multiple avalanche faces (Fig. 5E).
Star dunes occur in multidirectional or complex wind regimes
and are the largest dunes in many sand seas, reaching heights of
more than 300 m. Parabolic dunes (Fig. 5F) are characterized by
a U or V shape with a “nose” of active sand and two partly vegetated
arms that trail upwind. They are common in many coastal
dune fields and semi-arid inland areas, and they often develop
from localized blowouts in vegetated sand surfaces. Other important
dune types include nebkhas, or hummock dunes, anchored
by vegetation (common in many coastal dune fields); lunettes
(often composed of sand-sized clay pellets) that form downwind
of small playas; and a variety of topographically controlled dunes
(climbing and falling dunes, echo dunes).
Relations between dune types and wind regimes indicate
that the main control of dune type is the direction of the wind
(Fig. 6). Grain size, vegetation cover, and sediment supply play
subordinate roles in desert areas. In semi-arid and coastal areas,
vegetation cover plays a major role in aeolian dynamics.
Figure 5
Figure 5. Satellite images and aerial photographs of major dune types: (A) barchans and crescentic dunes, Namib Sand Sea; (B) compound crescentic dunes (Liwa, United Arab Emirates); (C) parabolic dunes, Casper, Wyoming; (D) simple linear dunes (Kalahari Desert); (E) complex linear dunes (Namib Sand Sea); (F) star dunes (Gran Desierto, Mexico). Figure from Lancaster (2005).
Figure 6
Figure 6. Relations between dune types and wind regimes. Figure from Lancaster (2005).

STRESSORS AND POSSIBLE CHANGES


The state of an aeolian geomorphic system is controlled
by the supply of sediment of a size suitable for transport by the
wind; the mobility of the supplied sediment, which is controlled
by wind conditions; and the availability of sediment for transport,
determined by vegetation cover and soil moisture (Kocurek and
Lancaster, 1999). Changes in these external drivers can be the
result of climate or human impacts. Climate change and variability
affects the mobility of sediment through variations in wind
strength; vegetation cover and soil moisture are directly influenced
by the amount of precipitation; the supply of sediment
may be affected by changes in wave energy, beach sediment budgets,
or river discharge. Changes to aeolian systems that can be
attributed to the effects of climate variability on annual to decadal
time scales include changes in the magnitude and frequency of
dust storms (Middleton, 1989), sand transport rates (Lancaster
and Helm, 2000), and activation or stabilization of areas of sand
dunes (Wolfe, 1997). Such changes are a good indication of the
response of a landscape to drought periods. In addition, human
impacts may affect vegetation cover by grazing pressure or trampling
by animals or people, and increase sediment availability of
soils due to disturbance by animals or off-road vehicles. Humans
can also directly or indirectly affect sediment supply from rivers
or the coastal zone.

VITAL SIGNS


Three main groups of vital signs for aeolian features and
processes have been identified. First, rates of sediment movement
by the wind give an indication of the magnitude and frequency
of aeolian transport events in an area, as well as changes in time
and space in response to stressors. Measurements or estimates
of rates of sediment transport (sand and/or dust) by the wind
provide information on the quantity of sediment transported in
this manner and therefore the likely contribution of wind action
to erosion and deposition. Second, dune field dynamics provide
information on how areas of dunes are responding to external and
internal stressors, including disturbance, changes in sediment
supply, and climate change and variability. Lastly, dune dynamics
provide information on how individual dunes or groups of
dunes are responding to stressors.
The level of effort and cost for each monitoring method are
summarized in Table 1.
Table 1
Table 1. Vital signs: Aeolian processes and features.
VITAL SIGN 1: FREQUENCY AND MAGNITUDE OF
DUST STORMS


The magnitude and frequency of dust storms is an index of
rates of wind erosion in the immediate vicinity of a defined location.
Reduction of visibility by blowing dust may also indicate an
influx of dust from neighboring upwind sources. In addition to the
methods described below, some information may also be obtained
from the visibility monitoring programs maintained by the Air
Resources Division of the National Park Service and its cooperators,
available at http://www2.nature.nps.gov/air/monitoring/.

Monitoring Methods

Level 1: Visual Observation of Dust Storms
Dust storms (Fig. 7) are defined as severe weather conditions
in which visibility is reduced to 1 km or less by blowing
dust. The frequency of dust storms is measured by the number
of such events in a given time period. The magnitude of dust
storms can be assessed by the duration of such conditions. Dust
events (also known as dust haze) are conditions when visibility is
reduced to 11.3 km or less by dust suspended in the air. Blowing
dust is a situation where dust is raised to a height of 2 m or more
by strong winds, but does not reduce visibility to less than 1 km.
Figure 8 provides an example of visibility reduction as a result
of a dust storm.
First order meteorological stations may record these conditions
as part of their normal schedule of hourly observations.
Such data have been used to assess the frequency of dust storms
in relation to climatic parameters, such as annual and seasonal
precipitation (e.g., Bach et al., 1996; Brazel, 1989; Goudie, 1983;
Goudie and Middleton, 1992; MacKinnon et al., 1990; Middleton,
1989). In many areas, dust storm frequency increases in the
period following years of lower than average rainfall, although
direct correlation between drought conditions and dust storm frequency
is not always evident.
Equipment required. No equipment is required.
Complexity. The complexity of this method is very low; it
can be conducted by a single observer
Cost. The cost is very low. No instrumentation is required,
except for wind speed measurements.
Methodology. Monitoring of dust storms can be undertaken
at selected locations where dust storms are known to occur and
suitable landmarks exist for visibility determinations. Landmarks
at 1 km and 11.3 km distance from the observation point should
be identified, and the time and date of dust conditions should be
recorded, as well as any relevant meteorological conditions (such
as wind speed) and site conditions (such as vegetation cover).
The number of dust events and their duration should be recorded
on a monthly and annual basis and compared to rainfall and antecedent
vegetation conditions.
Timing. Timing is event driven.
Figure 7
Figure 7. Dust Storm, Iraq.
Level 2: Camera Stations (Still and Video)
Remotely activated camera stations can be used to image the
time, location, and characteristics of dust plumes.
Equipment required. A camera station is required.
Cost. The cost is moderate—around $3,000 to $4,000 per
station. (All amounts listed herein are U.S. dollars.)
Complexity. Stations are moderately complex to set up and
maintain. Technical assistance is required for set up.
Methodology. Video cameras can be used as they are at
Owens Lake, California, by the Great Basin Unified Air Pollution
Control District, but their resolution and data capacity
are limited. For real-time images see http://www.gbuapcd.org/
dustcam.htm. Digital still camera stations have been used to
monitor dust storms in the Mojave National Preserve since 2000
(Tigges et al., 2001). The stations automatically acquire digital
color images of dust storms, with the cameras triggered by wind
speeds above a predetermined threshold. The images are used to identify the locations from which dust particles become airborne,
the direction and intensity of the dust event, and the meteorological
conditions at the time, in conjunction with Climate Impact
Meteorological (CLIM-MET) sites (http://climchange.cr.usgs.
gov/info/sw/clim-met/) in the area. The system is placed on top
of a mountain to provide views to sites of dust emission at distances
of 9–20 km. This system is made up of several off-the-shelf
components, and several components that were designed
and built in-house. Together they perform the task of automatically
recording digital images from an unmanned remote location,
with recording triggered by wind speed sensors, controlled
by a data logger. Images are recorded on a compact flash card
with date and time information, for subsequent correlation of
images and meteorological data.
The number of dust events and their duration should be
recorded on a monthly and annual basis and compared to rainfall
and antecedent vegetation conditions.
Timing. Timing is event driven.
Figure 8
Figure 8. Reduced visibility from  major dust storm in West Texas, 15 December 2003. Compare with a normal clear day (inset). Photographs by Jeff Lee, Texas Tech University.
Level 3: Visibility Sensors
Automated sensors can be used to estimate the reduction in
visibility due to blowing dust, and therefore provide information
on the timing, magnitude, and frequency of dust events.
Equipment required. A visibility sensor and a data logger
are needed. The instrument should be co-located with other meteorological
instruments, including anemometers, to establish the
conditions for dust generation and transport.
Complexity. This method requires technical assistance to set
up; it is simple to maintain thereafter.
Cost. This method is expensive. Sensors cost $10,000 (Vaisala),
plus $2,000 to $3,000 for setting up the station.
Methodology. A variety of visibility sensing devices measure
visibility automatically using the forward scattering of infrared
light in air over a range of 10–50,000 m. Normally used at airports
and other locations at which visibility measurements are made for
safety monitoring, such instruments are commercially available,
automated, and self contained. Applications specific to dust monitoring
are rare, but a visibility sensor has been deployed in association
with a U.S. Geological Survey/Desert Research Institute
Desert Winds site at Jornada Experimental Range, New Mexico,
for several years, and has routinely collected information on visibility
reduction resulting from increased dust content in the air.
Timing. The number of dust events and their duration should
be recorded on a monthly and annual basis and compared to rainfall
and antecedent vegetation conditions.
VITAL SIGN 2: RATE OF DUST DEPOSITION

The rate of dust deposition can indicate the rate of wind
erosion in areas that are upwind of a specific site. Wind-blown
dust may be derived from local sources, such as playas (dry or
ephemerally flooded lake beds), and/or more distant sources, like
far-traveled dust from the Sahara and Asia. The rate of dust deposition
is measured as mass/area/time.
Wind-blown dust is an important long-term contributor of
fine material and ions to soils in arid regions and adjacent areas,
where it also affects water quality and human health.
Deposition of dust may have a significant effect on the
composition and nature of soils in arid regions and beyond. Far traveled
dust may have a significant effect on soil chemistry and
nutrient status (Farmer, 1993). Rates of dust deposition can be
measured using a variety of active and passive samplers (Goossens
and Offer, 2000).

Monitoring Methods

Level 1: Dust Traps
A convenient and practical method of passive sampling
of atmospheric dust deposition has been developed by Marith
Reheis of the U.S. Geological Survey (USGS) (Reheis, 1997,
2003; Reheis and Kihl, 1995) and utilized in extensive networks
for sampling dust input to soils in the southwestern United States.
Similar techniques have been used to monitor dust deposition
rates in the Dry Valleys of Antarctica (Lancaster, 2002).
Equipment required. A dust trap and field and laboratory
supplies for cleaning the dust trap and collecting samples are
needed.
Complexity. The complexity is low.
Cost. The cost is low. Each trap costs less than $50 to set up.
Methodology. The method consists of installing a simple,
robust passive dust trap, which is cleaned and emptied periodically.
The trap is a coated angel-food cake pan painted black
on the outside and mounted on a post ~2 m above the ground
(Fig. 9). Glass marbles rest on a circular piece of metal mesh that
is fitted into the pan 3–4 cm below the rim. The 2-m height eliminates
most saltating sand-sized particles. The marbles simulate
the effect of a gravelly surface and prevent dust that has filtered
or washed into the bottom of the pan from being blown away. The
dust traps are fitted with two metal straps looped in an inverted
basket shape, and the top surfaces of the straps are coated with
a sticky material to discourage birds from roosting. At the chosen
monitoring interval, the deposited particles are removed by
rinsing the pan, the screen, and the marbles in de-ionized water
into a 1 L plastic bottle (see Reheis [2003] for full details of trap
construction and field procedures).
In the laboratory, the sample is slowly dried at ~35 °C in
large evaporating dishes or beakers; coarse organic material is
also removed during this process. The mineral matter remaining
can then be weighed. Subsequent physical and chemical analyses
on dust samples include: (1) moisture content, (2) organic matter,
(3) soluble salts, (4) total carbonate (calcite plus dolomite),
and (5) grain size. Other chemical analyses such as phosphorus
fractions, strontium and other isotopes, elemental and mineralogical
composition, and magnetic properties can be performed
on selected samples, depending on sample size and monitoring
or research needs.
Figure 9 (description follows)
Figure 9. US Geological Survey Reheis Dust Trap. Photo by Marith Reheis, USGS.
Following the definitions of Reheis and Kihl (1995), total aeolian flux is defined as the rate of deposition of material in grams per square meter per year (g m–2yr–1). This can be divided
into two components: “dust” flux, which comprises material <50 μm in diameter (silt and clay size), and “sand,” which is material >50 μm in diameter. The rate of aeolian deposition is
calculated as follows: aeolian deposition rate (g m–2yr–1) = mass of dust retained on filter (g) * 1/area of dust pan (m2) * time exposed (yr). Timing. Measurement frequency should be annual or semiannual (to distinguish seasonal changes in dust flux).
VITAL SIGN 3: RATE OF SAND TRANSPORT

The rate of sand transport on sandy surfaces is an indicator
of the activity of aeolian processes in an area. Rates of sand transport
in relation to wind speed have been determined empirically
in wind tunnel studies (Lancaster, 1995). Measurements of sand
transport rates in natural settings are much less common (except
perhaps in coastal areas), and long-term monitoring studies of
sand transport rates are rare.

Monitoring Methods

Level 1: Estimate Sand Transport from Available
Climatological Data

Providing that quality wind data are available, sand transport
rates may be estimated from wind speed data measured at
meteorological stations using one of a number of empirical and
theoretical equations (see reviews in Pye and Tsoar, 1990; Sarre,
1987). These rates are, however, potential rates, because actual
sand transport rates may be reduced by the availability of suitable
sediment for transport, presence of vegetation or other elements,
surface moisture, crusting, and cohesion of the surface.
Equipment required. This method requires access to preexisting
wind records.
Complexity. The complexity is low.
Cost. The cost is low.
Methodology. One of the most widely used equations for
estimating potential sand transport from wind data is the one
developed by the USGS (Fryberger, 1979), with modifications and
cautions discussed by Bullard (1997). The “Fryberger method”
also provides classification schemes for characterizing the energy
and directional variability of wind regimes. The method has been
widely used to characterize aeolian sand transporting conditions
(e.g., Sweet et al., 1988).
The Fryberger method only considers winds above a threshold
velocity for sand movement and weights these winds in recognition
of the fact that stronger winds are proportionately more
effective in transporting sand than weaker winds. Thus:

q∝V 2 (V −Vt)/100

where q is the rate of sand transport, V is the wind velocity at
10 m height and Vt is the impact threshold for transport measured
at 10 m. This weighting equation is then calculated for all wind
speed categories above the threshold and applied to the percentage
frequency of these wind speed categories so that:

Q∝V 2(V −Vt )t

where Q is the rate of sand drift (expressed in vector units), and t is
the percentage frequency of winds in that wind speed category.
The total Q for each wind direction and the total for a station
are obtained by summation to give the sand “drift potential,” or
DP; and the vector sum or resultant sand drift (RDP) magnitude
and direction are obtained by vectoral summation. The DP is
a measure of the total wind energy of a location, whereas the
ratio between RDP and DP is a measure of the directional variability
of the wind regime, which has been widely noted as a
major control of the type of sand dune in an area (Fig. 6). Full
details of the methods are contained in Fryberger (1979). The
original method was based on the use of wind speeds recorded
in knots. Bullard (1997) cautions the user and provides information
on the use of the weighting factors for winds recorded
in other units. Saqqa and Saqqa (2007) provide a simple computer
program for estimating sand transport potential. Figure 10
shows an example of a “sand rose” calculated using the Fryberger
method, as well as an example of monthly changes in
drift potential.
Timing. This method should be performed annually or
monthly.
Figure 10 (Description follows)
Figure 10. (A) Example of a "sand rose" developed using the Fryberger (1979) method. (B) Monthly variations in drift potential, Palm Springs, California.
Level 1: Abrasion Stakes
An estimate of the rate of wind transport of sand from different directions can be derived from observations of the removal of layers of paint from wooden or aluminum stakes, as used in Pangnirtung National Park, Baffi n Island, Canada (McKenna
Neuman and Gilbert, 1986).
Equipment required. 1–2-m-high aluminum or wooden poles are needed.
Complexity. The complexity of this method is low.
Cost. The cost of this method is low.
Methodology. The poles should be set up in areas known to
experience sand transport. Each pole is painted with eight layers
of exterior enamel paint of different colors. Exposure of paint of
different colors at different heights and orientation on the stake
gives a relative estimate of the intensity of sand transport by different
wind directions. This method will work best when sand
transport is at a high intensity and thus able to scour paint off
the poles.
Timing. Timing should be annual or seasonal.
Level 2: Sand Traps
A wide variety of sand traps have been developed to measure
rates of sand transport in laboratory and field settings. See Goossens
et al. (2000) and Nickling and McKenna Neuman (1997) for
a review of different trap designs.
Equipment required. Sand traps, a balance for weighing
sand, and collection bags are needed.
Complexity. The complexity is low to moderate.
Cost. The cost is low. Each Fryrear trap costs approximately
$80 (see below).
Methodology. Long-term monitoring of sand transport rates
using traps requires that the traps be robust and self-orienting into
winds from different directions. Although many different types of
traps have been developed, few can withstand long-term exposure
and maintain collection efficiency. An efficient and robust passive
sand trap was designed by the U.S. Department of Agriculture
(Fryrear, 1986) (Fig. 11). (See http://www.fryreardustsamplers.
com/BSNE.html.) This type of trap has been used extensively for
time-integrated measurements of sand-size particles moving in
saltation in harsh environments (Gillette et al., 1997b; Gillette,
1999; Gillette and Chen, 2001) and for long-term monitoring
(Lancaster and Helm, 2000; Tigges et al., 1999). These passive
collectors maintain a collection efficiency of ~90% for a wide
range of wind speeds (Shao et al., 1993).
Figure 11 (Description follows)
Figure 11. The Big Spring Number Eight sand trap (from http://www.fryreardustsamples.com/bsne.html).
The traps can be exposed at a single height (typically 10 cm)
or at multiple heights (spaced logarithmically, as in Fig. 11). They
may be emptied of sand at any desired interval, taking care that
the trap is not filled completely, and the contents weighed. The
total horizontal flux of sand can be calculated using the approach
of Gillette and Chen (2001). The sand traps should be co-located
with wind speed and direction sensors.
Timing. Timing should be weekly or monthly, depending on
the rates of sand transport to be expected.
Level 3: Electronic Sand Transport Sensors
Electronic sand transport sensors have been developed to
allow remote monitoring of sand transport rates in conjunction with
measurements of wind speed and other parameters. The devices
operate on a piezo-electric principle, in which sand grains impact
a protected crystal, which sends a signal recording the number and
kinetic energy of grain impacts (Gillette and Stockton, 1986). One
device is the Sensit™ (http://www.sensit.com/). In practice, the
kinetic energy signal from this device has been difficult to interpret.
The particle count signal is calibrated by the manufacturer or
in a wind tunnel with a known mass flux of sand.
Equipment required. A sand transport sensor, data logger,
anemometer, and wind vane are needed.
Complexity. The complexity is moderate to high. Technical
knowledge is needed to install and maintain devices and interpret
data.
Cost. The cost is moderate to high; Sensits are about $2,000
each, Safires about $700 each. A data logger and associated
instrumentation (e.g., anemometer and wind vane) are required,
adding a further $2,000–$3,000 to the cost.
Figure 12 (Description follows)
Figure 12. Sensit sand transport sensor at Owens Lake, California. Sensor is mounted so that height of crystal above surface can be adjusted if surface changes.
Methodology. The Sensit device has been widely used
for sand transport monitoring (Fig. 12). Examples include the
USGS Desert Winds (Tigges et al., 1999) and CLIMET programs
(http://climchange.cr.usgs.gov/info/sw/clim-met/) and the Great
Basin Unified Air Pollution Control District at Owens Lake,
California. Sensits should be co-located with anemometers and
wind vanes to ensure sand transport is monitored in conjunction
with relevant wind data. A newly developed alternative to the
Sensit are the Saltation Flux Impact Responders (Safires). Baas
(2004) evaluated the performance of these piezoelectric crystal
type instruments. He found that the Safire presents a minimal
obstruction to the wind flow and provides high-frequency omnidirectional
measurements at a relatively low cost compared with
other piezoelectric type sensors. Although Safires have been used
in coastal dune settings in the Netherlands (Arens, 1997), they
have not been used for long-term monitoring.
Timing. Data should be downloaded monthly.
VITAL SIGN 4: WIND EROSION RATE

The action of wind on exposed sediments and friable rock
formations causes erosion (abrasion) and entrainment of sediment
and soil particles. Wind erosion physically removes the
lighter, less dense soil constituents such as organic matter, clays,
and silts. Thus it removes the most fertile part of the soil and lowers
soil productivity (Leys, 1999). The rate of wind erosion of a
given area is a direct measure of the loss of the surface soil and
its contained nutrients, seeds, and soil materials. In addition to
lowering of the surface, some authors have noted changes in soil
texture as a result of wind erosion. Although these mainly apply
to agricultural fields, some coarsening of natural surfaces may
occur as a result of wind erosion.

Monitoring Methods

Level 1: Lowering of Affected Surfaces
Equipment required. Erosion pins (e.g., rebar) and a measuring
tape are needed.
Cost. The cost is low.
Complexity. The complexity level is low.
Methodology. The rate of lowering of affected surfaces, such
as sand sheets, playa surfaces, and alluvial flats may be monitored
using erosion pins at strategically located sites. Monitoring
of erosion using erosion pins can be as simple as using lengths
of rebar hammered into the ground and measuring the exposure
of the pin relative to the ground surface at regular intervals. Such
methods have been used to monitor wind erosion rates at Jornada
Experimental Range and Owens Lake (Fig. 13). When placing
the pins, care should be taken to choose a representative sample
of the area. Multiple pins per site are useful to estimate local variability
in rates. The resolution of this method is relatively low; it
is difficult to measure surface changes of less than 10 mm.
In addition, qualitative estimates of wind erosion can be
made via the degree of exposure of roots and creation of residual
pedestals of soil by wind erosion.
Timing. Timing should be monthly or annually.
Figure 13 (Description follows)
Figure 13. Erosion pins for monitoring dune dynamics, Namib Desert.
Level 2 and 3: Measurements of Dust Concentration
Downwind of Affected Area

Measurements of dust concentration in the air downwind
of an eroding area can provide information on the mass of fine
material eroded from that area.
Equipment required. Dust concentration measuring devices
(e.g., DustTrak) are needed.
Cost. Costs are very high. MiniVol is $3,500; DustTrak is
$3,000 to $4,000; TEOM is $25,000 or more.
Complexity. The complexity level is high to very high. Skilled
technical support is required to install and maintain TEOM instruments.
DustTrak can be operated with minimal training.
Methodology. The vertical profile of dust concentration
above the surface is a measure of the amount of fine particulate
matter being emitted from a surface (the vertical flux of dust).
Typically, dust concentration shows an exponential decrease in
height above the emitting surface, due to dispersion of the dust by
atmospheric turbulence. The gradient (slope) of the profile is proportional
to the rate of emission. Measurements of concentration
profiles require sophisticated instrumentation and are a research,
rather than a monitoring, technique. They apply only to the conditions
being studied (Gillette et al., 1997a; Gillette et al., 1997b;
Nickling et al., 1999).
An estimate of dust emission rates can be obtained via measurements
of particle concentrations downwind of the eroding
area. Such measurements are made using one of a wide variety of
devices used to measure ambient air quality and are usually targeted
toward a specific particle size (e.g., PM10, or 10 μm). Such
devices employ a pump, which draws air at a precisely calibrated
rate through a filter, on which the dust collects for subsequent
weighing and analysis. Typical examples of these devices are
the HiVoL and MiniVol samplers used to collect dust to ensure
compliance with federal (Environmental Protection Agency) and
state air quality standards (Chow, 1995). Newer devices include
the DustTrak (Fig. 14), which employs a laser beam to measure
dust concentrations in a chamber within the instrument (http://
www.tsi.com/Product.aspx?Pid=11). The TEOM, Tapered Element
Oscillating Microbalance, (http://www.rpco.com/products/
ambprod/amb1400/index.htm) is another, more costly device,
and it is the only real-time particulate monitor that directly and
continuously measures the mass of particulates collected on a filter
and provides continuous particulate concentration data. Such
devices can be installed in critical areas (such as Owens Lake,
California). They also provide information on the magnitude
and frequency of dust storms via changes in dust concentration
over time.
Timing. Timing should be event based, or data can be downloaded
weekly.
Figure 14 (Description follows)
Figure 14. DustTrak monitoring device deployed at Jornada Experimental Range, NM. (Photograph by W.G. Nickling.)
VITAL SIGN 5: CHANGES IN TOTAL AREA
OCCUPIED BY SAND DUNES


The total area occupied by sand dunes is an indication of the
long-term supply of sediment to an area. Changes in the total area
and location covered by dunes will, over time, reflect the long term
sediment budget of an area, as well as the degree to which
the dune field as a whole is migrating. Decreases in the area covered
by dunes as well as the size of dunes (see below) reflect a
negative sediment budget in which sediment is being lost from
the dune field faster than it is supplied. Conversely, an increase
in dune size or area of dunes may indicate a positive sediment
budget in which the supply of sediment exceeds losses.
Monitoring Methods

Level 1: Delineation on a Map of Area Occupied by Dunes
Measurements or estimates of the area occupied by dunes
are necessary steps to establishing a baseline for monitoring
changes in dune fields.
Equipment required. Maps and/or orthophotograph quads
and a planimeter are needed.
Cost. The cost is low.
Complexity. The complexity level is low.
Methodology. The most straightforward method of assessing
changes in dune area is to delineate the area(s) covered by
dunes on published topographic maps or similar products. The
total area(s) covered by dunes can be estimated using a planimeter
or by measuring dune field width and length. If maps compiled
at different dates are available, then comparisons of dune
field area and position can be made. In many cases, however, the
area(s) of dunes depicted on USGS 7.5′ quad (1:24,000 scale)
topographic maps is very generalized. A more accurate estimate
of dune field area can be obtained from the digital orthophoto
quarter-quadrangles (DOQQ), which are compiled directly from
aerial photographs. It should be noted that this method can only
provide information on dune field area for the dates when the
maps were compiled, which may be irregular or infrequent. If
a more frequent assessment of dune field area is required, then
Level 2 or Level 3 methods should be used.
Timing. Intervals should be determined by available maps
and frequency of revision.

Level 2: Global Positioning System (GPS) Survey
In recent years, low-cost, handheld GPS units have provided
a simple, rapid, and accurate way to document geographical
areas. Field traverses of the margins of dune fields can provide an
accurate outline of the area.
Equipment required. A handheld GPS unit and maps and/or
orthophotograph quads are needed.
Cost. The cost of this method is low.
Complexity. The complexity level is low to moderate.
Methodology. Field traverse of the perimeter of the area of
dunes are conducted. Coordinates of key points are determined
by GPS. The coordinates of these points can then be transferred
to a topographic map base or geographic information system
(GIS). This method may be time-consuming for larger dune
fields and is best suited to small areas of dunes where an accurate
mapping of the area of dunes is required at a frequency
greater than that of map revisions or new aerial photograph or
satellite coverage.
Timing. This method can be performed at annual or longer
intervals.

Levels 2 and 3: Comparison of Areas of Dunes on Aerial
Photographs and/or Satellite Images

In many areas, a long history (as much as 70 years) of dune
field dynamics can be compiled by comparing the area and position
of dune field margins on vertical aerial photographs or satellite
images taken at different times.
Equipment required. Aerial photographs, a scanner, access
to a GIS application are needed.
Cost. The cost is moderate to high, depending on the cost of
aerial photograph coverages.
Complexity. The complexity level is high; this method
requires knowledge of GIS applications.
Methodology. At the simplest level, transparency sheets
(made of mylar, for example) are laid over the photographs. The
area can be delineated on the transparency, and the information
transferred to a topographic map using visual comparison
to features common to both. More accurate and more valuable
information can be gained by scanning the images, correcting
their geometry in a GIS and compiling coverages of dune field
area at different times. Geometric corrections are necessary to
co-register the images in a common geographic reference frame.
The GIS can then be used to generate dune areas and to estimate
changes in area and/or position over time (Lancaster, 1997; Lancaster
et al., 2001).
Satellite image data (e.g., Landsat) can also be used to generate
data on dune field dynamics. Because their spatial resolution
is in tens of meters, they are best used for larger dune areas.
Figure 14. DustTrak monitoring device deployed at Jornada Experimental
Range, NM. (Photograph by W.G. Nickling.)
Aeolian features and processes 15
High-resolution satellite image data are available, but these data
are expensive and have a limited temporal and spatial availability.
Landsat satellite data are also only available back to 1973, so
information from this source is only relevant to the past 30 years
or so. Despite these limitations, satellite data have been used to
provide a long-term view of the dynamics of some dune fields,
such as Great Sand Dunes (Janke, 2002; Marîn et al., 2005).
Timing. Intervals should be determined by dates of aerial
photograph coverages.
VITAL SIGN 6: AREA OF STABILIZED AND
ACTIVE DUNES


The active area of dunes, bare sand surfaces or migrating
(mobile) dunes, compared to the inactive area (stabilized by
vegetation) is a valuable indicator of the response of both inland
and coastal dunes to changes in sediment supply and mobility. In
coastal dune areas, the primary control of dune activity is the supply
of sediment, because many coastal dunes are located in areas
where the climate permits growth of vegetation. Thus, many
coastal dunes are very active close to the supply of sediment at
the coast and become progressively less active inland as sediment
supply decreases.
In inland dune areas, the primary control of dune activity
is climatic. Dune mobility can be characterized by the ratio
between wind energy (W) and the effective precipitation (P/PE)
(Lancaster and Helm, 2000). Thus, dunes can be active in areas
that are characterized by windy conditions, although precipitation
can be quite high. The relations between wind energy and
effective precipitation for dune areas in the western United States
are shown in Figure 15.

Monitoring Methods

Level 1: Delineation of Area Occupied by Active and Inactive
Dunes on Topographic Maps

Equipment required. Aerial photographs and topographic
maps are needed.
Cost. Costs are low to moderate, depending on the cost of
aerial photograph coverages.
Complexity. The complexity level is moderate.
Methodology. The areas covered by active and vegetationstabilized
dunes can be interpreted from aerial photographs
or field survey and transferred to a topographic map base. On
most aerial photographs, bright tones indicate bare (active) sand,
whereas progressively darker tones indicate vegetation-stabilized
dunes and sand surfaces. Field checking is desirable to develop
accurate classification of areas. These methods have been used
to map areas of active and inactive dunes in many areas (Forman
et al., 2006; Lancaster, 1997; Paisley et al., 1991)
In some cases, historical records of dune conditions developed
by land surveys and explorations can be a valuable source
of information on very long-term trends (Muhs and Holliday,
1995).
Timing. Timing should be determined by the dates of aerial
photograph coverages.
Figure 15 (Description follows)
Figure 15. Dune mobility index values for locations in the western United States compiled from data provided by author and D. Muhs (US Geological Survey).
Level 2: GPS Survey
Equipment required. A handheld GPS unit is required.
Cost. The cost of this method is low.
Complexity. The complexity level is low to moderate (assuming
that a GPS unit is available).
Methodology. Field traverses of the margins of active and
inactive dune areas can provide an accurate outline of the area
of dunes in different states, once the coordinates of key points
are determined by GPS survey transferred to a topographic map
base. This method may be time-consuming for larger dune fields
and is best suited to small areas of dunes where an accurate mapping
of the area of dunes is required at a frequency greater than
that of map revisions or new aerial photograph coverage.
Timing. Intervals should be annual or decadal.
Levels 2 and 3: Comparison of Aerial Photographs and/or
Satellite Images

Equipment required. Aerial photographs or satellite images
and access to GIS application are needed.
Cost. The cost of this method is moderate, depending on the
cost of aerial photograph coverages and satellite images.
Complexity. This method is highly complex; it requires
knowledge of GIS applications and image analysis.
Methodology. In many areas, a long history (as much as 60
years) of dune field dynamics can be compiled by comparing the
area and position of areas of active and inactive dunes on vertical
aerial photographs. At the simplest level, transparency sheets
(made of mylar, for example) are laid over the photographs. The
area can be delineated on the transparency, and the information
transferred to a topographic map using visual comparison to features
common to both. More accurate and more valuable information
can be gained by scanning the images, correcting their
geometry in a GIS and compiling coverages of dune field area at
different times. Geometric corrections are necessary to co-register
the images in a common geographic reference frame. The GIS
can then be used to generate dune areas and to estimate changes
in area and/or position over time. Satellite image data can also be
classified to develop information on vegetation and land cover
characteristics and to determine changes in the area of active and
inactive dunes. For example, Janke (2002) was able to show that
dune grasses were being replaced by semi-desert scrub on the
west side of Great Sand Dunes, thereby reducing sand mobility.
Timing. Intervals are determined by dates of aerial photograph
and/or satellite image coverages.

VITAL SIGN 7: DUNE MORPHOLOGY AND
MORPHOMETRY


Dunes occur in a variety of morphological types in self-organized
patterns as a response to the wind regime (especially
its directional variability) and the supply of sand. Parabolic dunes
and nebkhas are controlled by the presence of vegetation. Sand
dunes occur in four main morphologic types (Figs. 5 and 6;
Table 2): Crescentic (transverse), linear, star, and parabolic.
The type of dune is therefore an indicator of the characteristics
of the wind regime and the amount of sand available for dune
construction. Changes in dune morphology over time can provide
valuable information about the long-term response of the
dune field to climate and sediment supply. For example, a change
from parabolic to crescentic dunes could indicate a reduction in
vegetation cover, whereas development of parabolic dunes from
crescentic types is indicative of increased vegetation cover, as in
coastal areas of Israel (Tsoar and Blumberg, 2002). Changes in
dune size are a clear indication of increases or decreases in sediment
supply and changes in sediment budgets.
Table 2. Morphological Classification of Dunes
Table 2. Morphological Classification of Dunes
Monitoring Methods

Level 1: Describe Major Dune Types and Their
Characteristics

Equipment needed. Maps, aerial photographs, and satellite
images of the dune area are needed, as well as access to any previously
published work.
Cost. The cost of this method is low.
Complexity. The level of complexity is low.
Methodology. Identification and description of the major
dune types occurring in a dune field is a necessary first step to
understanding the dynamics of a dune system. The different
dune types present should be identified using well-accepted
lassification schemes (e.g., McKee, 1979a) (Fig. 5, Table 2), and
described in terms of their height, width, and spacing. There are
numerous studies of dune morphology done in this way (see Lancaster,
1995, for examples), but many dune fields in the United
States have not been systematically described.
Timing. This method should be used as needed. Most dune
types do not change significantly over time periods of years to
decades, and many have remained similar for thousands of years.

Level 2: Map Dune Types and Their Distribution using Aerial
Photographs or Satellite Images

Equipment required. Maps, aerial photographs, and satellite
images of the dune area are needed.
Cost. Costs are moderate to high, depending on the cost of
aerial photograph coverages or satellite images.
Complexity. The complexity level is moderate to high; this
method requires knowledge of GIS applications.
Methodology. The goal at this level is to accurately map the
different dune types using aerial photographs or satellite images,
using a classification scheme as above. In this way, the area occupied
by different dune types can be estimated, and changes in dune
distribution and/or morphology can be assessed using sequential
aerial photograph series. There are numerous examples of the
application of these techniques (Andrews, 1981; Lancaster, 1990,
1993; McKee and Moiola, 1975; Sweet et al., 1988).
Timing. Intervals are determined by dates of aerial photograph
coverages.

Level 3: Use Digital Elevation Models (DEMs) to Estimate
Dune Size and Sediment Volumes

Equipment required. Computing resources and GIS applications
are needed.
Cost. Costs are low to moderate, depending on the cost of
image data.
Complexity. This method is highly complex; it requires
knowledge of GIS applications and data processing.
Methodology. DEMs can be used to estimate dune size,
spacing, and sediment volume using GIS software. With these
data, it is possible to accurately monitor changes in sand volume
that may be occurring as a result of changes in sediment supply.
Data may include online digital data (e.g., http://seamless.usgs.
gov/) or high-resolution LIDAR (light detection and ranging)
data, which may be available from state or local governments, or
specifi cally commissioned.
Timing. Intervals are determined by the dates of DEM or
LIDAR coverages.

VITAL SIGN 8: DUNE FIELD SEDIMENT STATE

Dune fields form part of the well-defined regional- and local scale
sediment transport systems in which sand is moved by wind
from source areas (e.g., distal fluvial deposits, sandy beaches) via
transport pathways to depositional sinks. Dune fields accumulate
downwind of source zones at points where wind speed and directional
variability change, so that the influx of sand exceeds outflux,
resulting in deposition and growth of a dune field. Over long
periods of time (decades to centuries and longer) the dynamics of
the system are determined by changes in the supply of sediment
of a size suitable for transport by the wind; the availability of this
sediment for transport, determined by vegetation cover and soil
moisture; and the mobility of this sediment, controlled by wind
strength. The interactions between these variables can be evaluated
in terms of the state of the aeolian system and the limiting
factors identified (Kocurek and Lancaster, 1999). Monitoring of
the current and past sediment state of a dune field is an aid to
understanding how it is responding to stressors.

Monitoring Methods

Level 1 and 2: Identify and Describe the Sources, Transport
Pathways, and Depositional Sinks of the System

Equipment required. Maps, aerial photographs, and published
reports are needed.
Cost. The cost is moderate to low, depending on the cost of
image data. Google Earth is also a good source of data.
Complexity. The complexity level is moderate to high. Some
expert knowledge may be required for interpretation of data.
Methodology. Monitoring of basic parameters and how
they change over time is essential to assessing the state of any
system. A regional survey of the primary and secondary sources
of sediment, the transport pathways, and the sinks for sediment
(depositional areas) is also valuable for addressing impacts on
the system. For example, knowledge of these parameters in the
Coachella Valley, California, was a necessary prerequisite for
developing a habitat conservation plan for the Coachella fringetoed
lizard (Griffiths et al., 2002).
The sources of sediment, transport pathways and sediment
sinks can be identified from published literature and maps, field
survey, and aerial photographs, supplemented by mineralogical
analyses of sand. Good examples of this approach are Griffiths
et al. (2002) and Sharp, (1966).
Timing. This method should be used at decadal intervals.

Level 3: Use Remote Sensing Data to Identify and Track
Sand Sources, Transport Pathways and Sinks

Especially in large, complex dune fields, it may be difficult to
assess sand sources, transport pathways, and sinks using published
studies and limited field surveys. Recent advances in both remote
sensing technologies and methods of analysis allow the identification
and monitoring of aeolian systems remotely, thus saving
many months of field research. These approaches were pioneered
in the Gran Desierto of Mexico (Blount and Lancaster, 1990;
Blount et al., 1990), and have been followed by more detailed
studies of sand sources for Kelso dunes, California (Ramsey et al.,
1999) and the Coachella Valley, California (Katra et al., 2009).
Equipment required. Satellite image data, computing
resources, and image analysis software are needed.
Cost. Costs are moderate, assuming image data are available.
Complexity. This method is highly complex; expert knowledge
is required for image analysis and interpretation.
Methodology. Primary minerals have distinctive characteristics
that can be identified in multispectral image data (such
as Landsat). This approach uses spectral information on the
sub-pixel scale to identify mineral composition and the relative
abundance of different primary minerals. Formerly, this was a
research technique, but some available image analysis software
applications (such as ENVI) include these techniques as part of
their suite of image analysis routines. Care should be taken with
interpretation of results.
Timing. This method should be performed at decadal
intervals.

VITAL SIGN 9: RATES OF DUNE MIGRATION

The rate of dune migration is inversely proportional to dune
height and directly proportional to wind speed and sand transport
rates. Monitoring rates of dune migration provides valuable and easily understood information on the dynamics of the aeolian
system. If the potential exists for dunes to move into areas of
concern (by crossing roads or migrating into critical habitats, for
example), then monitoring of migration rates can provide valuable
information for resource management. There is a long history
of studies of dune migration rates and several well-established
methodologies, as discussed below.

Monitoring Methods

Level 1: Field Survey of Dune Position Over Time
Equipment required. Marker stakes and a tape measure
are needed.
Cost. The cost is very low.
Complexity. The complexity level is low.
Methodology. Where dunes are well defined, rates of migration
may be monitored by comparing their position relative to fixed
markers, such as stakes driven into the ground. These markers may
be placed around the perimeter of isolated dunes or adjacent to the
lee face of transverse or parabolic dunes. The position of the dune
can be compared to the original stake positions and rates of change
determined on a seasonal or annual basis. Such methods have been
used at White Sands, New Mexico (McKee and Douglass, 1971),
and in Namibia (Bristow and Lancaster, 2004), and elsewhere. The
disadvantage of field surveys is the need to continually revisit the
monitored dune over the years, and the probability that monitoring
stakes may be buried or left behind as the dune advances.
Timing. This should be done annually.

Level 2: GPS Survey of Dune Positions
Equipment required. A differential GPS unit is needed.
Cost. Costs are low, providing that a GPS unit is available.
Complexity. This method is moderately complex; training in
use of GPS units is required.
Methodology. Dune migration rates can be determined and
monitored very easily with high-precision GPS surveys using a
differential GPS unit. Using this methodology, the coordinates
(latitude and longitude or UTM coordinates) of the leading edge
of a dune (usually the base of a slip face) can be determined with
a precision of less than 1 m, which is more than sufficient for
annual surveys of dune migration rates. The coordinates for the
position of the dune in successive years can then be compared
to determine any advance. This methodology has been used to
determine dune migration rates in Egypt (Stokes et al., 1999).
The outline of the dune can be also surveyed using this method,
providing a record of changes in dune morphology over time.
Timing. This should be done annually.

Level 3: Comparison of Dunes on Aerial Photographs or
Satellite Images of Different Dates

Equipment required. Aerial photographs and topographic
maps are needed.
Cost. Costs are low to moderate, except for computing
resources.
Complexity. This method is highly complex; it requires specialist
knowledge of GIS and data processing.
Methodology. In this method, the position of dunes at
different times is compared using aerial photographs taken at
selected intervals. At the simplest level, transparency sheets
(made of mylar, for example) are laid over the photographs.
The area can be delineated on the transparency, and the information
transferred to a topographic map using visual comparison
to features common to both. The change in position of the
dunes can then be measured on the map and divided by the
number of years between the aerial photograph coverages to
provide an estimate of dune migration rates. This method has
been used extensively in southern California (Haff and Presti,
1995; Long and Sharp, 1964; Sweet et al., 1988) and elsewhere
(Finkel, 1959; Hastenrath, 1967; and Slattery, 1990). It
works best when the dunes are well-defined and moving fairly
rapidly. In general, dune migration rates vary inversely with
dune size.
More precise and more valuable information can be gained
by scanning the images, correcting their geometry in a GIS and
compiling coverages of the position of the dunes at different
times. The GIS can then be used to generate maps of dunes at
different times and to estimate migration rates. This methodology
was used to examine dune and dune field migration rates in
the Christmas Valley, Oregon, in support of management of this
area by the Bureau of Land Management (Lancaster et al., 2001),
and at Great Sand Dunes, where changes in dune migration rates
were compared to climate data (Marîn et al., 2005).
Timing. Intervals are determined by the dates of the aerial
photographs.

VITAL SIGN 10: EROSION AND DEPOSITION
PATTERNS ON DUNES


The pattern of erosion and deposition on dunes provides a
record of the response of the dune to airflow patterns and vegetation.
Valuable information on the dynamics of the dunes and their
response to changes in climate and vegetation can be generated
in this way.

Monitoring Methods

Level 1: Repeat Photography
Many changes in the topography and morphology of dunes
are complex, and require careful, quantitative topographic survey.
A qualitative monitoring of seasonal, annual, or multi-annual
changes in dunes can be achieved using repeat photography from
fixed camera stations (Livingstone, 1987).
Equipment required. A digital camera and a GPS unit are needed.
Cost. The cost of this method is low.
Complexity. The complexity level is low.
Methodology. Critical areas of dunes, such as advancing
dune fronts, are identified, and a camera station with an unobstructed view
is established. The camera station is permanently
marked and its GPS location recorded. Clear information is
needed on the date and time of the photographs, the camera system
and focal length of lens used. Photographs or panoramas are
repeated on a regular basis.
Timing. This method should be repeated annually.

Level 2: Erosion Pins
Transect lines or grids of erosion pins can be set up across
dunes to measure erosion and deposition patterns at certain points
on the dunes (Fig. 13). These patterns can then be compared to
winds and, if relevant, vegetation cover. This method has been
used to monitor changes on a dune in Namibia for over 20 years
(Livingstone, 1989, 1993, 2003). Other examples include studies
of coastal dune systems (e.g., Arens et al., 2004; Gares, 1990;
Gares and Nordstrom, 1995; Jungerius and Verheggen, 1981).
Equipment required. Erosion pins (or stakes) and tape
measure(s) are needed.
Cost. The cost is low (after initial set up).
Complexity. The complexity level is low to moderate.
Methodology. Grids or transects of erosion pins are set up
across the dune using pins at intervals of 5 or 10 m (or at critical
points, such as the base of the slip face). If possible, positions
of pins should be surveyed. Measurements from tip of pin
to surface should be recorded, as well as the height (exposure) of
pin. Changes in the exposure of the erosion pins (less exposure =
deposition; increased exposure = erosion) provide a record of
dune dynamics.
Timing. This method can be used weekly, monthly, or annually.
Shorter intervals provide more precise information and are
easier to relate to winds and vegetation conditions.

Level 3: Topographic Survey
Detailed topographic surveys, with a contour interval of
1 m or less can provide very useful data for monitoring dune
changes and dune dynamics. These techniques have been used to
assess dune changes in several studies, in Oman (Warren, 1988),
Namibia (Livingstone, 2003; Ward and von Brunn, 1985); and
in coastal blowout dunes in the Netherlands (Arens, 1997; Arens
et al., 2004).
Equipment required. Survey instruments (total station) or a
differential GPS unit are needed.
Cost. The cost is moderate, assuming that equipment can be
borrowed or rented.
Complexity. The level of complexity is moderate to high.
Training in surveying is required; analysis of results requires a
GIS expert.
Methodology. This type of survey can be carried out using
a total station, which downloads coordinates to a computer for
subsequent plotting in a contouring and mapping program such
as Surfer. If a differential GPS unit is available, then similar, but
slightly less precise, data can be generated from a detailed GPS
topographic survey. Either type of survey can generate data for a
digital elevation model, or DEM. Changes can be assessed quantitatively
by comparing digital elevation models for different time
periods, generating information on areas where changes have
occurred and on the volumes and rates of erosion and deposition
in these areas.
Timing. This data can be generated at seasonal to annual
intervals.

SUMMARY AND RECOMMENDATIONS FOR
MONITORING OF VITAL SIGNS


This section provides a statement of the most effective methods
for monitoring of the vital signs identified for aeolian processes
and landforms.

Vital Sign 1: Frequency and Magnitude of Dust Storms

Providing that personnel are available to record visibility
reduction caused by blowing dust, visual observation and recording
is the preferred and most cost effective method for monitoring
of dust events. In cases where the site is remote, then automated
camera systems are the preferred methodology.

Vital Sign 2: Rate of Dust Deposition

The USGS dust trap method is reliable and simple, and provides
a valuable record of dust deposition over periods of years
to decades.

Vital Sign 3: Rate of Sand Transport

Estimation of potential sand transport rates from wind data
is a necessary first step for monitoring of sand transport rates.
This also provides data that can be compared with other areas.
Long-term field monitoring of transport rates using the Big
Spring Number Eight (BSNE) trap provides a valuable record, if
the site(s) are carefully chosen.

Vital Sign 4: Wind Erosion Rate

Use of erosion pins and other topographic data can provide a
good documentation of wind erosion rates for specific areas.

Vital Sign 5: Changes in Total Area Occupied by
Sand Dunes


Although more complex and expensive, a GIS approach is
far superior to other methods for estimation of dune field changes,
providing quantitative data that can be used in conjunction with
climate records to understand long-term aeolian dynamics.

Vital Sign 6: Area of Stabilized and Active Dunes

Mapping of active and stabilized dunes using satellite image
data is an excellent method. When used in combination with a GIS, this approach is far superior to other methods for estimation
of dune field changes, providing quantitative data that can be
used in conjunction with climate records to understand long-term
aeolian dynamics.

Vital Sign 7: Dune Morphology and Morphometry

Use of aerial photographs and/or satellite images is the
preferred method for describing dune morphology and documenting
any changes that may occur. Although more complex
and expensive, a GIS approach is far superior to other methods,
providing quantitative data to understand long-term aeolian
dynamics.

Vital Sign 8: Dune Field Sediment State

Valuable data on sediment state can be obtained using a
descriptive approach, with limited analyses of samples for bulk
mineralogy.

Vital Sign 9: Rates of Dune Migration

Rates of dune migration are best determined using repeated
GPS surveys, or if a long-term historical record is needed, by
comparison of dune positions on aerial photographs or satellite
images. In either case, a GIS approach for data recording and
synthesis is desirable.

Vital Sign 10: Erosion and Deposition Patterns on Dunes

Repeat photography and simple field surveys can provide
valuable information and are simple to set up and repeat.

STUDY DESIGN

General Principles

Any study design should consider the goals of the monitoring,
and therefore what process or landform will be studied,
why it should be monitored, and for how long. Short-term
observations of change are useful, but long-term monitoring
is very valuable, though it involves a long-term commitment
of resources. The personnel and other resources available will
largely determine the methods employed. In general, simple
methods regularly applied will yield good results. As far as
possible, monitoring programs should strive for quantitative
and reproducible results. All data gathered should be assessed
critically after two or three measurement intervals to determine:
(1) whether changes can be detected; and (2) whether
the data can be explained and understood using knowledge of
the process or landform being monitored. Adjustments to the
monitoring program then can be made as needed, but radical
changes should be avoided. It is always useful to have an outside
“expert” to act as a consultant.

An Example Program to Monitor Movement of Small- to
Moderate-Size Inland Dunes

Monitoring of the rates of movement of inland dunes can
provide a sensitive overall assessment of the activity of an aeolian
sand system and its response to natural and anthropogenic stressors.
An ideal program will combine both short-term (months
to years) and long-term (years to decades) monitoring of dune
migration rates. Migration rates should be compared to climatic
data on all time scales.

Major Project Milestones
1. Determine resources available for monitoring and select
appropriate methods to be used. As discussed above, the
best techniques for short-term monitoring are field survey
using fixed markers or a differential GPS survey; for
long-term measurements, use aerial photographs or satellite
images taken on different dates.
2. Ensure that relevant hourly wind speed and direction data
are available for the monitoring site. Upgrade existing
weather stations or install new equipment for the monitoring
program.
3. For short-term measurements: select dunes to be monitored.
Dunes should be selected to be representative of
the size and morphological type found in the study area.
If dune types vary significantly, then choose measurement
sites for each type. Ensure that monitoring sites are
easy to access and not likely to be disturbed by animals
or people. Set up fixed points and benchmarks. Allocate
resources for monthly or seasonal measurements.
4. For long-term monitoring: Acquire baseline image data
and incorporate into a GIS system.
5. For short-term monitoring: make monthly or seasonal
measurements and enter these data into a database. Produce
graphics showing changes over time.
6. For long-term monitoring: compare dunes on images
acquired annually. Produce maps of changes from year
to year.
7. After one year, assess short-term monitoring data (if data
are collected monthly). Compare rates of movement to
wind and other climate data. Determine trends. Are there
seasonal differences in migration rates? Do these relate to
variations in wind speed and/or direction over the year?
Determine optimal timing of measurements and adjust
program accordingly. It may be that dune movement is
slow enough that annual or seasonal measurements are
sufficient. Report results to scientific and management
communities. Provide public outreach.
8. After 5 years, assess long-term monitoring data on migration
rates by comparing image data from different years.
Determine trends, if any. Are there inter-annual differences
in migration rates? Do these relate to variations in
wind speed and/or direction, or to changes in precipitation
(and therefore vegetation cover)? Report results to
scientific and management communities.
CASE STUDIES

There are relatively few long-term monitoring studies of aeolian
processes and landforms, in part because of the remote nature
of many desert regions, and in part because of the perception that
geomorphic change is slow in deserts. Good examples include
the USGS Desert Winds project (now in part administered by the
Desert Research Institute), the USGS dust trap–monitoring study
in the southwestern United States, and a decades-long monitoring
of cross-sectional and morphologic change on a Namibian
linear dune. Common to the latter two projects is a relatively simple
and robust methodology. This simplicity ensures that ongoing
costs of monitoring are low, using a clearly targeted process or
landform and a dedicated principal investigator, who has maintained
the monitoring network over many years. Many of the
problems encountered by the Desert Winds project stem from the
overabundance of data collected and the lack of a clearly defined
purpose for the monitoring.

Long-Term Monitoring of Sand Transport and Climatic
Parameters: The Desert Winds Project


The Desert Winds Project was set up by the USGS in the early
1970s and was designed to permit remote monitoring of aeolian
processes (McCauley et al., 1984). Goals of the monitoring network
were: (1) to provide a long-term database for understanding
the range of environmental conditions that can be expected to
occur normally in arid and semiarid areas of the desert southwest;
(2) to acquire baseline data to assess changes in the desert such
as changes in vegetation, migration of sand, and increased dust
storms that may occur due to climate change in desert regions;
and (3) to acquire data for field-checking remotely sensed image
data of various surfaces, so that regional models can be developed
for monitoring land surface changes over time.
Each station was equipped with anemometers at three
heights (1.2, 2.7, and 6.1 m), a wind vane at 6.1 m, temperature
and humidity sensors at 1.2 and 6.1 m, and a tipping bucket rain
gauge. Transport of sediment by wind is monitored by BSNE
sand traps (Fryrear, 1986) mounted at 0.05 or 0.15 m, 0.50 m,
and 1.0 m, together with a Sensit piezo-electric sand transport
sensor at 0.05 or 0.15 m above the surface. Full details of the
instrumentation and operation of the stations are given in Tigges
et al. (1999). With the exception of the BSNE traps, all sensors
are scanned at one-second intervals with 6-minute averages of
the anemometer, wind vane, and Sensit readings, together with
12-minute average temperature, as well as hourly humidity, and
precipitation. Data were uploaded via the GOES satellite each
hour until the mid 1990s, when the original equipment was
replaced by data loggers, which are downloaded on a monthly
basis. Currently, only two of the original stations are operating:
Gold Spring, Arizona, and Jornada, New Mexico. In addition to
the meteorological data, there is repeat photography for the sites
at a series of marked camera stations, which provides an indication
of changes in vegetation cover over time.
Some preliminary results of the project are discussed in
Breed and Reheis (1999), but most of the data have never been
analyzed in a systematic fashion. An example of the application
of this unique monitoring network to understanding variability
of sand transport rates in relation to climatic parameters is documented
in (Lancaster and Helm, 2000). The data on long-term
variations in sand transport rates provide a record of the response
of sand transport rates to external stressors, including drought
periods and changes in the composition of vegetation communities
over a period of two decades at Gold Spring and Jornada
(Fig. 16). The Gold Spring data show the effects of heavy rainfall
on vegetation cover and sand transport rates in the period 1992–
1993 and subsequent droughts, while the Jornada data show an
order of magnitude increase in sand flux since the mid 1990s,
likely as a result of the change from a grassland to a mesquite-dominated
landscape.
Figure 16 (Description follows)
Figure 16. Changes in sand flux over time at Gold Spring and Jornada Desert Wind sites. Sand flux measured using Big Spring Number Eight traps.
Figure 17 (Description follows)
Figure 17. Changes in dust flux in the Mojave and Great Basin deserts (after Reheis, 2006). (A) Dust flux and precipitation at northern sites. (B) Dust flux and precipitation at southern sites.
Dust Deposition in the Southwestern United States

This is a good example of a monitoring project that provides
quantitative information on an important geologic process, as
well as data that lead to a greater understanding of how the process
responds to stressors. The project is ongoing and is designed
to monitor dust deposition rates in areas of the Great Basin and
Mojave deserts of the southwestern United States. Goals of the
project are to determine the rate and composition of dust inputs
to soils, and to relate dust accumulation to climatic patterns,
especially the amount and seasonal distribution of rainfall, as it
affects different dust source areas, including playas and alluvial
areas. Results of the project are summarized in Reheis, (1997,
2003, 2006) and Reheis and Kihl (1995). The methodology used
is briefly described above under Vital Sign 2, and discussed in
detail in Reheis (1999, 2003).
Thirty-five dust trap sites in the eastern Mojave Desert and
southern Great Basin have been monitored since 1984. Rates of
deposition of silt and clay, clay, carbonate, and soluble salts have
been determined on an annual or two-year basis, and compared
to data on annual and seasonal precipitation at nearby weather
stations (Fig. 17). Additional data on the chemical and mineral
composition of the deposited dust were also generated.
The data show that generation and accumulation of dust
is affected by the amount and seasonal distribution of rainfall.
However, different source types (alluvium, dry playas, and wet
playas) respond in different ways. A major factor in determining
dust generation is the condition of surface sediments, especially
their moisture content. For example, the flux of silt and clay and
soluble salt increased following the El Niño events of 1987–1988
and 1997–1998 at sites close to playas with a shallow depth to
groundwater. In this case, evaporative concentration of salts disrupted
surface crusts and increased the susceptibility of surface
sediment to wind erosion. The silt and clay flux increased during
drought periods at sites downwind of alluvial sources and playas
with deeper groundwater. This was the result of reduced vegetation
cover on alluvial sediments, and local runoff events that delivered fresh
sediment to playa margins and the distal portions
of alluvial fans (Reheis, 2003, 2006). Reheis (2006) also noted
geographical differences in the response of dust sources to precipitation
variability, with a greater range of dust fluxes noted in
southern (mostly Mojave Desert) sites.
Figure 18 (Description follows)
Figure 18. Changes in linear dune morphology (simplified from Livingstone, 2003).
Topographic Change on a Namibian Linear Dune

Most long-term studies of desert dunes have concentrated
on monitoring rates of movement of small crescentic or barchan
dunes via comparison of the position of dunes on aerial photographs
taken at different time intervals. Studies of the dynamics
of individual dunes are relatively rare. This project has monitored
surface topographic change on a large linear dune in the central
Namib Desert since 1980. Erosion pins were set up in 1980 and
monitored weekly for the first four years (Livingstone, 1989).
Subsequently, the dune was resurveyed in 1993, providing information
on dune change over a decade (Livingstone, 1993), and
again in 2001 (Livingstone, 2003). Methods used varied over the
period of monitoring, from direct measurement of erosion pins in
the intensive phase to use of a total station in 2001. Some of the
erosion pins placed in 1980 survived to provide fixed points for
subsequent surveys. Results of the intensive monitoring showed
that the crest region of the dunes is the most active. The crestlines
migrate over a lateral distance of as much as 14 m over a
12-month period, but with little net change over periods of years
due to changes in seasonal wind directions. On the dune studied
by Livingstone, the crest area changed from a relatively high
single crest in the 1980s to a slightly lower double crest form in
the 1990s, and then back to single crest form by 2001, regaining
much of its original height (Fig. 18). The lower, or plinth, areas
of the dune showed little change over the period of study. Livingstone
(2003) attributed the changes in dune crest characteristics
to changes in the relative magnitude and frequency of strong easterly
winter winds, which increased in the late 1980s. The studies
also suggested that these large dunes are not migrating because
no lateral movement was detected. More recent studies show,
however, that the rate of lateral migration of these dunes over
periods of hundreds to thousands of years is only 0.13 m/year
(Bristow et al., 2007), so the net migration over the period studied
by Livingstone would have been ~0.25 m, and therefore difficult
to detect.
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Last updated: August 3, 2017