George M. Hornberger and Justin E. Lawrence1
Address: ESPM Ecosystem Sciences Div.,
of the Interior
The hydrology of Big Meadows is driven by seasonal changes in climate, rainfall and snowfall events, and pumping for water supply. Groundwater levels respond to all of these driving variables. The level of water in the fens at Big Meadows, and therefore their lateral extent in terms of saturated or nearly saturated areas, likewise vary. Knowledge about the variations in moisture levels and groundwater levels across the open meadow and through time can be gained by use of monthly water balance models, both standard climatological water balances and balances estimated by using soil-moisture and groundwater models. The primary data necessary to develop and calibrate such models are meteorological data, soil-moisture data, and data on groundwater levels. The work in this project concentrated on collating existing data, on collecting additional data on soil moisture and groundwater, on analyzing these data, and on synthesizing the data through the use of models. The technical and scientific details of the work, as described in the body of this report, lead to the following inferences that can be useful in planning for water supply and environmental protection at Big Meadows.
(1) The Thornthwaite water balance (TWB), implemented in an Excel spreadsheet, provides a useful summary of monthly climatology for Big Meadows. An important output from the model is moisture surplus (groundwater recharge). A simple time-series (regression) model accurately relates the TWB surplus to discharge at Lewis Spring, which is the primary water-supply source for operations at Big Meadows. Thus, by using seasonal climate forecasts, it would be possible to predict Lewis Spring flows for months into the future. Such scenario forecasts could be useful in planning ahead for the peak summer season.
(2) The backup water supply in cases where flows at Lewis Spring are inadequate to meet demand is currently a well (BM14) about a mile southeast of the Visitors Center. Use of a well about a third of a mile north of the Visitors Center was discontinued in the late 1980s when the USGS reported that it was hydraulically connected to the nearby swamp. A critical question is whether use of BM14 might affect water levels in the fen in the southeastern part of Big Meadows. Measured spatial patterns of soil moisture and water levels in shallow wells suggest that the fen is a perched system, vertically separated from the underlying groundwater flow system by unsaturated material and therefore unaffected by groundwater pumping; i.e., pumping from BM14 is unlikely to have an effect on the fen.
(3) Models developed under this project indicate that impacts of future climate change on the hydrology of Big Meadows will lead to more extended areas and periods of low soil moisture, and to lower spatial variability as normally wet areas become dry, and to more frequent years of low flow at Lewis Spring. These changes may affect terrestrial ecosystems, especially in the fen in the southeastern meadow, and will certainly affect the reliability of the water supply from Lewis Spring.
The entire report is available to view or download in PDF file format. Using PDF files requires the free Adobe Acrobat Reader. If you do not already have it installed on your computer, you may download it now. Download Reader.
To download a pdf file, click on this icon in the toolbar of the pdf window: . This will allow you to save the file on your computer. If you want to copy or print only a small part of the saved file, click on this icon to select the desired text: .