Precipitation regime is the most important climatic factor for determining the structure and function of southwestern ecosystems. The availability of moisture drives (or limits) basic ecological processes, such as primary production, nutrient cycling, and plant reproduction (Miller 2005). Precipitation events in arid and semi-arid southwestern grasslands and shrublands are spatially variable and punctuated by extended dry periods, creating systems with high potential for evapotranspiration and limited opportunities for soil-water recharge. Seasonality, area, intensity (amount of moisture per unit of time), and form (snow versus rain) of precipitation events strongly influence which plant species and communities dominate grassland and shrubland ecosystems (Miller 2005).
Summer monsoon precipitation is an important source of moisture for arid and semi-arid grasslands in the American Southwest. Moist air masses originating in the Gulfs of Mexico and California are believed to be the source of the Southwest monsoon, which extends north from central Mexico to portions of the southwestern and central United States. Summer monsoon storms are typically high-intensity convective storms of short duration. The intensity of a precipitation event, in combination with soil surface characteristics, determines how much infiltration will occur and how much moisture will be lost as runoff (Whitford 2002). Summer monsoons of short duration may trigger soil surface processes, but they may not be sufficient to initiate seed germination, photosynthesis, or soil-water recharge (Miller 2005). A larger proportion of precipitation is lost as surface water runoff during summer monsoon storms than during winter storms, when moisture from melting snow can penetrate to greater soil depths. In contrast to monsoon precipitation, winter storms are lower-intensity, of longer duration, and cover larger areas. Winter precipitation can recharge soil water and affect the growth of perennial, deeply-rooted plants.
A second important climatic factor—wind—affects the structure and function of grassland and shrubland ecosystems by influencing wildfire behavior and rates of evapotranspiration. Wind is also a major driver of erosion, redistributing surface soil and soil-forming materials (Whicker et al. 2002). Disturbances such as wildfire and livestock grazing can contribute to dramatically increased levels of wind erosion.