Submitted to: American Meteorological Society of the Conference on Hydrology Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/5/1997
Publication Date: N/A
Interpretive Summary: In the southwestern United States, nearly 2/3 of the annual precipitation occurs during the summer months. This is the period when thunderstorms bring the most intense showers and cause normally dry washes to flow and ponds to fill. This is also the period when rangeland vegetation, grasses and shrubs, produce foliage and make use of the water for growth. These two orepresentative vegetation types compete for limited water resources and both are subject to climate and human-induced impacts. To gain a better understanding of the water use, or evapotranspiration (ET), of these two plant communities, which will help explain the dynamics of the relative preponderance of one or the other, continuous measurements of ET for many years are needed. The USDA Agricutural Research Service has been using a variety of indirect methods to measure and estimate ET on two watersheds in southeast Arizona since 1990. Energy balance (EB) techniques use measurements of the total available energy to estimate the amount of energ that is being used for ET. Water balance (WB) techniques use measurements of rainfall, runoff and water held in the soil to estimate ET. Preliminary results of ET estimates using micrometeorological and hydrological techniques for several monsoons indicated that EB estimates were higher than the WB estimates. The relative ET observed in a grass dominated watershed was 15% higher than that observed in a shrub dominated watershed.
Technical Abstract: In the Sonoran and Chihuahuan desert regions of the southwestern U.S., nearly 2/3 of the annual precipitation occurs during the summer or "monsoon" period typically covering the months of July-September. This is the most dynamic period both from a hydrological and ecological perspective, yet few hydrometeorological measurements exist covering several monsoons to evaluate the water use by shrub and grass dominated communities. As these two ecosystems compete for available water and nutrients, they are strongly affected by climate variations and anthropogenic impacts, such as grazing. Therefore, to understand the factors which cause changes in vegetation cover over time, long term measurements are required to monitor vegetation responses. Since 1990 hydrometeorological data, including precipitation, runoff, weather data, surface energy fluxes and soil moisture profile measurements, have been collected at the USDA-ARS Walnut Gulch Experimental Watershed in southern Arizona. Preliminary results of evapotranspiration estimates using micrometeorological and water balance techniques for several monsoons indicated that energy balance estimates were higher than the water balance estimates. A difference of 15% in relative water use between the shrub and grass communities was observed.