Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/1996
Publication Date: N/A
Interpretive Summary: Interpretive Summary Over 90% of the precipitation that falls on semi-arid rangeland areas that cover much of the Western United States is returned back to the atmosphere through evaporation from the soil and transpiration from plants (evapotranspiration), with only a small fraction remaining for runoff and ground-water recharge. Thus, in order to understand the hydrology of these areas and to forecast the water supply to streams and reservoirs, it is vital accurately determine the evapotranspiration, which can vary tremendously across a landscape depending on the type of vegetation. Yet, methods to determine evapotranspiration from rangeland areas have not been fully developed or tested. The Simultaneous Heat and Water (SHAW) model is a detailed computer model with the ability to track available soil water for plant use and was shown to accurately simulate evapotranspiration from different plant communities. Having successfully tested the capabilities of the model, it can be incorporated into other computer simulation models to forecast water supply to streams and reservoirs, recharge to ground water, and forage production on rangelands.
Technical Abstract: Abstract Transport of mass and energy between and within soils, canopies and the atmosphere is an area of increasing interest in hydrology and meteorology. On arid and semi-arid rangelands, evapotranspiration (ET) can account for over 90% of the precipitation, making accurate knowledge of the surface energy balance particularly critical. Recent advances in measurement and modeling have made it possible to obtain accurate estimates of ET and the entire surface energy balance. The Simultaneous Heat and Water (SHAW) model, a detailed physical process model originally developed to simulate soil freezing and thawing, was applied to two years of data collected for three vegetation types (low sagebrush, mountain big sagebrush and aspen) on a semi-arid watershed. The SHAW model is capable of simulating the effects of a multi-species plant canopy on heat and water transfer. The plant canopy may be divided into several layers, and transfer of water vapor and energy are solved for each layer within the canopy. ET was measured for each vegetation type using the Bowen ratio energy balance method. Timing and magnitude of ET from the three sites differed considerably. Detailed simulations of the energy balance using the SHAW model were compared with Bowen ratio