Modeling evapotranspiration and surface energy budgets across a watershed

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 semiarid 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 the accurate estimate of ET and the entire surface energy balance possible. The Simultaneous Heat and Water (SHAW) model, a detailed physical process model capable of simulating the effects of a multispecies plant canopy on heat and water transfer, was applied to 2 years of data collected for three vegetation types (low sagebrush, mountain big sagebrush, and aspen) on a semiarid watershed. Timing and magnitude of ET from the three sites differed considerably. Measured and simulated ET for approximately 26 days of measurement in 1990 were 41 and 44 mm, respectively, for the low sagebrush, 74 and 69 mm for the mountain big sagebrush, and 85 and 89 mm for the aspen. Simulated and measured cumulative ET for up to 85 days of measurement at the three sites in 1993 differed by 3– 5%. Simulated diurnal variation in each of the surface energy balance components compared well with measured values. Model results were used to estimate total ET from the watershed as a basis for a complete water budget of the watershed.