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United States Department of Agriculture

Agricultural Research Service

Title: Simulating Evapotranspiration on Semi-Arid Rangeland

Submitted to: North American Water and Environment Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 3, 1995
Publication Date: N/A

Interpretive Summary: Transport of mass and energy between and within soils, canopies and the atmosphere is an increasing area of interest in hydrology and meteorology. Recent advances in measurement and modeling have made it possible to get accurate estimates of evapotranspiration (ET) and the entire surface energy balance. On arid and semi-arid rangelands, ET can account for over 90% of the precipitation, making accurate knowledge fo the surface energy balance particularly critical. However, soils and vegetation vary considerably on rangeland, complicating efforts to estimate the ET portion of the water budget on even small rangeland watersheds. The SHAW model is a detailed process model which simulates heat and water movement through a plant-snow- ue-soil system. The model has the capability to simulate heat and water transfer through a multi-species canopy and directly computes soil evaporation separately from transpiration. The purpose of this paper was to demonstrate the ability of the model to simulate the temporal surface energy balance of varying types of vegetation from two very different rangeland ecosystems. The SHAW model was applied to a 400-cm profile for five types of vegetation from two very different rangeland ecosystems. Diurnal variation in the energy balance was simulated with reasonable accuracy. and temporal variation in evapotranspiration and the surface energy balance for a wide diversity of vegetation from widely varying ecosystems.

Technical Abstract: Detailed simulations of the surface energy balance using the Simultaneous Heat and Water (SHAW) model were compared with measurements collected at the Reynolds Creek Experimental Watershed in Idaho and the Walnut Gulch Experimental Watershed in Arizona. Simulations were conducted for three vegetation types (low sagebrush, mountain big sagebrush and aspen) in Idaho oand two vegetation types (grassland and creosote bush) in Arizona. Simulated diurnal variation in each of the surface energy balance components compared will with measured values for all sites. Timing and magnitude of evapotranspiration (ET)from the sites differed considerably. Measured and simulated ET for approximately 25 days of measurement at the Idaho sites 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. Measured and simulated ET for 20 days of measurement at the Arizona sites were 66 and 48 mm for the creosote bush and 74 and 69 for the grass dominated site. The variation in hourly simulated evapotranspiration accounted for by the model ranged from 56% for the creosote bush site in Arizona to 78% for the aspen site in Idaho.

Last Modified: 7/24/2014
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