Modeling Evapotranspiration and Surface Energy Budgets Across a Watershed

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 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. Pierson, 1991Pierson, , 1996, snowmelt and soil freezing [Flerchinger and Saxton, 1989;Flerchinger and Hanson, 1989;Flerchinger et al., 1994a;Hayhoe, 1994], and evaporation. However, the ability of the model to simulate ET and the entire surface energy balance has never been adequately tested. The primary purpose of this paper was to test the ability of the model to simulate the temporal surface energy balance of three types of vegetation across a small semiarid watershed. A secondary objective was to obtain an estimate of total ET for the watershed to be used in computing a water balance for the watershed. Numerous studies have been conducted to test various aspects of the SHAW model, including variability of soil temperature and moisture due to vegetation effects [Flerchinger and

Surface fluxes and water balance of spatially varying vegetation within a small mountainous headwater catchment

“…Thus, model simulations from the Simultaneous Heat and Water (SHAW) model (Flerchinger et al, 1996) were used to: assess the representativeness of the measured flux data within the 10 to 20 day periods used to compare fluxes from the three sites; and extrapolate the measured periods to monthly ET estimates. The SHAW model has been tested and applied extensively over a range of vegetation types in semi-arid and arid environments, particularly in the surrounding RCEW (Flerchinger et al, 1996(Flerchinger et al, , 1998, and Link et al (2004) previously validated the model for fir forest canopies. The model simulates the surface energy balance, evapotranspiration and fluxes within a multispecies plant canopy using detailed physics of heat and water transfer through the soil-plant-atmosphere continuum, making it ideal for use in this study.…”

Balanço hídrico em área de pastagem no semiárido pernambucano

“…A evapotranspiração (ET) depende bastante da disponibilidade de água e energia e também das características do local (Li et al, 2007;Mitchell et al, 2009), sendo uma das principais formas de transferência de água no sistema soloplanta-atmosfera e em pastagens áridas e semiáridas, podendo ser responsável por mais de 90% do consumo da precipitação (Flerchinger et al, 1996).…”

Modelling the dynamics of the evapotranspiration process using genetic programming

“…potential evapotranspiration). The complexity of the model varies depending upon the uniformity of the surface, canopy properties, and baseline assumptions (see Shuttleworth & Wallace, 1985;Choudhury & Monteith, 1988;Granger & Gray, 1989;Flerchinger et al, 1996;Biftu & Gan, 2000). Among the wealth of local and global evapotranspiration models, the Penman-Monteith (PM) (Monteith, 1965) equation is perhaps the most widely adopted evapotranspiration model (Abbott et al, 1986).…”