Calculating Evaporation from Native Grassland Watersheds

EVAPORATION from soil and plant surfces in many natural grassland sites will often be less than potential evaporation because of sparse vegetation or soil water deficits in the rootzone. A computer model developed for calculating daily evaporation from row crop surfaces with partial cover was modified for use on native grasslands. Daily evaporation is computed by adding independently calculated soil evaporation and transpiration. Potential evaporation is calculated from commonly available atmospheric information. Soil evaporation rate is related to soil hydraulic properties, mulch cover, and potential evapora-tion rate. Transpiration rate is related to potential evaporation through the green leaf area index and the soil water status of the rootzone. Daily evaporation was calculated for three small native grassland watersheds where runoff, rainfall, and soil water content were measured. Seasonal changes in soil water content were calculated based on drainage rates computed from the amount of infiltra-tion in excess of the upper limit of extractable soil water and calculated evaporation rates. Calculated changes were usually within 5 cm of measured soil water during a 1-yr period. Water may be transported from soil to the atmosphere directly as evaporation from soil or as transpira-tion through plants. In many native grassland sites, actual evaporation rate (soil evaporation and transpira-tion) will depend on atmospheric, soil, and plant interactions. A model that describes how these factors combine to determine evaporation from crop plants has been described by Ritchie (1972) and Richardson and Ritchie (1973). Evaluations of evaporation from grasslands by Garwood and Williams (1967), Aase and Wight (1972), Nkemdirim and Yamashita (1972), and Nkemdirim and Haley (1973) indicate that evaporation response to atmos-pheric demands and soil water deficits of many grassland species is similar to that of many crop species. In this paper we report a modification of the cropland model for calcu-lating evaporation from native grass-lands and identify specific soil, plant, and atmospheric information needed for calculating grassland evapo-transpiration.