Implementation and Validation of GLEAMS-SF

GLEAMS (Leonard et al., 1987; Knisel et al., 1992; Knisel, 1993) is a continuous, field-scale hydrology and chemical transport model like Opus, simulating the transport and transformations of nutrients and pesticides in the root zone and in surface runoff. It runs on a daily time step, using daily rainfall information. GLEAMS uses a modification of the SCS Curve Number method (William and La Seur, 1976) to predict runoff and a modification of the USLE (Onstad and Foster, 1975) to predict erosion. Potential ET is estimated in GLEAMS by either the Priestly-Taylor (1972) method, using daily temperature and radiation data interpolated from fitting mean monthly data (Kothandaraman and Evans, 1972), or the Penman-Montieth method (Jensen et al., 1990), which uses wind movement and dew point temperature fitted from monthly data (Kothandaraman and Evans, 1972) in addition to the temperature and radiation data. Actual soil evaporation and plant transpiration are simulated separately by the method developed by Ritchie (1972) for incomplete cover. Separate components of ET are needed to partition chemical movement upward in the soil and into the transpiration stream for plant uptake. Plant transpiration is extracted from the active root zone using an exponentially decreasing function.