DEVELOPMENT AND CALIBRATION OF A POTENTIAL EVAPOTRANSPIRATION MODEL FOR SITE OR REGIONAL STUDIES

A computer model of daily potential evapotranspiration (PET) was developed and calibrated using temporally and spatially variable measurements of solar radiation and potential evapotranspiration. The model can be run as a continuous multi-year simulation for a single site, watershed, or region (California was modeled using a 270-meter grid spacing). The model was written in FORTRAN 90 for single or parallel processors using a portable message-passing system (MPICH: Message Passing Interface, Chameleon) for Windows 2000® and Linux® beowulf clusters. The model uses the Priestley-Taylor formulation of PET, daily maximum and minimum air temperature mapped using a gradient inverse-distance-squared technique, calculated net radiation, and incoming short-wave radiation (modeled to account for the topographic shading of direct and diffuse radiation, receipt of ground-reflected radiation from surrounding topography, and the temporal and spatial distribution of atmospheric ozone, precipitable water, turbidity, circumsolar radiation, and ground albedo). The short wave radiation model compares well with 30 years of data collected at 29 solar radiation stations located in California, Arizona, Nevada, and Oregon. The ground-reflected shortwave radiation was calculated using albedo data spatially interpolated from the 29 stations and MODIS data. Incoming and outgoing longwave radiation was calculated from daily air temperature data, and ground heat flux was calculated from changes in monthly air temperature. The model was compared to the estimates from the California Irrigation Management Information System and the Arizona Meteorological Network, both of which calculate reference, or PET. The model was calibrated to actual daily values of incoming solar radiation and PET using corrections in long- and short-wave radiation and changes in cloudiness, which were calculated from differences in minimum and maximum daily air temperature.