Date of this Version
In a recent survey conducted by the University of Nebraska's Center for Agricultural Meteorology and Climatology of Agricultural Network (AGNET) users, the results of potential evapotranspiration (ETp) projections (calculated using the Blaney-Criddle approach, which employs "normal" climatic data to project ETp estimates up to three days into the future) were labeled "unrealistic." To improve these projections, National Weather Service (NWS) forecast variables were used as input into the Blaney-Criddle and Penman equations. ETp projections calculated according to the Penman equation, with data measured by automated weather stations as input, were assumed to represent the "best" attainable. ETp projections calculated using NWS forecasted values were compared with the "best" projections for the summer of 1985. Increased accuracy in ETp projections due to increased accuracy in the individual forecasted input variables was evaluated.
Overall, daily ETp projections made with the Blaney-Criddle equation were substantially improved using the NWS forecasted temperature in place of normal temperature; over a growing season, however, accurate estimates resulted from using normal temperatures. The use of NWS-forecasted variables as input into the Penman equation offers the greatest potential for improving ETp projections. "Over" forecasting of all variables (relative to the estimation of ETp) limited the ability of the Penman equation in this study. For greatest improvement in ETp projections using the Penman equation, efforts should be concentrated on improving forecasts of relative humidity and solar radiation.