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Improving Projected Potential Evapotranspiration Estimates Using National Weather Service Forecasts

Steven J. MeyerCenter for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln, Nebraska

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Kenneth G. HubbardCenter for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln, Nebraska

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Donald A. WilhiteCenter for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln, Nebraska

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Abstract

In a recent survey conducted by the University of Nebrask'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 projection National Weather Service (NWS) forecast variables were used as input into the Blancy-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 Blancy-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.

Abstract

In a recent survey conducted by the University of Nebrask'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 projection National Weather Service (NWS) forecast variables were used as input into the Blancy-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 Blancy-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.

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