Estimating pan evaporation using limited meteorological observations

Pan evaporation measurements have been used worldwide as a means of estimating evapotranspiration and free water surface evaporation. The availability of pan evaporation measurements is critical to many applications including irrigation scheduling, hydrologic modeling, and irrigation system design. The ability to predict daily pan evaporation based on limited meteorological observations is highly desirable. In many situations it is advantageous to calculate, rather than measure, pan evaporation. This is often the case in developing nations or remote locations where costs are prohibitive. Many relationships that predict pan evaporation or potential evapotranspiration as a function of limited meteorological observations have been developed. The relationships are often empirical, thus they suffer from lack of transferability. The relationship between potential evapotranspiration and pan evaporation facilitates local calibration of established equations using measured pan evaporation data. This calibration may then be extrapolated to locations that have similar climatic characteristics. A calibration procedure is presented and demonstrated by calibrating three equations using data from several sites located in the southeastern United States.