Estimation of Reference Evapotranspiration by an Energy Balance Approach

The sustainability of irrigated agriculture depends primarily on efficient water use. Efficient irrigation decisions can be expressed as a function of potential atmospheric demand, which is expressed by reference evapotranspiration (ETo) governing either the ideal amount of water taken up by the plants or irrigation frequency of agronomic crops at a given site. All ETo estimation methods refer to daily values including night evaporation losses. Night time losses are only substantial for a few days after rain or irrigation. A method is proposed for estimating ETo based on the local energy balance from limited meteorological data monitored in an automated weather station throughout daylight periods. To validate the current method, climatic data and lysimetric measurements from Piracicaba, São Paulo, Brazil were used. Regression analyses revealed that a modified Bowen method provided results similar to the Penman–Monteith method and similar to measurements made by weighing lysimeters with load cells. The modified balance energy approach tended slightly to overestimate evapotranspiration by about 5%, whereas the Penman–Monteith method tended to underestimate atmospheric demand at a rate close to 7%. Given the high coefficients of determination and ease of estimation, the method based on daily net radiation, mean air temperature and relative humidity is recommended for assessment of crop water use for the region studied. Further investigations are required to examine the performance of the modified Bowen approach at other specific sites under different climatic conditions.