COMPARISON OF THE BOWEN-RATIO ENERGY-BALANCE AND THE WATER-BALANCE METHODS FOR THE MEASUREMENT OF EVAPOTRANSPIRATION

An experiment was conducted in a 36 ha alfalfa field in Logan, Utah, USA, during three growth-harvest cycles of alfalfa in April-July 1992. A Campbell Scientific (CS) Bowen Ratio System was used to measure continuously 20 min means of micrometeorological elements such as air and dew point temperatures at 1 and 2 m, solar and net radiation, wind speed and direction at 2 m, soil heat fluxes at 8 cm (two locations), and soil temperatures at 2 and 6 cm during the three consecutive growing cycles. The gravimetric method (for 0-10 cm) and the Campbell Pacific Nuclear (CPN) Hydroprobe (for 20-150 cm) were used to measure soil moisture daily at 09:00 h. The 24 h evapotranspiration (LE, using the Bowen ratio-energy balance method) was computed by adding the 20 min mean LE values from 09:00 h until 09:00 h of the next day for comparison with the water balance method. Evapotranspiration measured by the water balance method was 98% of that measured by the Bowen ratio-energy balance method during the experimental period (r = 0.987).