Measuring evapotranspiration by hydrological methods

The measurement of evapotranspiration using the methods of micrometeorology requires a sophisticated application of advanced technology. The simpler alternatives, though they are not always less costly, are discussed, namely (1)precipitation-minus-stream discharge, (2)estimates of the changes of the water storages, either in the water table aquifer or in the unsaturated zone of the soil, (3)lysimeters, and (4)material balance methods. The accuracies attainable by these methods vary. The most precise estimate can be got from a well-designed, weighable lysimeter, but the representativeness of the exposure is always a worry for the experimenter. The least precise estimate, on a month-to-month basis, is afforded by the precipitation-minusstream discharge method, but this method can give a good estimate of annual total evapotranspiration. The range of evapotranspiration rates probably spans 0.7 mm d-1 in mid-winter, on occasions, to 5 mm d-1 in late spring if the season is a wet one. For reasonable definition of these rates on a daily basis, we are looking for a method that is accurate to ±0.2 mm d-1. Only the weighing lysimeter can achieve this. The other methods variously provide mean rates for periods of five days or longer.