Present Day Evaporation Measurement Techniques

The energy-budget and mass-transfer methods are currently being used by the United States Geological Survey to measure evaporation from lakes and reservoirs. Because the field equipment is expensive and data processing and analysis are time consuming, the use of the energy-budget method is limited to those reservoirs where the need for detailed information warrants the expense. Ordinarily the mass-transfer coefficient for a selected reservoir is determined using the energy-budget method over a period of 15 months or longer. After this, the energy-budget equipment can be removed, and the mass-transfer method, which requires a minimum of equipment and office work, can be used to compute evaporation on a continuing basis. During 1965-67 a cooperative study by the International Boundary and Water Commission and the Geological Survey showed that evaporation from Falcon Reservoir on the Rio Grande was 189 cm and 167 cm for two successive years.