Development of Evaporation and Evapotranspiration Coefficients

In the 1980s, the U.S. Geologic Service, in cooperation with the U.S. Bureau of Reclamation, developed a water balance approach for estimating water consumption along the Lower Colorado River. Major estimated water consumption components were evaporation (E) and evapotranspiration (ET). These values were combined with a water balance applied to each of the four reaches of the river defined by the major dams, between Hoover Dam and the border with Mexico. The method is called the Lower Colorado River Accounting System (LCRAS). In 1990, Reclamation assumed responsibility for continued development of LCRAS and developed a strategy for improving the system. The strategy involved improving the accuracy of flow measurement, use of higher resolution remotely sensed digital satellite data, and improving the accuracy of E and ET estimates. In 1998, monthly evaporation coefficients were developed for use with reference ET from AZMET and CIMIS micro-meteorological weather stations. Surface water temperatures were related to water temperatures measured below the dams creating the reservoirs. The Penman-Monteith (P-M) equation was used to estimate open water evaporation taking into account the advection of heat energy in the water inflows and outflows for each reach. The temperature of water leaving Hoover Dam is nearly constant year round ranging from 10 to 13 C. Average water temperatures increase in each of the four reaches until approaching mean air temperatures in the last of the four reaches below Imperial Dam.