Estimation of daily Class A pan evaporation from meteorological data
Evaporation is an important process in many hydrological, physiological and atmospheric processes. It is controlled by the availability of water and the meteorological conditions over the evaporating surface. The Class A pan evaporimeter is a popular instrument for the measurement of evaporation from a free water surface. Evaporimeter data are used primarily to estimate evapotranspiration from crop surfaces, and water requirements for irrigation. They are an important input parameter for the soil moisture computer program used at Iowa State University to estimate soil moisture and to forecast yields for corn;The Class A pan evaporimeter has drawbacks. Data are lost due to overflow during heavy rainfall and it is expensive to operate due to the requirement of a skilled operator for regular management. Also, the number of sites is very limited giving inadequate areal coverage. Because of these shortcomings, a network of automated weather stations was set up at three locations and meteorological data were collected to see if this parameter could be estimated more effectively. Using combination equations and some other empirical equations, daily Class A pan evaporation was estimated from meteorological data. Daily Class A pan evaporation was also estimated from a simplified empirical equation which used only commonly observed meteorological parameters. The simplified method assumed that minimum temperature represents the dew point temperature;Methods were evaluated by comparing the estimated values with measured pan evaporation. The combination equation gave the best performance giving most unbiased and precise results compared to other methods tested. The use of minimum temperature instead of dew point temperature on the tested empirical equations gave almost similar results.