The effect of soil moisture on pan evaporation

Abstract

Pan evaporation observations are usually higher than potential evaporation computed from meteorological data, especially for drier periods or areas. To interface estimates of potential evaporation made with meteorological data from an increasing number of real-time reporting stations with historical daily pan evaporation measurements, pan evaporation was interpreted as a function of the computed potential evaporation and the plant available moisture in the top soil under maize ( Zea mays L.) in the vicinity of the weather station. If the soil is at full moisture holding capacity, the pan evaporation is close to the potential evaporation, but if the surrounding area is dry, the pan evaporation is considerably higher than the potential evaporation computed by the Penman (1948) or Van Bavel (1966) methods. A multiple regression of daily pan evaporation on Penman potential evaporation and available soil moisture in the top 15 cm was associated with about half of the variance in the pan evaporation measurements at the West Lafayette 6NW weather station from 1973 to 1975, when only non-synoptic observations of pan evaporation and the four meteorological variables necessary for computing potential evaporation were available. The relation was tested in 1976, when hourly meteorological data were available. Potential evaporation and soil moisture were associated with 75% of the variance in the daily pan evaporation data. For each 10% decrease in available soil moisture, pan evaporation was estimated to increase 0.2 mm per day over potential evaporation.