Comparison of precise water depth measurements on agricultural storages with open water evaporation estimates

A simple technique to determine evaporation and seepage losses of agricultural water storages is described. Evaporation is calculated from automatic weather station (AWS) variables using the Penman–Monteith equation, and seepage is determined as the difference between this and accurate water depth measurements made using a pressure sensitive transducer (PST). The accuracy of the PST devices (±1 mm) was far greater than any flow metering equipment available, so analysis only took place when there was no pumping in and out of the dam. Calibration tests were carried out during the summer of 2004/2005 at a dam site where seepage was very close to zero, as total evaporation plus seepage loss over the winter months there was independently determined to be less than 1 mm/day. Summertime PST depth traces were compared to the Penman 1948 equation, Penman–Monteith (PM) ET0 calculated according to the FAO56 method and Penman–Monteith (PM) with surface resistance set to zero to simulate a open water surface. The first two produced the best correlations (within 10% agreement with the water depth trace), but PM open water over predicted by 40%. This technique has provided a useful tool to more accurately apportion total water loss into evaporation and seepage components. Similar to the evapotranspiration of different crop types, it is suggested that that the evaporation of open water can be similarly related to the international standard FAO56 PM via a simple dam factor.