Sensitivity of Reservoir Sizing to Evaporation Estimates

Net reservoir evaporation is defined as the open water evaporation less the original evapotranspiration from the reservoir site. These rates are estimated on an average monthly basis by the method of Morton using sparse climatological data that include average annual precipitation, air temperature, dew‐point temperature, and sunshine ratio. Sunshine ratio data are substituted with the more generally available cloud cover data with little loss in accuracy. Generalized reservoir surface area‐volume relationships are derived by regression analysis separately for large and small reservoirs in Australia. They represent the surface area‐volume characteristics of a reservoir confined in a hypothetical valley whose shape is an averaging of the shapes of a large number of actual sites. The sensitivity of estimated storage sizes to errors in the evaporation rates and the reservoir surface area‐volume relationships is tested for five catchments located in the southeastern extremity of Australia. With regard to these parameters, estimated reservoir sizes are shown to be surprisingly robust.