A Simple Framework for Examining the Interannual Variability of Land Surface Moisture Fluxes

An equation that describes the partitioning of annual mean precipitation into annual mean evaporation and runoff, developed decades ago by Budyko, is used to derive a second equation that relates the interannual variability of evaporation to gross characteristics of the atmospheric forcing. Both Budyko’s original equation and the new variability equation perform well when tested against results from a 20-yr GCM simulation. In these tests, using knowledge of the climatological mean precipitation and net radiation alone, the authors predict the ratio of annual evaporation to annual precipitation with a standard error of 0.10 in nondesert regions, and they predict the ratio of the standard deviation of annual evaporation to that of annual precipitation there with a standard error of 0.14. In analogy with Budyko’s conclusion for the mean hydrological cycle, water and energy availability appear to be the critical factors controlling the interannual variability of surface moisture fluxes. The derived equations suggest, and the GCM results confirm, that the ratio of an evaporation anomaly to the corresponding precipitation anomaly tends to be significantly less than the ratio of mean evaporation to mean precipitation.