Water balance - derived precipitation and evapotranspiration

The map shows isolines for the annual precipitation and the annual evapotranspiration (in millimetres). Water is normally supplied by precipitation and lost through the processes of evapotranspiration and runoff or it may remain as surface or subsurface storage. Over long periods, generally several years or more, changes in water storage for any particular region may be neglected, and the precipitation (P) is "balanced" by evapotranspiration (E) and runoff (N). The concept of a water balance was first introduced in the literature by Thornthwaite in 1944 as a means of calculating evapotranspiration. Since that time, the water balance concept has been used in a variety of ways; herein it is viewed conceptually by the following equation: P = N + E. In this form, the water balance equation can serve a dual purpose. It can provide estimates of any of the components P, N, or E as long as the other two are known, or it can serve as a check when each of the components has been measured independently. The derived annual precipitation and annual evapotranspiration fields, shown on the map, are provisional. Efforts are continuing on improved methods of correcting the precipitation underestimate, involving experimental evidence and a recalculation of the evapotranspiration field based on available energy and moisture supply. A more careful analysis of the runoff data, especially in isolated areas, is also necessary.