Thornthwaite's Method for the Computation of the Water Balance

Abstract

Evapotranspiration is one of the crucial processes on water balance under the effect of meteorological and environmental factors and soil and vegetation characteristics. Thornhtwaite's method for the evaluation of the water balance at a given place is related to the temperature, precipitation data at monthly or daily intervals and on the information on the water holding capacity of the soil at that place. The method is based on equating the sum of evapotranspiration and runoff to the sum of precipitation and soil moisture. In Thornthwaite's method, most steps of the calculation are expressed by mathematical formulas but the values of the amount of water retained in the soil as a function of potential evapotranspiration (PE), for each value of the water holding capacity (WHC) of the soil are given by tables. In this paper we present an overview of the method and we present mathematical formulas that fit the data of the of these tables, to express the relation between water retained in soil and potential evapotranspiration (PE) for different water holding capacity (WHC).