A theoretical development of the evapotranspiration estimation method in forests without shrub strata, using daily soil moisture variation of root horizon to estimate evapotranspiration, is presented. Since the incrop coefficient of forests is unknown, the use of the estimation of evapotranspiration incrop coefficient of natural prairies is proposed. The coefficient values are rectified iteravely, till the anomalous variation curve of soil moisture changes form and fulfils two requisites, first: the values of estimated moisture contents are statistically the same as the data records of the forest soil, and second: the curve fulfils the principle of equality in minimum soil moisture contents of the year. The rectified Kc value will be the forest's incrop coefficient. The methodology uses the HIDROSUELO simulation model to estimate the daily soil moisture variation by means of the daily evaluation of the universal water balance formula. A water balance formula for the forest its proposed.