Bouchet's theory on actual and potential evaporation can be used as a basis to develop semi-empirical relationships between precipitation, actual evaporation, and radiation. These relationships are curvilinear, and the derived precipitation-runoff relationships are also curvilinear. Since the variation of annual radiation from one year to another in tropical-equatorial regions is not very significant, the average value of annual radiation can be approximated using direct measurements or indirect estimates from data taken over a period of from two to five years. This value can be used with the semi-empirical relationship developed in the paper to obtain fair estimates of the annual variation of actual evaporation and runoff from data on annual precipitation and good estimates of the long-term averages of these hydrologie characteristics. Bouchet's theory can be used to analyze evaporation and runoff for periods of less than one year and for estimating lake and reservoir evaporation. However, in this case, the assessment of the effect of heat and moisture storage requires further investigation.