The theoretical base of the formulas used in this application isdescribed in this section. Note that, for example, if the water needsof the crops must be estimated, it is necessary to consider, amongothers, the determination of the free water surface evaporation andthe extraterrestrial radiation values. Hence, in the next subsectionswe analyze the solar, the clear-sky solar, and the extraterrestrialsolar radiation as well as the equivalent evaporation, following theguidelines for computing crop water requirements described byFAO (Allen et al., 1998). These guidelines have been widely appliedby the researcher community in agriculture.