ABSTRACT
Because ET is a major component of the hydrologic cycle, this chapter explores the processes by which ET occurs. In addition, we discuss techniques for estimating the amount (depth) and rate (depth/time) of ET. You may be asking yourself why you would ever need to calculate ET. If you work with water or plants, chances are you will need to estimate ET. The rate and amount of ET is the core information needed to design irrigation projects, and it is also essential for managing water quality and other environmental concerns. Policymakers need to know how estimates of ET are determined because these methods are used in litigation and in negotiations of contracts and treaties involving water. To predict meltwater yields from mountain watersheds or to plan for forest re prevention, estimates of ET are needed. In urban development, ET calculations are used to determine safe yields from aquifers and to plan for ood control. Anyone involved with resource management will likely need to understand the methods available for estimating ET. Like precipitation, there is great regional variation of ET, but the two are in general negatively correlated: the greatest potential ET is in regions with less precipitation.
