Introduction

The question of assessing surface evaporation has been considered since at least the time of Dalton in the early nineteenth century. Literally hundreds of methods have been described in the literature. These differ greatly with respect to the essence of the method, the assumptions utilized, the conditions when applicable, the units of the constants and variables, and the degree of empiricism involved. This section gives only a broad overview of the methods in widespread use. For the details, the reader is referred to summaries in sources such as Sellers (1965), Rosenberg (1974), Oke (1987), Dunne and Leopold (1978), Brutsaert (1982), Jackson (1989), Lowry and Lowry (1989), Schmugge and André (1991), Dingman (1994), and Raupach (2001).

The total evapotranspiration from the earth’s surface is a composite of water flux derived from five types of surfaces: open water, bare soil, vegetation, and snow and ice. The factors and processes in the exchange of vapor with the atmosphere are different in each case and hence the basic surface types will be examined separately in this chapter. However, snow and ice will not be considered, since these surfaces are rare in most of the dryland regions considered in this text.