Abstract
This chapter describes atmospheric water demand and various approaches commonly used to estimate reference evapotranspiration (ET) (RefET). RefET is a more precise definition of potential evapotranspiration (PET). It describes the ET of a well-watered short grass and corresponds to a very commonly used estimate of the atmosphere’s ability to absorb water from the land surface. We describe some of the main physical and biological mechanisms controlling evaporation from soils and transpiration from plants. We briefly trace some of the history of this subject, and present several formulations for estimating RefET. In water-limited settings the total amount of water vapor that evaporates and transpires from the land surface (actual ET or AET) will be less than RefET. Hence, RefET provides a very useful upper limit for AET. We conclude with a discussion of the complementary relationship, which describes the inverse relationship between RefET and AET, water and energy limitations on AET, and how these aspects of atmospheric water demand relate to temporal and spatial variations in vulnerability and exposure.
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