Goals of This Chapter
In this chapter, we aim to support the movement toward an improved treatment of the evaporative aspects of drought. Our main thrusts are to describe the various problems with the current definition, estimation, and use of both ET and E0 in operational drought monitoring and/or forecasting, to outline the current state of the science, and to suggest ways forward to address the issues, highlighting some of the advantages of using either ET or E0, or both. We will demonstrate that ET and E0 are important in drought dynamics and that their accurate and fully physical representations offer new opportunities to improve monitoring and forecasting. This will necessitate our showing that E0 sets the stage for drought—for example, the potential for vegetation stress in agricultural drought—while ET shows the onset of actual stress and that the specific interrelations of ET and E0 show promise in drought early warning, spotting flash drought onset, and monitoring of ongoing drought. Further, we will show that forecasting E0 leads to the opportunity to forecast stress from E0 and hence, in part, to achieve the long-sought goal of forecasting drought itself. With respect to drought and climate change, we caution that E0 must be estimated properly, and that we must understand what trends in ET and E0 mean for drought vulnerability under a changing climate.