The Issue of Potential Evaporation and Real Evaporation:

Evaporation off bare ground vs Evapotranspiration

So in a follow-up discussion with some hydro engineers, there are some things to clarify with respect to evaporation and precipitation.  The first is the difference between potential evaporation and actual evaporation when assessing your site.  The ratios I gave for our site in Al Baydha and for death valley are potential evaporation, which is how much water would evaporate if it were just sitting on the surface.  Actual evaporation in real deserts is much lower—often times approaching zero—because there isn’t any water in the soil to evaporate, except in itinerant rain events.  

When you plant trees in the desert, you are actually increasing real evaporation, but in a way that is fundamentally different from evaporation from bare soil.   

Evaporation from bare soil over time results in increasing salinization of the land, leading to drops in fertility and making it very very difficult to restore the land to production.  

Evaporation off of trees is called evapotranspiration, and in addition to water vapor includes volatile organic compounds in the form of aerosols,  terpenes, sterenes, and other compounds.  

These compounds when they reach the atmosphere often times provide the nuclei for clouds to condense on and for rain to fall.  This is one of the primary mechanisms affecting rainfall and cloud formation in forested areas. 

To put it simply, evaporation from bare soil makes the soil salty, whereas evaporation from trees incorporates all the trees’ affects on soil, and can increase rainfall and cloud cover.  (Side Note:  you can’t control where that rain is going to fall!)