Water where there is no water – Atmospheric water captured by world deserts

Atmospheric water, or non-rainfall water inputs (NRWIs) are an important source of water in arid areas. Considering the large surface area of arid and extremely arid regions, NRWIs are a critical, albeit largely overlooked, component of the global hydrological cycle.  Water vapor adsorption is not only the least studied form of NRWI but likely the most common one in arid areas. The amount of water vapor adsorption mainly depends on the gradient between water vapor pressure between the air (e_a) and the soil (e_s).  Sea breeze, which carries moist air from the sea landward, can result in a significant daily increase in e_a in desert areas. 

We have examined the diurnal cycle of soil water content derived by water vapor adsorption and evaporation in two very different deserts: the Negev (loess soil, ~100 mm y^-1) and the Namib (sand dunes, ~20 mm y^-1).  Water vapor adsorption into the Negev’s loess soil has been established as the dominant NRWI (with 0.3-0.5 mm night^-1). Even in the Namib, which is known as a fog desert, even on nights with fog, at least half of the water accumulation occurred via water vapor adsorption, before the onset of fog (0.1-0.2 mm night^‑1).