Modeling reference water vapor adsorption in desert soils

Non-rainfall water inputs (NWRIs; i.e., dew, fog, and water vapor adsorption (WVA)) are significant sources of water in arid environments. Amongst all NRWIs, WVA is likely the most common, yet it is the least studied. There is increasing evidence that water vapor adsorption occurs in many arid and hyper-arid regions, that together occupy 26% of the earth’s terrestrial surface. Quantifying WVA is therefore essential to fully understand the water cycle in these regions. While some studies quantified WVA as a function of the surface properties, they were either laboratory trials or limited to a specific location. No studies, to date, have presented a general model to quantify WVA. Given the complexity of the process, we propose an initial step towards bridging this knowledge gap, with the introduction of a new “reference water vapor adsorption” (A_o). A_o is the adsorption of water vapor from the atmosphere to a reference surface, conceptually similar to the “reference evapotranspiration” (ET_o) that quantifies the evapotranspiration rate from a reference surface. We propose to calculate A_o as A_o = r_aC_p(e_a-e_s)/lgr_a where ρ_a is the density of air, C_p is the specific heat capacity of air, e_a and e_s are the water vapor pressure in the air and in the air-filled pores, respectively, γ is the psychrometric constant, and r_a is the aero dynamic resistance. Assuming a completely dry surface (similarly to assuming well-watered crop to calculate ET_o), e_s is set to zero. To test this new concept, we conducted measurements in the Negev desert, Israel, from July to October 2025. A_o was calculated from continuous measurements of temperature and relative humidity at 2m height, and wind speed at two heights (3 and 0.8 m). In parallel, A_o was directly measured every two hours during multiple 24-h campaigns by exposing dry silica gel to the atmosphere. The calculated A_o followed closely the trend of measured A_o, encouraging further development of this index, and potentially allowing mapping of reference adsorption based on simple meteorological measurements.