Arid Region Vegetation Carrying Capacity Constrained by Rainfall Fraction Sheltered from Surface Evaporation

Abstract:

Arid lands represent one third of terrestrial surfaces with ecosystems uniquely adapted to water limitations. Arid regions are characterized by low rainfall and sparse vegetation with potential evapotranspiration (ET₀) exceeding annual rainfall (P) and surface evaporation dominating water losses. We hypothesize that the fraction of rainfall sheltered from surface evaporation could be used to estimate arid region vegetation carrying capacity. A recently developed surface evaporation capacitor (SEC) model was used to quantify surface evaporation based on the climatic record of rainfall and potential evaporation and soil properties. The SEC uses soil-specific active evaporation depth where only rainfall events that exceed its critical capacitance leak into deeper layers. The “leakage” below a critical depth becomes protected from surface evaporation and may support vegetation or inter-annual storage. It is a complex function of rainfall characteristics and soil type. We focus on arid regions (aridity index P/ET₀< 0.2) to highlight the strong correlation between evaporation-protected leakage and regional gross primary productivity (GPP) using typical values of water use efficiency. SEC-estimated GPP values were in good agreement with observations and predictions by ecohydrological model (T&C). Evaporation-protected soil water storage is generated during a few large rainfall events and it increases with increasing rainfall variability (primarily large events). The results suggest that vegetation carrying capacity may increase with enhanced rainfall variability without changes in mean rainfall by diverting larger fraction of the rainfall from surface evaporation to vegetation-supporting “leakage”. Changes in vegetation carrying capacity are greatly influenced by (i) legacy of rainfall variability and large rainfall events, (ii) soil type, and (iii) surface features that concentrate or divert runoff. The implications of this mechanism for global greening of arid lands and woody plant encroachment (decoupled from CO₂ fertilization effects) will be discussed.