Global Distribution of Climatic Aridity

Aridity exerts fundamental controls on terrestrial hydrology with impacts on ecosystems and society. Here we show that the global distribution of dryness index is dominated by the exponential variability of precipitation. The resulting distribution of the dryness index only has one parameter describing the balance between global mean atmospheric water demand and supply. Its shape, with a minus two power-law tail, is well supported by observations and appears to be robust to future changes in aridity, making it a possible benchmark for Earth System Models. While water availability is of great importance to the ecosystem and society, modeling its distribution at the global scale has large uncertainties. Here we quantify water availability as the dryness index defined as the ratio of long-term averages of atmospheric water demand (potential evapotranspiration) to supply (precipitation). We find the global distribution of the dryness index is essentially controlled by the spatial variations of precipitation. Such distributions did not have significant change in the past century and their shape appears to be robust even for the drier future climates as projected by climate models. Exponential variability of precipitation is an essential constraint on the global pattern of climatic aridity as measured by dryness indexDistribution of dryness index has one parameter quantifying the overall balance between global mean atmospheric water demand and supplyDryness index distribution will likely retain its form in future drier climates as projected by Earth System Models