The aridity index is widely used to indicate water availability on land. Balancing climatic water supply (precipitation, P) against demand (potential evapotranspiration, PET), it is often expressed as the P/PET ratio1 or humidity index. Water also flows laterally by rivers and groundwater, from hills to valleys and from mountains to plains, subsidizing the receiving lowlands2. Here, we show that this lateral subsidy reduces aridity in the receiving lowlands. We first estimate monthly subsidies (Qlat) by surface and groundwater at 30″ global grids with a global hydrology model. We then calculate the conventional global humidity index (GHI) as P/PET and a new GHI including Qlat as (P + Qlat)/PET. Termed GHI_topo, the latter reflects land topography, higher in hydrologically convergent lowlands. It also exhibits a delayed and dampened seasonality (relative to P) owing to delayed and diffused Qlat arrival at the receiving lowlands. Such spatiotemporal features of Qlat, arising from both the climate and the terrain, make GHI_topo a more realistic indicator of local water availability in downgradient societies and ecosystems, enabling life in arid locations and times. Global land area with GHI_topo ≥ 1 (supply meets or exceeds demand) is 33% greater than GHI ≥ 1 and far higher in arid and season-arid climates.
