A global humidity index with lateral hydrologic flows.

Area of Science:

• Hydrology
• Climatology
• Environmental Science

Background:

• The aridity index, typically P/PET, quantifies land water availability.
• Lateral water flow (rivers, groundwater) from higher to lower elevations is often overlooked.
• This lateral flow provides a crucial water subsidy to lowlands.

Purpose of the Study:

• To quantify the impact of lateral water subsidies (Qlat) on global aridity.
• To introduce a new topography-inclusive humidity index (GHI_topo).
• To assess how GHI_topo improves the representation of water availability.

Main Methods:

• Global monthly surface and groundwater subsidies (Qlat) were estimated using a global hydrology model on 30″ grids.
• The conventional humidity index (GHI = P/PET) was calculated.
• A new index, GHI_topo = (P + Qlat)/PET, was computed to include lateral flow.

Main Results:

• Lateral water subsidies reduce aridity in receiving lowlands.
• GHI_topo is higher in topographically defined lowlands.
• GHI_topo exhibits delayed and dampened seasonality compared to precipitation (P) alone.
• The land area with GHI_topo ≥ 1 is 33% larger than for GHI ≥ 1.
• This effect is pronounced in arid and season-arid climates.

Conclusions:

• Lateral water subsidies are a significant factor in determining regional water availability.
• GHI_topo provides a more accurate assessment of water availability, particularly in topographically influenced areas.
• This improved index highlights areas where life can be sustained despite arid conditions or seasonality.