An arid climate is characterized by extremely low annual precipitation, typically less than 250 millimeters, and high evaporation rates. Arid regions cover about one-third of the Earth's land surface.
An arid climate, classified under the Koppen climate classification system as type BW (hot desert) or BWk (cold desert), is defined by annual precipitation below 250 mm and potential evapotranspiration that far exceeds rainfall. These conditions result in sparse vegetation cover, limited surface water resources, and heavy reliance on groundwater for human water supply. Major arid regions include the Sahara, Arabian, Gobi, and Sonoran deserts. Water resources in arid climates face unique challenges including high evaporative losses from reservoirs and irrigation systems, episodic flash flooding from intense but infrequent rainfall events, and declining groundwater tables from over-extraction. Precipitation in arid climates tends to be highly variable both spatially and temporally, making water supply planning particularly challenging. Climate change projections suggest that many arid regions will become even drier, while some semi-arid zones may transition to arid conditions, potentially affecting billions of people. Water management strategies in arid regions emphasize conservation, efficient irrigation technologies such as drip systems, water reuse and recycling, desalination where coastal access permits, and artificial groundwater recharge.