Drylands

Drylands are regions where low and variable rainfall limits water availability, encompassing arid, semiarid, and dry subhumid areas. Their ecosystems and livelihoods are shaped by water scarcity and strong rainfall variability.

Aridity index: A ratio of mean annual precipitation to potential evapotranspiration used to classify climatic dryness. Its precipitation-to-demand ratio distinguishes dryland climate zones.
Arid climate: A climate with very low precipitation relative to atmospheric water demand. Arid areas form the driest part of the dryland range.
Dryland farming: Crop production in regions where rainfall is the primary water source and irrigation is limited or absent. Farming practices must match crop demands to scarce, variable rainfall.
Drought: A prolonged period of unusually low water availability relative to typical conditions. Dryland rainfall variability can turn ordinary water scarcity into severe drought.
Potential evapotranspiration: The estimated atmospheric demand for water from a well-watered surface under specified conditions. It helps explain why limited rainfall may not meet local atmospheric water demand.
Semiarid climate: A climate with limited precipitation, generally more than arid climates but insufficient for consistently moist conditions. Semiarid areas are a central dryland zone with marked water constraints.
Pastoralism: A livelihood system centered on raising and moving livestock to use grazing resources. Mobile livestock keeping can adapt to patchy forage and shifting water access.
Biodiversity of drylands: The diversity of organisms and ecological communities adapted to dryland conditions. Specialized organisms persist despite water scarcity and episodic resource pulses.
Rainfall variability: Variation in precipitation across time or space, including changes in amount, timing, and intensity. Unpredictable rainfall makes water availability fluctuate sharply between seasons and years.
Dry subhumid climate: A relatively moist dryland climate where precipitation remains limited compared with potential evapotranspiration. This climate forms the wetter boundary of many dryland classifications.

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