1. Arid, semi-arid, and dry subhumid areas (UNCCD).
2. All the terrestrial regions where water scarcity limits the production of crops, forage, wood and other ecosystem provisioning services, i.e. where the aridity index (AI), defined as the long-term ratio of mean annual precipitation to mean annual evaporation demand (expressed as potential evaporation), is less than 0.65 (Millenium Ecosystem Assessment). Formally, the definition encompasses all lands were the climate is classified as dry subhumid (0.5<AI<0.65), semiarid (0.2<AI<0.5), arid (0.05<AI<0.2) or hyper-arid (<0.05). Precipitations in drylands are characterised by their high variability and unpredictability. (from Kéfi 2008, Ph.D. Thesis, p.9)