Drylands are areas with limited water resources. This first aspect of drylands is therefore based on their climatic character. Rainfall is scarce, unreliable and concentrated during a short rainy season with the remaining period tending to be relatively or absolutely dry. High temperatures during the rainy season cause much of the rainfall to be lost in evaporation, and the intensity of tropical storms ensures that much of it runs off in floods. Water supply is not only meagre in absolute terms but also of very limited availability for human and natural uses. The two dominant characteristics of dryland climates are aridity and variability. Several classifications of drylands have been developed. The FAO typology for example, is based on agroclimatic zones defined according to the Length of Growing Period - LGP (production perspective) - arid (<75 days/year); semi-arid (<120 days/year) and dry sub-humid (<180 days/year).