Other approaches utilise a more general working definition based on the idea of any area where rainfall is a problem owing to its amount, distribution and unreliability. UNEP on the other hand, refers to a classification based on the aridity index (climatic perspective). It is important to note therefore, that the different approaches and classifications can produce very divergent assessments of the range and extent of drylands thoroughout the world. Nonetheless, there is no doubt that drylands are present in all the continents and cover more than a third of the Earth's land surface. Likewise, all the classifications agree that 90% of drylands areas have tropical climates.
These extreme climatic conditions strongly influence the natural and socio-economic environments of drylands. Ecosystems, including their flora and fauna, adapt their behaviour according to water availability and distribution. Seed germination and growth cycles for example, closely follow the variations in rain and moisture content in soils. Plants and wildlife developed mechanisms to survive long periods of both water and food scarcity. This adaptation process has been traditionally applied to human activities linked to natural resources management, such as agriculture and animal husbandry. Cropping patterns, farming systems, management of water resources, range management, harvest of wood and natural plants, wildlife hunting, food processing and storage have been relying traditionally on dryland agro-climatic conditions, turning the constraints of dryland natural diversity into an adapted social and economic diversity. Although the direct relationship between aridity and population density is still the object of debate, indigenous populations have often developed traditional social and economic systems that reflect the capacity of local ecosystems to provide the necessary nutrition, housing materials, shelter, energy and social welfare, which has led to inevitabe consequences in terms of settlement patterns, population mobility, land tenure systems, nutrition patterns and rural livelihoods in general.
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).