"Drylands" - what criteria do we use?
In 2000, the United Nations Convention on Biological Diversity established a programme of work on drylands covering: "dryland, Mediterranean, arid, semi-arid, grassland, and savannah ecosystems which may also be known as the programme on 'dry and sub-humid lands'."
This broad view of drylands has been adopted by SEPASAL and includes both the climatically defined dryland zones defined by the United Nations Environment Programme (Middleton, 1997), and other Mediterranean, grassland and savannah ecosystems showing characteristics of dryland habitats.
SEPASAL focuses on tropical and subtropical drylands, omitting areas
defined by UNEP as subhumid, humid or cold. SEPASAL covers the following
geographical areas, with a focus on Africa in line with the United Nations
Convention to Combat Desertification in Countries Experiencing Serious
Drought and/or Desertification, Particularly in Africa (UNCCD):
Horn of Africa
parts of Zimbabwe
parts of Madagascar
parts of Pakistan
parts of Mexico
Aridity may be defined in several ways. UNEP uses an Aridity Index (AI), calculated as the ratio of annual precipitation (P) / potential evapotranspiration (PET), derived from surface climate maps. AI values of <1.0 indicate an annual moisture deficit.
Hyperarid environments (P/PET < 0.05) have very limited and highly variable rainfall amounts both interannually (up to 100%) and on a monthly basis such that there is no seasonal rainfall regime. In virtually all cases where data are available, year-long periods without rainfall have been recorded. These areas offer limited opportunities for human activity.
Arid areas (0.05 >/= P/PET < 0.20) have mean annual precipitation values up to about 200 mm in winter rainfall areas and 300 mm in summer rainfall areas, but more importantly interannual variability in the 50-100% range. Pastoralism is possible, but without mobility or the use of groundwater resources is highly susceptible to climatic variability.
Semi-arid areas (0.20 >/= P/PET < 0.50) have highly seasonal rainfall regimes and mean annual values up to about 800 mm in summer rainfall areas and 500 mm in winter regimes. Interannual variability is nonetheless high (25-50%) so despite the apparent suitability for grazing of semi-arid grasslands, this and other sedentary agricultural activities are susceptible to seasonal and interannual moisture deficit.