Definitions and distribution of arid and semi-arid lands

The growth and yield of trees, as of other plants, in arid areas depend on several limiting factors including low and irregular rainfall with frequent drought periods, high temperature and intense radiation, and poor soil conditions, including low moisture-holding capacity, high infiltration rate, and low content of organic carbon. An integrative discussion of their physiological effects was given in Perry and Goodall (1979), but together these are expressed conveniently in terms of aridity

The bioclimatic aridity of a site depends on the relative amounts of water gained from rainfall and lost by evaporation and transpiration. Various ratios of these climatic measurements have been used, but they will not be reviewed here. On a worldwide scale the most useful representation is that used by UNESCO (1977) in the preparation of a map of the world distribution of arid regions; this used the ratio of mean annual precipitation (P) to the mean annual evapo-transpiration (EPT). Four main classes of aridity were recognized: hyper-arid (P/EPT < 0.03), arid (0.03 < P/EPT < 0.20), semi-arid (0.20 < P/EPT < 0.50), and subhumid (0.50 < P/EPT < 0.75). Temperature and its annual fluctuations were recognized in subdivisions, and rainfall regimes were represented by colours.

Within the hyper-arid zone annual rainfall is less than 100 mm with virtually 100 per cent inter-annual variability; vegetation is either absent or ephemeral; tree planting without irrigation is unlikely to succeed; and the most common land use is for oil and mineral mining (and construction of gambling casinos), unless subterranean water is available.

In the arid zone annual rainfall is approximately in the range 100-400 mm, with an inter-annual variation of 50-100 per cent; vegetation comprises mainly sparse annual grasses with some low shrubs; non-irrigated tree planting is possible. Extensive animal breeding occurs, and rainfed agriculture is increasing, although irrigated agriculture may be considered more logical.

The semi-arid zone receives 400-600 (occasionally 800) mm of annual precipitation with 25-50 per cent variation between years; the total differs between areas receiving winter rainfall (e.g., north Africa, 400-500 mm), and those receiving summer rainfall (e.g., east Africa, 700-800 mm). Vegetation is taller and denser with more trees than in the arid zone.

While the UNESCO map provides at small scale a base document for the world, it does not delineate exactly the distribution of arid zones within a given country. Further the systems used by national agencies for site classification do not necessarily agree with those of UNESCO, and in any case no meaningfully precise delimitation is possible because of annual variation in rainfall and temperature.

Various countries consider 300 mm rainfall the upper limit of the arid zone and 500-800 mm the upper limit of the semi-arid zone. For the purpose of this report an exact definition is unnecessary because tree-planting techniques and constraints do not change abruptly. (Ghosh 1977, in his treatment of afforestation techniques in India, grouped all tropical dry forests that occur in areas receiving less than 1,250 mm.) The areas considered here are those recognized as arid (350-500 mm rainfall) and very arid (200-350 mm) in Kenya (Government of Kenya 1979) and as arid in India (less than 500-600 mm, mainly in parts of Gujarat and Rajasthan; e.g., see ICRISAT 1978), but to a large extent the discussion applies equally to the drought prone areas of Andhra Pradesh, Bihar, Gularat, Haryana, Jammu and Kashmir, Karnataka, Madhya Pradesh, Maharashtra, Orissa, Rajasthan, Tamil Nadu, Uttar Pradesh, and West Bengal (Government of India 1978). However, most of the examples are taken from Gujarat, where tree planting is actively pursued. The Kenyan acronym ASAL (arid and semi-arid lands) is adopted as an all-embracing term (Government of Kenya 1979).

These areas are illustrated in figures 1 (India) and 2 (Kenya). Although there are considerable variations in climate, soil, topography, and demography within these areas, many of the obstacles to tree planting are common within a country.