General Introduction

Abstract

The English term “drylands” was defined by Barrow (1992) as “environments which are permanently, seasonally or temporarily subjected to a significant deficit in moisture”. One third of the emerged land mass — 35–37% of our planet or some 45 million km² — is dry, and is inhabited by 15–20% of the world’s population. Being azonal, these lands are found in polar, temperate, subtropical or tropical latitudes. For these reasons the subject “Mankind, aridity and droughts in drylands” seems worthy of consideration, further justified by some basic facts:

• Water, the lack or scarcity of which forms the core of this book, paradoxically covers 70% of our planet, but 90% of it is neither available nor amenable for human consumption. This may be because of its salinity (seawater) or its location either in the polar ice caps or in deep groundwaters which also may be saline; the accessible freshwaters are not always directly consumable because of being polluted.

• The treatment of water entails a high consumption of energy and chemicals, the residues of which are difficult to handle.

• The desalination of salt water is costly because of its energy-intensive nature. Modern techniques using salty water for agricultural purposes witness to a new approach and economic progress, the water no longer being desalinated, but used in its saline state for irrigation.

• Tapping deep fossil aquifers by pumping is irreversible on a historic time scale and leads to compaction of the aquifers, subsidence of the soil surface and the ingression of seawater into the continents. Such subsidence is at its worst in Mexico City and in the San Joaquin Valley of California. In Denver (Colorado) pumping has led to settlement with effects similar to those of an earthquake. Bangkok and Venice are other examples that come to mind.

• Storage of water in large dams is expensive as far as investment is concerned and because of the loss due to evaporation which may only be avoided by underground storage which is still more expensive.

• The transport of water is also costly in terms of investment and management and its effects on the environment are difficult to predict.

• Urbanization and other land uses involving deforestation increase the loss of water runoff, flash-floods and inundation, the latter sometimes assuming catastrophic dimensions as in France in 1988 in Nimes, in 1992 in Vaison-la-Romaine, Valréas and Bollène, and also along the Mississippi in 1993.

• Any change in land usage, in the use of soils, i.e. any modification of cultural practices and vegetation (e.g. changes in land tenure) modify the hydrological parameters and affect the local hydrological equilibrium.