Hydrology and Best Practices for Managing Water Resources in Arid and Semi-Arid Lands

Introduction

The natural resources of the Arid and Semi-arid lands (ASALs) – in particular soil and water- are limited, and are often in a delicate environmental balance. It is also worthwhile noting that easily- developed land in many areas of developing countries in Africa is already exploited. Attention therefore is increasingly turning to ASALs to relieve population pressure and to provide more food for the ever increasing human population. Desert encroachment due to lack of conservation planning, and the dangers of destroying or depleting beyond recovery these productive resources, are evident at the present time and may be disastrous if development is based on short term expediency rather than long-term environmental stability. In general, the distinctive features of ASALs include (1) High levels of incident radiation, (2) generally large diurnal and seasonal temperature variations, (3) low humidity, (3) strong winds with dust and sand storms, (4) sporadic rainfall of high temporal and spatial variability and (5) high sediment transport. What then can the ASALs offer? In agriculture, they can provide a certain well-known range of crops and animals adapted to the range of ASALs climates. In industry, plants and industrial complexes can benefit from the relatively easy availability of extensive areas at comparatively low prices, at least initially, and a warm climate with clear skies, ample sunlight and largely unused solar energy. The most obvious negative features are poor distribution or lack of water. Abundant groundwater may be available in some areas but with small natural recharge and high evaporation, this raises the problems of exploiting a non-renewable resource. Other problems in development of ASALs may arise from the sudden introduction of modern technology to societies unprepared for it. For instance traditional water extraction from the ground has been based on manual methods using limited power. The rate of withdrawal of groundwater and natural recharge was therefore achieved without substantial use of groundwater storage and field efficiency was relatively high. Electrical or engine- driven pumps have greatly increased the capacity of the wells. The absence of effective controls on drilling and pumping has resulted in groundwater harvesting with the consequences of decreasing yields, deteriorating water quality, and wastage of water. On the other hand, climate change which has set in is even complicating if not worsening the very existence of an already vulnerable and fragile ecosystem. The difference between climate variability and climatic change is a delicate point that arouses much debate. However, in the real world, what matters is the fact that the climate is indeed never static and that a study of the past is still the best basis on which to plan for future. This is because the prediction of the climate is too uncertain. In fact even a small “change” of the climate, such as an increase in the rainfall variability about the mean even if the mean remains unchanged can be sufficient to tip the balance against- or for- say the cultivation of a certain crop in an area where rainfall is marginal. Tough irrigation provides a form of risk insurance, the effects of even a small climatic “change” may be significant too. Surface run-off in rivers and recharge to groundwater represent a small residual between two much larger terms in the hydrologic equation, rainfall and evaporation. It therefore follows that a small change in one of these factors will have a significantly greater effect on water availability in the ASALs ecosystems.