Recharge of groundwater

In regions with annual rainfall less than 250 mm, the recharge of aquifers is very small for all except the most permeable soils. The amount of recharge depends on the permeability and retention capacity of the soil, and the distribution of rainfall in relation to the evaporative demand. A more detailed analysis of groundwater exploitation and recharge is presented in Chapter 6.

The retention capacity of the soil controls the quantity of soil water that is held in storage. This quantity of water is depleted during dry periods, and the difference between it

and that needed to saturate the soil is the amount of rainfall required to satisfy the soil-moisture deficiency before recharge can take place. If rain occurs in a number of events during the wet season and dry weather intervenes between them, then a substantial part of the total rainfall would be used to refill the depleted soil moisture storage, so leaving only a very small quantity, if any, for ground water recharge. This generally, is not the case for sands and sand dunes and alluvial stream channels which have a low specific retention of less than 5 percent. These allow penetration of rain deep enough and beyond the zone of seasonal drying so that it becomes recharge to the aquifer.

The channel beds are normally the areas through which most of the recharge takes place. Recharge also occurs in flood plains at zones where there is pronounced presence of coarse alluvial sediments, at zones where the outcrop is weathered, and at fractured exposures of bedrock. Runoff in arid regions is often heavily laden with sediments. If the water is not too turbid and if the channel bottom is permeable enough, water will flow into the gravel and sand of the channel bottom. Part of this water will return to the atmosphere by evaporation and part will go into recharge of the underlying aquifer. Most of the recharge tends to take place at points where the channel is constricted rather than where the flow spreads out over a large area. This is because at narrow points of the channel the sediments are more permeable, being of larger size due to the velocity potential of flow.

In arid regions the usually great depth to the water table in the upland areas and the very small hydraulic gradient suggest there will be only relatively small amounts of recharge in these regions.

3.3.2.3. Groundwater

Most of the distinctive hydrogeologic features of arid regions are related to the quantity and quality of available ground water. In dry regions, groundwater is a very important source of freshwater for domestic, agricultural, and industrial use, and it may be the only source of water supply over large parts of the year.

Groundwater resources play a significant role in the hydrologic cycle and the water balance of a region since they act as a buffer, providing significant amounts of water during years of low precipitation or storing a significant part of runoff on the occasion of high precipitation. In view of meagre recharge in areas of low rainfall, aquifers are destined for ultimate depletion unless prudent and very conservative development is carried out. Such development should not seek to extract more than the long-term amounts of recharge.

Due to the great quest for more water in these regions, aquifers are in many cases overexploited, and suffer much degradation, such as lowering of the water level, salinisation and intrusion of marine water, and mineral and organic pollution. This unbalanced situation is typical of man induced water shortage and desertification. The very strong demand exerted on the most easily accessible aquifers brings about salt accretion to groundwater that becomes a serious problem near the coast or in the vicinity of aquifers with low quality water. This is more pronounced in areas of increasing water scarcity where water from different aquifers of varying quality is imported and blended with surface waters to maintain existing land uses and to meet other demands.

The exploitation of aquifers in regions of low natural recharge and scarce surface water is to a large extent controlled by the low rainfall and the relatively small amounts of

runoff, which is often associated with high erosion when the vegetative cover is poor and flash floods are the rule.

Extensive aquifers found in arid regions are often the result of past climates which were considerably more moist than the present conditions. Other aquifers at the fringe of high mountain ranges may owe their replenishment to higher rainfall in the uplands.

Closed basins filled with fine sediments of recent origin, are often highly saline especially in areas where water remains at the surface for some time before it evaporates.

Aquifers with good quality water are only found along the margins of such basins or in deeper aquifers not affected by present arid conditions. The most significant aquifers are formed by river deposits that are usually a mixture of poorly sorted, and of generally low permeability, sediments. The most permeable zones are found only where the runoff persists long enough to sort out and deposit only the coarser material in the streambed.

These conditions can normally be expected only in former narrow valleys and channels and they are normally of limited extent.

Furthermore, due to the high erosion experienced in arid regions the thickness of alluvial material is limited and the bedrock in many cases is exposed or quite near to the surface. On the other hand, widespread consolidated and semi-consolidated aquifers are sometimes found in regions of water scarcity with considerable quantities of ground water in storage which was recharged in past geologic times under different climatic conditions.

The mining of these aquifers also entails increasingly heavy constraints, even if the estimated storage is huge compared with the annual consumption. Their development and use must be based on sound hydrologic studies since overexploitation will inevitably result in their depletion and extinction.

3.3.3. Sediments

In arid and semi-arid regions, several environmental conditions favour water and wind erosion. On the one hand, soil erodibility tends to be high because lack of organic matter and the presence of salinity lead to low stability of the soil aggregates. On the other hand, rainfall and wind erosion are quite strong. Rainfall events are often very intense and wind storms also have a high potential to detach the soil particles and transport them large distances. Erosion is aggravated by the reduced protection offered by vegetation, which is mostly sparse and offers little soil coverage, and by the lack or insufficiency of conservation measures in agricultural lands. Then, detached soil, of silt and clay size particles, is easily transported by overland runoff to the ephemeral streams, thus providing a ready supply of sediments to the streamflow.

In low rainfall regions, runoff can be generated from even small rainfall events falling on rock outcrops without vegetation or on hard clay surfaces. However, only in the very low intensity rain events is the runoff likely to be free of suspended sediments. More commonly, in low rainfall regions, depending on the soil types, the relative lack of vegetation and the sudden bursts of short but intensive storms cause the runoff to erode large amounts of soil resulting in mudflows and highly turbid water.

In addition to the intensity and conditions of a rain event, the amount of erosion depends considerably on the type and state of the land surface. The extent of plant cover, roughness and micro-relief of the surface, porosity, texture, structure, and salinity of the

soil and the soil moisture content are some of the basic controlling factors. In low rainfall areas, the lack of vegetative cover, the moisture deficiency and surface crusting are critical.

Problems are worse when the topography comprises steep, long slopes, and the lithologic features encourage sheet erosion and the transport of detached particles by overland flow.

Gullies are very often formed under these conditions. Therefore, in water scarce areas, surface flows are very often associated with large sediment charges, and the deposition of fine sediments in flat areas and stream beds make the infiltration, that would provide for groundwater recharge, more difficult.

Erosion and salinity are among the main problems affecting soils in water scarce regions. This physical degradation, together with the soil moisture stress and loss of fertility associated with the soil losses, is among the most important causes of desertification. Other causes are the unbalanced land and water uses, which are very often much above the potential capability of those natural resources. When too much pressure is placed on their exploitation, their resilience is not sufficient for recovery and unbalances become evident and, in many cases, quite permanent.