The amount of water stored in the soil is fundamentally important to agriculture and is an influence on the rate of actual evaporation, groundwater recharge, and generation of runoff. Soil moisture contents are directly simulated by global climate models, albeit over a very coarse spatial resolution, and outputs from these models give an indication of possible directions of change. Gregory et al. (1997), for example, show with the HadCM2 climate model that a rise in greenhouse gas (GHG) concentrations is associated with reduced soil moisture in Northern Hemisphere mid-latitude summers. This was the result of higher winter and spring evaporation, caused by higher temperatures and reduced snow cover, and lower rainfall inputs during summer.
The local effects of climate change on soil moisture, however, will vary not only with the degree of climate change but also with soil characteristics. The water-holding capacity of soil will affect possible changes in soil moisture deficits; the lower the capacity, the greater the sensitivity to climate change. Climate change also may affect soil characteristics, perhaps through changes in waterlogging or cracking, which in turn may affect soil moisture storage properties. Infiltration capacity and water-holding capacity of many soils are influenced by the frequency and intensity of freezing. Boix-Fayos et al. (1998), for example, show that infiltration and water-holding capacity of soils on limestone are greater with increased frost activity and infer that increased temperatures could lead to increased surface or shallow runoff. Komescu et al. (1998) assess the implications of climate change for soil moisture availability in southeast Turkey, finding substantial reductions in availability during summer.
Groundwater is the major source of water across much of the world, particularly in rural areas in arid and semi-arid regions, but there has been very little research on the potential effects of climate change. This section therefore can be regarded as presenting a series of hypotheses.
Aquifers generally are replenished by effective rainfall, rivers, and lakes. This water may reach the aquifer rapidly, through macro-pores or fissures, or more slowly by infiltrating through soils and permeable rocks overlying the aquifer. A change in the amount of effective rainfall will alter recharge, but so will a change in the duration of the recharge season. Increased winter rainfallas projected under most scenarios for mid-latitudesgenerally is likely to result in increased groundwater recharge. However, higher evaporation may mean that soil deficits persist for longer and commence earlier, offsetting an increase in total effective rainfall. Various types of aquifer will be recharged differently. The main types are unconfined and confined aquifers. An unconfined aquifer is recharged directly by local rainfall, rivers, and lakes, and the rate of recharge will be influenced by the permeability of overlying rocks and soils. Some examples of the effect of climate change on recharge into unconfined aquifers have been described in France, Kenya, Tanzania, Texas, New York, and Caribbean islands. Bouraoui et al. (1999) simulated substantial reductions in groundwater recharge near Grenoble, France, almost entirely as a result of increases in evaporation during the recharge season. Macro-pore and fissure recharge is most common in porous and aggregated forest soils and less common in poorly structured soils. It also occurs where the underlying geology is highly fractured or is characterized by numerous sinkholes. Such recharge can be very important in some semi-arid areas (e.g., the Wajir region of Kenya; Mailu, 1993). In principle, rapid recharge can occur whenever it rains, so where recharge is dominated by this process it will be affected more by changes in rainfall amount than by the seasonal cycle of soil moisture variability. Sandstrom (1995) modeled recharge to an aquifer in central Tanzania and showed that a 15% reduction in rainfallwith no change in temperatureresulted in a 4050% reduction in recharge; he infers that small changes in rainfall could lead to large changes in recharge and hence groundwater resources. Loaiciga et al. (1998) explored the effect of a range of climate change scenarios on groundwater levels in the Edwards Balcones Fault Zone aquifer in Texas, a heavily exploited aquifer largely fed by streamflow seepage. They show that, under six of the seven GCM-based scenarios used, groundwater levels and springflows would reduce substantially as a result of lower streamflow. However, they use 2xCO2 scenarios that represent changes in temperature that are considerably greater than those projected even by the 2080s under current scenarios (Carter and Hulme, 1999), so the study considerably overstates the effect of climate change in the next few decades.
Shallow unconfined aquifers along floodplains, which are most common in semi-arid and arid environments, are recharged by seasonal streamflows and can be depleted directly by evaporation. Changes in recharge therefore will be determined by changes in the duration of flow of these streamswhich may locally increase or decreaseand the permeability of the overlying beds, but increased evaporative demands would tend to lead to lower groundwater storage. In semi-arid areas of Kenya, flood aquifers have been improved by construction of subsurface weirs across the river valleys, forming subsurface dams from which water is tapped by shallow wells. The thick layer of sands substantially reduces the impact of evaporation. The wells have become perennial water supply sources even during the prolonged droughts (Mailu, 1988, 1992).
Sea-level rise will cause saline intrusion into coastal aquifers, with the amount of intrusion depending on local groundwater gradients. Shallow coastal aquifers are at greatest risk (on Long Island, New York, for example). Groundwater in low-lying islands therefore is very sensitive to change. In the atolls of the Pacific Ocean, water supply is sensitive to precipitation patterns and changes in storm tracks (Salinger et al., 1995). A reduction in precipitation coupled with sea-level rise would not only cause a diminution of the harvestable volume of water; it also would reduce the size of the narrow freshwater lense (Amadore et al, 1996). For many small island states, such as some Caribbean islands, seawater intrusion into freshwater aquifers has been observed as a result of overpumping of aquifers. Any sea-level rise would worsen the situation.
It will be noted from the foregoing that unconfined aquifers are sensitive to local climate change, abstraction, and seawater intrusion. However, quantification of recharge is complicated by the characteristics of the aquifers themselves as well as overlying rocks and soils.
A confined aquifer, on the other hand, is characterized by an overlying bed that is impermeable, and local rainfall does not influence the aquifer. It is normally recharged from lakes, rivers, and rainfall that may occur at distances ranging from a few kilometers to thousands of kilometers. Recharge rates also vary from a few days to decades. The Bahariya Oasis and other groundwater aquifers in the Egyptian Desert, for example, are recharged at the Nubian Sandstone outcrops in Sudan; such aquifers may not be seriously affected by seasonal or interannual rainfall or temperature of the local area.
Attempts have been made to calculate the rate of recharge by using carbon-14 isotopes and other modeling techniques. This has been possible for aquifers that are recharged from short distances and after short durations. However, recharge that takes place from long distances and after decades or centuries has been problematic to calculate with accuracy, making estimation of the impacts of climate change difficult. The medium through which recharge takes place often is poorly known and very heterogeneous, again challenging recharge modeling. In general, there is a need to intensify research on modeling techniques, aquifer characteristics, recharge rates, and seawater intrusion, as well as monitoring of groundwater abstractions. This research will provide a sound basis for assessment of the impacts of climate change and sea-level rise on recharge and groundwater resources.
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