4.3.4. Soil Moisture
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.
4.3.5. Groundwater Recharge and Resources
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