Environmental Engineering Reference
In-Depth Information
aquifer; these cavities also facilitate ground-
water discharge in the form of springs.
Subsurface geology influences discharge
processes as well as recharge processes. If the
rate of discharge from an aquifer is less than
the recharge rate, water storage within the
aquifer increases. Aquifer storage can reach a
maximum at which point additional recharge
cannot be accepted, regardless of the amount of
precipitation. This condition typically leads to
enhanced runoff.
Geophysical techniques have a wide range
of uses in geologic and hydrologic studies, pro-
viding information on the electrical, physical,
and chemical properties of surface and sub-
surface sediments. In regards to quantifying
groundwater recharge, geophysical methods
are most useful for determining soil-water con-
tent (
Section 5.2.1
) and changes in subsurface
water storage (
Section 2.3.3
). However, informa-
tion obtained from the application of geophys-
ical techniques is also useful in a qualitative
sense. Geophysical techniques can be used to
infer aquifer geometry and hydraulic proper-
ties, important information for shaping con-
ceptual models of hydrologic systems and for
constructing computer models of groundwater
flow (Robinson
et al
.,
2008a
).
upslope portions as well as precipitation. Local
relief, orientation, and altitude of mountain
ranges are additional topographic factors that
can affect recharge processes (Stonestrom and
Harrill,
2007
).
1.4.5 Hydrology
A conceptual model of recharge processes needs
to consider the surface-water and groundwater
flow systems and how they are linked. Are
streams in the area perennial or ephemeral?
Are streams gaining (receiving groundwater
discharge) or losing (providing recharge)? A sin-
gle stream could conceivably be losing water to
an aquifer in one reach, but gaining water in
another reach; the difference between ground-
water and surface-water elevations, according
to Darcy's law, determines whether water is
moving to or from the subsurface. These are
key questions, the answers to which will help
shape the conceptual model.
The depth to the water table also is import-
ant. If the unsaturated zone is thin, infiltrat-
ing water can quickly travel to the water table;
recharge may be largely episodic, occurring
in response to any large precipitation event.
However, shallow water tables are also sus-
ceptible to groundwater discharge by plant
transpiration. Therefore, water that recharges
shallow subsurface systems may only reside
in the saturated zone for a short time before
it is extracted by plant roots and returned to
the atmosphere. Thick unsaturated zones are
less likely to have episodic recharge events;
recharge would be expected to be seasonal or
quasi-steady because wetting fronts moving
through the unsaturated zone tend to slow
with depth and multiple fronts may coalesce
and
1.4.4 Surface topography
Land-surface topography plays an important
role for both diffuse and focused recharge.
Steep slopes tend to have low infiltration rates
and high runoff rates. Flat land surfaces that
have poor surface drainage are more conducive
to diffuse recharge; these conditions also favor
flooding. Small, often subtle depressions can
have a profound influence on infiltration rates.
Delin
et al
. (
2000
) showed that, even with highly
permeable soils, slight depressions in an appar-
ently uniform agricultural field caused runoff
to be focused in certain areas, with the result
that infiltration (and recharge) in those areas
was substantially greater than that in the rest
of the field. Even with uniform surface char-
acteristics, apparent infiltration rates increase
in the downslope direction along a long hill
slope (Dunne
et al
.,
1991
) because downslope
portions of the hill are exposed to runoff from
become
indistinguishable
from
each
other.
1.4.6 Vegetation and land use
Vegetation and land use can have profound
effects on recharge processes. Types and
densities of vegetation influence patterns of
evapotranspiration. A vegetated land surface
typically has a higher rate of evapotranspir-
ation (and, hence, less water available for
recharge) than an unvegetated land surface
