Environmental Engineering Reference
In-Depth Information
rain storms, when the total precipitation for a
day exceeds the daily evapotranspiration rate
(Ahuja and El-Swaify,
1979
).
In arid regions, focused recharge from
ephemeral streams and playas is often the
dominant form of recharge. The frequency and
duration of streamflow play important roles in
the recharge process. The frequency of stream-
flow in Rillito Creek in Tucson, Arizona, coin-
cides with the frequency of recharge events.
Pool (
2005
) showed that interannual variabil-
ity in recharge from the creek is linked to the
El Niño/Southern Oscillation climate trend.
Years dominated by El Niño conditions (high
winter precipitation rates) produced signifi-
cantly higher streamflow and recharge rates
than years dominated by La Niña conditions.
120
90
60
30
0
Recharge
Evapotranspiration
Precipitation
Figure 1.4
Average monthly recharge,
evapotranspiration, and precipitation for the 2-year
period beginning in April 1950 for the Beaverdam
Creek watershed in eastern Maryland. Recharge occurs
throughout the year, but most of it occurs in the months of
November through March when evapotranspiration rates
are low (after Rasmussen and Andreasen,
1959
).
1.4.3 Soils and geology
Permeabilities of surface and subsurface
materials can greatly affect recharge proc-
esses. Recharge is more likely to occur in
areas that have coarse-grained, high-permea-
bility soils as opposed to areas of fine-grained,
low-permeability soils. Coarse-grained soils
have a relatively high permeability and are
capable of transmitting water rapidly. The
presence of these soils promotes recharge
because water can quickly infiltrate and drain
through the root zone before being extracted
by plant roots. Finer-grained sediments are
less permeable, but are capable of storing
greater quantities of water. Thus, in areas of
finer-grained sediments, one would expect
decreased infiltration, enhanced surface
runoff, increased plant extraction of water
from the unsaturated zone, and decreased
recharge relative to an area of coarser-grained
sediments. Permeability also is important in
terms of focused recharge. High-permeability
streambeds facilitate the exchange of surface
water and groundwater. In the Black Hills of
South Dakota, most recharge to the Madison
Limestone aquifer occurs at high elevations
as focused flow from streams that cross rock
outcrops (Swenson,
1968
; Downey,
1984
). In
karst regions, dissolution cavities or sinkholes
that have developed in the geologic material
can rapidly channel streamflow directly to an
the unsaturated zone to the water table when
precipitation rates exceed evapotranspiration
rates. In regions outside of the tropics, eva-
potranspiration rates follow a seasonal trend,
with highest rates occurring during summer
months and lowest rates in winter months. If
precipitation rates are fairly uniform through-
out the year, the most likely time of the year
for drainage to occur is winter through spring,
when precipitation rates exceed evapotran-
spiration rates. At a site in the eastern United
States, Rasmussen and Andreasen (
1959
) esti-
mated that 62% of recharge over a 2-year
period occurred in the months of November
through March (
Figure 1.4
); precipitation was
relatively uniform throughout the year, but
evapotranspiration rates were lowest during
these months.
Duration and intensity of individual pre-
cipitation events can have a large influence on
recharge in some settings. On the humid, wind-
ward side of the Hawaiian Islands, precipita-
tion and evapotranspiration rates are relatively
uniform throughout the year. Recharge occurs
at any time of the year in response to intense
