Environmental Engineering Reference
In-Depth Information
differences in this field are minute, apparently
random; to someone standing in the field dur-
ing a rain storm, the systematic pattern in
recharge is obvious.
Recharge also varies temporally. Seasonal,
multiyear, or even long-term trends in climate
affect recharge patterns. Because of its close
link to climate, temporal variability of recharge
is addressed more thoroughly in
Section 1.4.2
.
Changes in land use or in vegetation type and
density can also result in large changes in
recharge rates over time.
T The i mp or t a nc e of s p at i a l a nd t e mp or a l v a r i -
ability of recharge must be considered within
the context of study objectives. Spatial variabil-
ity may not be critical for groundwater resource
evaluation if an average rate of recharge can
be determined for an entire aquifer. Spatial
variability is important, though, for assessing
aquifer vulnerability to contamination; there-
fore, methods that provide point estimates
of recharge may be appropriate. Historically,
many groundwater-flow models were devel-
oped under the assumption that recharge was
constant in time. Current model applications
typically allow recharge to vary over time
but hold it constant for periods of months or
years. Recent advances in incorporating land-
scape features into combined surface-water
and groundwater flow (
Section 3.6
) will allow
impacts of climate, land-use, and vegetation
change on water resources to be examined at
unprecedented levels of temporal and spatial
variabilit y.
Simulated recharge (mm/yr)
0- 1
1-2
2-5
5-10
10-30
30-50
50-00
100-200
Figure 1.3
Map of average annual recharge rate for the
state of Texas (Keese
et al
.,
2005
).
display trends similar to those in statewide
maps of annual precipitation. The concept of
recharge rates increasing with increasing pre-
cipitation rates is certainly intuitive - recharge
cannot occur if water is not available. The
random factor in recharge variability can be
viewed as local-scale variability that can be
attributed, for example, to natural heterogen-
eity in permeability in surface soils or variabil-
ity in vegetation. Any of the factors addressed
below can contribute to apparent random vari-
ability. Delin
et al
. (
2000
) found that annual
recharge varied by more than 50% within what
appeared to be a uniform 2.7-hectare agricul-
tural field simply because of slight differences
in surface topography; the total relief in the
field was less than 1.5 m. It could be argued that
this difference in topography was not random;
indeed, distinguishing between systematic
and random patterns of recharge is sometimes
a matter of scale. In the context of the entire
upper Mississippi River valley, the topographic
1.4.2 Climate
Climate variability is often the most important
factor affecting variability in recharge rates.
Precipitation, the source of natural recharge, is
the dominant component in the water budget
for most watersheds. T The relation bet ween spa-
tial trends of precipitation and recharge has
been noted in
Section 1.4.1
. Temporal variabil-
ity in precipitation also is important. Seasonal,
year-to-year, and longer-term trends in pre-
cipitation, as well as frequency, duration, and
intensity of individual precipitation events
also affect recharge processes. Conditions are
most favorable for water drainage through
