Environmental Engineering Reference
In-Depth Information
Initial Steps
Review previous studies
Accumulate and analyze existing data
Formulate Conceptual Model
Consider all
methods
Importance of focused vs. diffuse recharge.
Where, when, why does recharge occur?
At what rate?
Construct water budget for aquifer, watershed.
Apply numerical model of aquifer, watershed.
Compare results
from different
methods
Select
appropriate
methods
Determine
- What data
to collect?
- Where?
- For how
long?
Assess uncertainties
and sensitivities
Analyze
data-measurement
errors
spatial/temporal
variabilities
Generate
estimate of
recharge or
drainage
Collect
data
Examine
spatial/temporal
variabilities
Figure 1.2 Schematic showing iterative process for
developing a conceptual model of recharge processes.
for determining sustainable yields in many
groundwater systems (Sophocleous et al ., 2004 ;
Devlin and Sophocleous, 2005 ), but recharge
rates by themselves are not sufficient for deter-
mining sustainability (Bredehoeft et al ., 1982 ;
Bredehoeft, 2002 ). The effects of changes in
groundwater levels on groundwater discharge
rates and aquifer storage must also be consid-
ered. From a hydrologic perspective, sustain-
able yield is best studied within the context of
the entire hydrologic system of which the aqui-
fer is a part, but decisions as to what constitutes
a sustainable yield often involve more than just
hydrologic considerations. Ecological, cultural,
economic, and other considerations should help
to determine the acceptability of any effects
related to groundwater development (Alley and
Leake, 2004 ).
recharge estimates are obtained. As noted by
Lerner et al . ( 1990 ), a good method for estimat-
ing recharge provides not only an estimate of
how much water becomes recharge, but also
explains the fate of the remaining water that
does not become recharge. Water-budget ana-
lyses serve that purpose. In addition, a water-
budget equation provides a convenient context
for the analysis of assumptions inherent in vari-
ous estimation techniques.
Although recharge is important in water-
supply studies, recharge rates are sometimes
incorrectly equated with the sustainable yield
of an aquifer (Meinzer, 1923 ; Bredehoeft et al .,
1982 ; Bredehoeft, 2002 ; Alley and Leake, 2004 ).
The term sustainable yield or safe yield refers
to the rate at which water can be withdrawn
from an aquifer without causing adverse
impacts. Those impacts could be in the form of
decreased discharge to streams and wetlands,
land subsidence, or induced contamination of
groundwater, for example, by seawater intru-
sion. The notion that recharge is equivalent to
sustainable yield is based on an incomplete or
incorrect conceptual model of a hydrologic sys-
tem. Knowledge of recharge rates is important
1.4.1 Spatial and temporal variability
in recharge
Recharge rates vary in space in both system-
atic and random fashions. This is true for both
focused and diffuse r e c h a r g e . S y s t e m a t i c t r e n d s
often are associated with climatic trends,
but land use and geology are also important.
Statewide maps of estimated annual recharge
for Texas ( Figure 1.3 ; Keese et al ., 2005 ) and
Minnesota (Lorenz and Delin, 2007 ) both
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