Environmental Engineering Reference
In-Depth Information
several sources of error, and they come into play
in all segments of the estimation process. The
most serious errors arise due to formulation of
an incorrect conceptual model of recharge proc-
esses. Development of a conceptual model and
estimation of recharge rates are iterative proc-
esses. Therefore, it is natural that revisions be
made to the conceptual model as more informa-
tion becomes available. Continual reevaluation
of the conceptual model is critical.
Inappropriate application of a method and
violations of one or more assumptions of a
method are other common sources of error. For
example, estimates of recharge by the water-
table fluctuation method are based on rises in
groundwater levels. As explained in
Chapter 6
,
water levels can rise because of factors other
than the occurrence of recharge. Failure to
account for rises due to changes in barometric
pressure or entrapped air or changes in pumpage
will lead to inaccuracies in predicted recharge
rates. Measurement errors also contribute to
overall error. The Darcy method requires meas-
urement or estimation of hydraulic conductiv-
ity; however, hydraulic conductivity is highly
variable in space and is also very difficult to
measure accurately. Errors also can accrue if
spatial and temporal variability are not prop-
erly accounted for.
Water-budget methods and modeling
approaches are amenable to classical error ana-
lysis, as described in
Chapters 2
and
3
. Such
analyses are useful, but they have to be viewed
with some caution because they are based on
the assumption that the conceptual model is
correct. If the conceptual model is incorrect,
the error analysis is meaningless. Application of
multiple methods will not necessarily improve
estimation accuracy in a quantitative sense.
However, having multiple estimates of recharge
is beneficial in several respects. Consistency
among estimates lends support to validity of
the estimates. Inconsistency among estimates
can provide insight on measurement errors and
invalid assumptions and thus indicate in which
manner the conceptual model could be revised.
Many methods can be applied by drawing on
the vast amount of existing data without the
need to collect additional data.
9.3 Comparison of methods
Previous chapters have presented details of
individual methods. Here the main features
of all methods are discussed and summarized
in
Tables 9.1
and
9.2
. Information provided in
these tables represents the views of the author
and should not be viewed as strict guidelines.
The tables can assist readers in identifying
potentially useful methods and pointing them
toward the chapters that contain details on the
methods. The information contained in the
tables is not all self explanatory. A full under-
standing of the content of the tables requires a
careful reading of this section.
Table 9.1
indicates whether a method can be
applied for focused and/or diffuse recharge esti-
mates.
Table 9.1
also indicates whether a method
estimates an actual recharge rate (R), a rate of
drainage (D) through the unsaturated zone, or
base flow (B). According to the definitions given
in
Chapter 1
, the term
recharge
applies to water
that actually arrives at the water table. The
term
drainage
applies to a vertical water flux
within the unsaturated zone beneath the zero-
flux plane (bottom of the root zone). Draining
water will eventually become recharge when it
arrives at the water table. Methods that use data
collected in the unsaturated zone usually gen-
erate estimates of drainage. For methods that
assume the recharge is steady in time, such
as the Darcy unsaturated-zone unit-gradient
method, drainage and recharge are equivalent
terms. Water-budget and modeling methods
can generate estimates of either recharge or
drainage, depending on assumptions made for
specific applications.
Table 9.1
contains information on time
scales, i.e. the intervals over which a recharge
estimate can be calculated. For example, a water
budget of a soil column can be calculated on an
event (or daily) basis or on the basis of weekly
data. But it is generally impractical to calculate
a recharge estimate based on average annual
values for precipitation and evapotranspiration.
Annual, multiannual, and decadal estimates of
recharge can still be obtained with the water-
budget method by performing the necessary
