Environmental Engineering Reference
In-Depth Information
weighing lysimeter, relies on the principle
that changes in mechanical load (attributed
to changes in water storage) are reflected in
changes in groundwater pressures. Accurate
measurements of groundwater levels are
required, and the measurements must be cor-
rected for the effects of atmospheric pressure
and Earth tides. Barr
et al
. (
2000
) indicated
that results obtained from measurements at a
depth of 34.6 m in a single well were represen-
tative of an area of about 10 ha, a much larger
area than that represented by a typical weigh-
ing lysimeter. Geological weighing lysimeters
do not provide direct estimates of recharge;
instead, they provide estimates of changes in
surface and subsurface water storage, much
like gravity measurements (
Sections 2.3.3
and
2.5.3
). The use of geological weighing lysim-
eters for calibration of gravity measurements
was suggested by Bardsley and Campbell
(2007). Estimated water storage changes could
be a useful part of a water-budget approach for
estimating recharge and evapotranspiration.
widely used because of the ease with which
they can be applied and the abundance of avail-
able data in local, state, and federal databases.
Recent technological advances have improved
the accuracy, reduced the expense, and eased
the complexity of making automatic measure-
ments and recordings of groundwater levels.
Within any watershed, recharge rates vary
with location because of differences in precipi-
tation, vegetation, soil properties, land use,
geology, and other factors. Obtaining an average
recharge rate requires multiple wells that are
distributed across the watershed in a manner
that accounts, as much as is practical, for this
variability. Groundwater levels can be affected
by phenomena other than recharge, including
atmospheric pressure, pumping, evapotranspir-
ation, tides, and surface loads. Measured water
levels should be adjusted to account for these
phenomena. Difficulties in estimating specific
yield contribute to the overall uncertainty
of the water-table fluctuation method, yet it
remains one of the most widely used methods
for estimating recharge. In light of the uncer-
tainty inherent in any method for estimating
recharge, the desirability of applying multiple
estimation methods, and the favorable fea-
tures cited above, any hydrologic study would
likely benefit from careful analyses of available
groundwater-level data.
6.6 Discussion
Recharge estimation methods that are based
on measurements of groundwater levels are
