Environmental Engineering Reference
In-Depth Information
If a stream rises above its banks in the course
of a runoff event, flood waters could extend over
large areas and provide a significant source of
infiltration (potential recharge). Recharge from
a losing stream is typically viewed as focused
recharge. The widespread nature of infiltration
from flood waters imparts more of a diffuse
nature to the potential recharge. Flood waters
may eventually flow back through the subsurface
to the stream, but this process can take a long
time, especially in flat, swampy areas. Climate,
geology, soils, land use, land surface topography,
and water use all play a role in the interaction
between groundwater and surface water.
includes pumping and interaquifer flow. (A dis-
cussion of the uncertainty inherent in the
derivation of Equation (
4.1
) and in the meas-
urement of individual components of the equa-
tion is provided in
Chapter 2
.) Assuming that
recharge is equal to base flow is equivalent to
assuming that all other terms on the right-hand
side of Equation (
4.1
) are negligible - there is
no change in aquifer storage, no exchange of
water with underlying aquifers, no underflow,
no withdrawal or injection of groundwater, and
no groundwater loss to evapotranspiration.
The magnitude of the neglected terms in
Equation (
4.1
) should be assessed with any avail-
able data; if any terms are determined to be
important, recharge estimates can be adjusted
accordingly. Groundwater levels, if measured
with sufficient frequency, can be used to esti-
mate change in aquifer storage (
Section 6.2
).
In undisturbed natural hydrologic systems,
weather patterns result in seasonal fluctua-
tions in groundwater levels and, therefore,
aquifer storage. But over the course of years,
in the absence of significant climate change,
aquifer storage tends to remain constant. The
magnitude of interaquifer flow can be assessed
by using Darcy/Hantush methods (
Section 6.3
),
but these methods require water-level meas-
urements in both aquifers and an estimate of
hydraulic conductivity. Direct measurements of
groundwater withdrawal and injection rates can
be difficult to obtain, but rates can sometimes
be estimated on the basis of permits issued by
regulatory agencies, irrigation demands deter-
mined by agricultural agencies, and records of
utilities and municipal water suppliers. Direct
evapotranspiration of groundwater occurs in
areas with shallow water tables and is gener-
ally more prevalent in humid regions, such as
the eastern United States, than in arid regions.
However, phreatophytes within riparian zones
in arid and semiarid regions can extract signifi-
cant quantities of groundwater (White,
1932
).
For many watersheds, the difference between
base flow and actual recharge may be within
the margin of measurement uncertainty for
base flow.
As to the question of whether streamflow
at low-flow levels (i.e. in the absence of surface
4.1.2 Base flow
Base flow is defined in
Chapter 2
as ground-
water discharge to a stream or other surface-
water body. For clarity, it should be noted that
this term has been used by others to represent
all streamflow that is not derived directly from
surface runoff or interflow and can include, in
addition to groundwater discharge, release of
water from snowpacks, bank storage, and other
features (Meyboom,
1961
; Linsley
et al
.,
1982
;
Halford and Mayer,
2000
). Some of the methods
described in this chapter provide estimates of
base flow (groundwater discharge to streams).
Base flow is usually associated with diffuse
recharge that occurs somewhat uniformly in
space as infiltration from precipitation or irri-
gation. That infiltrating water travels through
the subsurface to the water table and eventually
discharges to a stream. To determine whether
base flow is a good approximation to recharge,
two questions must be addressed: (1) Does all
water that recharges an aquifer ultimately dis-
charge to a stream? (2) Does streamflow (at cer-
tain low flows) consist entirely of groundwater
discharge?
The water-budget equation for an aquifer
presented in
Chapter 2
provides a basis for ana-
lysis for the first question:
R
= ++ + −
∆
S
Q
ET
(
Q
Q
)
(4.1)
gw
bf
gw
gw
gw
off
on
where
R
is recharge, Δ
S
gw
is change in ground-
water storage,
Q
bf
is base flow,
ET
gw
is evapo-
transpiration of groundwater, and (
Q
gw
off
-
Q
gw
on
)
is net groundwater flow out of the aquifer and
