Environmental Engineering Reference
In-Depth Information
Figure 1.1
Vertical cross section
showing infiltration at land surface,
drainage through the unsaturated
zone, diffuse and focused recharge
to an unconfined aquifer, flow
between the unconfined aquifer
and an underlying confined aquifer
(interaquifer flow), and the zero-
flux plane.
Land
surface
Evapotranspiration
Precipitation
Zero-flux plane
Infiltration
Stream
Drainage
Water table
Focused
recharge
Diffuse recharge
Aquifer
Flow to or from aquifer
(Interaquifer flow)
of papers on recharge processes and case stud-
ies of recharge in arid and semiarid regions of
the southwestern United States.
the unsaturated zone to the water table; dif-
fuse recharge is sometimes referred to as
local
recharge (Allison,
1987
) or
direct
recharge
(Simmers,
1997
).
Focused
recharge is the move-
ment of water from surface-water bodies, such
as streams, canals, or lakes, to an underlying
aquifer. Focused recharge generally varies more
in space than diffuse recharge. A distinction
between different types of focused recharge has
been proposed by Lerner
et al
. (
1990
), with
local-
ized
recharge defined as concentrated recharge
from small depressions, joints, or cracks, and
indirect
recharge defined as recharge from map-
pable features such as rivers, canals, and lakes.
Groundwater systems receive both diffuse and
focused recharge, but the importance of each
mechanism varies from region to region and
even from site to site within a region. Generally,
diffuse recharge dominates in humid settings;
as the degree of aridity increases, the impor-
tance of focused recharge in terms of total aqui-
fer replenishment also tends to increase (Lerner
et al
.,
1990
). Some methods addressed in this
topic are designed to estimate diffuse recharge;
others are specific to focused recharge.
Iniltration
is the entry of water into the sub-
surface. Infiltrating water can be viewed as
potential recharge
; it may become recharge, but it
may instead be returned to the atmosphere by
evapotranspiration, or it may simply remain in
storage in the unsaturated zone for some period
of time. The
zero-lux plane
(ZF P) is the hor izontal
1.2 Terminology
Recharge
is defined, herein, as the downward
flow of water reaching the water table, adding
to groundwater storage. This definition is simi-
lar to those given by Meinzer (
1923
), Freeze and
Cherry (
1979
), and Lerner
et al
. (
1990
). Strictly
speaking, this definition does not include water
flow to an aquifer from an adjoining ground-
water system (such as water movement from
an unconfined aquifer across a confining bed
to an underlying aquifer); we refer to this flow
as
interaquifer flow)
. Others include this flow
in their definition of recharge. Interaquifer
flow has also been referred to as groundwater
underflow. Regardless of terminology, methods
for estimating interaquifer flow are included
in this text. Recharge is usually expressed as
a volumetric flow, in terms of volume per unit
time (L3/T),
3
/T), such as m
3
/d, or as a flux, in terms
of volume per unit surface area per unit time
(L/T), such as mm/yr.
Recharge occurs through diffuse and
focused mechanisms (
Figure 1.1
).
Diffuse
recharge is recharge that is distributed over
large areas in response to precipitation infil-
trating the soil surface and percolating through
