Environmental Engineering Reference
In-Depth Information
plane at some depth within the unsaturated
zone that separates upward and downward
moving water; the ZFP is sometimes equated
with the bottom of the root zone (
Figure 1.1
).
Water above the ZFP moves upward in response
to evapotranspiration demand; water beneath
the ZFP drains downward, eventually arriving
at the water table. The depth of the ZFP changes
in response to infiltration and evapotranspira-
tion, ranging from land surface (for the case
of downward water movement throughout the
unsaturated zone) to some depth beneath the
water table (for the case of groundwater eva-
potranspiration). Water draining beneath the
ZFP in the unsaturated zone is referred to as
drainage
,
percolation
, or
net infiltration;
; it becomes
actual recharge when it arrives at the water
table. Some techniques described in this topic
provide estimates of potential recharge; others
provide estimates of drainage; and some meth-
ods provide estimates of actual recharge.
For clarity, we use the term
groundwater
to
refer to water beneath the water table (within
the saturated zone) and the term
pore water
to
refer to water above the water table (within the
unsaturated zone). A
point
estimate pertains to
recharge at a specific point in space or time,
whereas an
integrated
estimate refers to a value
of recharge that is averaged over some larger
space or time scale.
Different climatic regions are referred to
throughout the text. Climatic regions are clas-
sified on the basis of annual precipitation. An
arid
climate is one with annual precipitation
of less than 250 mm; a
semiarid
region has pre-
cipitation rates between 250 and 500 mm/yr; a
subhumid
climate refers to precipitation rates
between 500 and 1000 mm/yr; and
humid
cli-
mates have annual precipitation rates that
exceed 1000 mm.
from that of Lerner
et al
. (
1990
) and Wilson
(
1980
), who chose to organize methods on the
basis of source of recharge (precipitation, riv-
ers, etc.). While there is perhaps no ideal format
for this presentation, the format used in this
text has proved workable within the classroom
over the decade and a half that we have taught
this material. Examples are given to show how
methods can be applied for different sources of
recharge water.
This first chapter provides an introduction
to the topic, emphasizing the importance of
developing a conceptual model of recharge
processes for the area of interest.
Chapters 2
through
8
are the heart of the topic. They pro-
vide in-depth analysis of methods for estimat-
ing recharge. The format for each presentation
is similar: discussion of theory and assump-
tions, advantages and limitations of the meth-
ods, and description of example case studies.
Each chapter is devoted to a particular family of
methods. Water-budget methods (
Chapter 2
) are
presented first to emphasize the importance of
water budgets in all studies.
Water-budget methods are widely used;
indeed, most methods for estimating recharge
could be classified as water-budget methods. To
avoid making
Chapter 2
too long, its content is
limited to the use of the residual water-budget
method, whereby a water-budget equation is
derived for a control volume, such as a water-
shed or an aquifer. All components within that
equation, except for recharge, are measured
or estimated; recharge is then set equal to the
residual in the equation. Other methods that
can be categorized as water-budget methods
(e.g. the water-table fluctuation method, the
zero-flux plane method, and modeling methods)
are described in other chapters. Remote-sensing
tools are described in
Chapter 2
, although they
may be useful in other methods as well.
Discussion in
Chapter 3
is devoted to the use
of models for estimating recharge. A general
approach to modeling, applicable to all models,
is presented first; a brief description of inverse
techniques is included. Unsaturated zone water-
budget models, watershed models, groundwater
flow models, and integrated surface- and subsur-
face-flow models are then discussed. Because of
1.3 Overview of the text
This text is organized by methods, which are
grouped on the basis of types of required or avail-
able data (e.g. methods based on water budgets,
or on data obtained from the unsaturated zone,
or on streamflow data). Our approach differs
