Environmental Engineering Reference
In-Depth Information
2
Water-budget methods
single chapter. This chapter is limited to discus-
sion of the “residual” water-budget approach,
whereby all variables in a water-budget equa-
tion, except for recharge, are independently
measured or estimated and recharge is set
equal to the residual. This chapter is closely
linked with
Chapter 3
, on modeling meth-
ods, because the equations presented here
form the basis of many models and because
models are often used to estimate individual
components in water-budget studies. Water
budgets for streams and other surface-water
bodies are addressed in
Chapter 4
. The use of
soil-water budgets and lysimeters for determin-
ing potential recharge and evapotranspiration
from changes in water storage is discussed in
Chapter 5
. Aquifer water-budget methods based
on the measurement of groundwater levels are
described in
Chapter 6
.
Water budgets are fundamental to the con-
ceptualization of hydrologic systems at all
scales. Building a preliminary water budget
from existing data is an easy and logical first
step in any study, regardless of whether water-
budget methods will actually be used to esti-
mate recharge. Initial analysis of a water budget
can provide insight as to the suitability of any
recharge estimation technique. Refinement of
the water budget and the conceptual model of
the system throughout the duration of a study
can further guide study efforts. Evaluation
of the water budget at the conclusion of a
study is a particularly useful exercise, serv-
ing as a postaudit or a check of study results.
2.1 Introduction
A water budget is an accounting of water move-
ment into and out of, and storage change within,
some control volume.
Universal
and
adaptable
are adjectives that reflect key features of water-
budget methods for estimating recharge. The
universal concept of mass conservation of water
implies that water-budget methods are applic-
able over any space and time scales (Healy
et al
.,
2007
). The water budget of a soil column in a
laboratory can be studied at scales of millim-
eters and seconds. A water-budget equation is
also an integral component of atmospheric gen-
eral circulation models used to predict global
climates over periods of decades or more. Water-
budget equations can be easily customized by
adding or removing terms to accurately portray
the peculiarities of any hydrologic system. The
equations are generally not bound by assump-
tions on mechanisms by which water moves
into, through, and out of the control volume of
interest. So water-budget methods can be used
to estimate both diffuse and focused recharge,
and recharge estimates are unaffected by phe-
nomena such as preferential flow paths within
the unsaturated zone.
Water-budget methods represent the largest
class of techniques for estimating recharge.
Most hydrologic models are derived from a
water-budget equation and can therefore be
classified as water-budget models. It is not feas-
ible to address all water-budget methods in a
