Water-budget methods - Estimating Groundwater Recharge - page 16

Inconsistencies between the conceptual model
of a hydrologic system and recharge estimates
generated during the study may indicate the
need for further evaluation of both the model
and the estimates.
The following section presents a general
analysis of water-budget equations. Methods for
measuring and estimating components in typ-
ical water-budget equations, including precipi-
tation, evapotranspiration, runoff, and water
storage, are presented in subsequent sections.
Temporal and spatial scales of interest play an
important role in selecting appropriate meth-
ods. For convenience, methods are organized
by spatial scales: local, corresponding to a small
field plot where point measurements of indi-
vidual components of the water-budget equa-
tion are sufficient - measurement techniques
for the individual components are addressed
in Section 2.3 ; mesoscale, corresponding to a
watershed where either upscaling of multiple
point measurements is required or estimating
equations must be used for each component -
estimation techniques are stressed in Section
2.4 ; and macroscale, corresponding to regional,
continental, or global scales where remote-
sensing tools are usually applied - these tools
are reviewed in Section 2.5 .

known or can be easily measured or estimated
make good boundaries for a control volume. For
purposes of estimating recharge, watersheds,
aquifers, and one-dimensional soil columns, or
some part of these, are the most widely used
control volumes; and for that reason, they are
emphasized in this chapter.
A simple water-budget analysis used in
many hydrological studies is based on a soil
column that extends downward from land sur-
face to some depth, L ( Figure 2.1 ) as the control
volume:

(2.1)

P =+++ ET ∆ S R D

off

where P is precipitation; ET is evapotranspir-
ation, which includes evaporation and plant
transpiration; Δ S is change in water storage in
the column; R off is direct surface runoff (pre-
cipitation that does not infiltrate); and D is
drainage out of the bottom of the column. All
components are given as rates per unit surface

P

ET

2.2 Water budgets

R off

The first step in a water-budget analysis is to
select one or more control volumes for study.
A control volume can be a volume of earth
or atmosphere or a hydrologic structure - a
laboratory soil column, a lake or stream, an
aquifer, a watershed, a country or state, and
the Earth itself are all examples of control vol-
umes for which water budgets can and have
been constructed. Any control volume whose
water-budget equation contains recharge as a
component can be used to estimate recharge.
Insight on the hydrology of the system is useful
in selecting a control volume. Where and when
recharge occurs, whether recharge is focused
or diffuse, what types of data are available -
these are important considerations in selecting
a control volume. Locations where fluxes are

Soil
column

D

D = P - ET - R off - ∆ S

Figure 2.1 Schematic diagram showing water budget of a
one-dimensional soil column. D is drainage out the bottom
of the column, P is precipitation, ET is evapotranspiration,
R off is runoff, and Δ S is change in storage.