Environmental Engineering Reference
In-Depth Information
5
Physical methods: unsaturated zone
based on the Darcy equation. Unsaturated-zone
water budgets relate changes in the amount of
water stored in the unsaturated zone to infiltra-
tion, drainage, and evapotranspiration. These
methods include the zero-flux plane method
and lysimetry. Included in the section on the
zero-flux plane method is a discussion on meas-
urement of water storage and change in storage
within the unsaturated zone. Darcy methods
require measurement or estimation of the
hydraulic gradient and hydraulic conductivity
at the ambient water content. Natural variabil-
ity in hydraulic conductivity complicates the
application of the Darcy method. Lysimeters
can provide precise measurements of drain-
age rates, but the instruments can be expen-
sive to install and maintain. A brief overview
of techniques for measuring water content,
pressure head, water-retention characteristics,
and hydraulic conductivity of unsaturated-zone
sediments, provided in Section 5.2 , lays the
groundwork for discussions of the zero-flux
plane method ( Section 5.3 ), the unsaturated-
zone Darcy method ( Section 5.4 ), and the use of
lysimeters ( Section 5.5 ).
5.1 Introduction
Estimates of recharge can be obtained from
measurement of downward water flux or change
in water storage within the unsaturated zone.
Methods based on physical (as opposed to chemi-
cal) data collected within the unsaturated zone
are not among the more commonly used tech-
niques for estimating recharge, but they offer
some distinct advantages. The methods actually
produce estimates of drainage rates below the
depth of measurement within the unsaturated
zone. The usual assumption is that the draining
water will eventually reach the water table, at
which time it can properly be called recharge.
But there can be a long lag time over which water
traverses that depth interval. Estimates generated
by these methods are referred to as drainage in
this chapter. In general, these methods produce
point estimates of drainage. The question of how
representative a measurement at a single point is
of flux through the unsaturated zone as a whole
requires careful consideration. These methods
can be costly to implement and require intensive
instrumentation that is susceptible to measure-
ment inaccuracies. Nonetheless, under certain
circumstances, such as rapid movement of a wet-
ting front from land surface to a shallow water
table, these methods have a unique ability to pro-
vide detailed insight into recharge processes and
factors that influence recharge rates.
The methods can be divided into two
classes: water-budget methods and methods
5.2 Measurement of unsaturated-
zone physical properties
5.2.1 Soil-water content
Soil-water content can be measured gravi-
metrically or by one of many geophysical tech-
niques, some of which are listed in Table 5.1 .
 
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