Environmental Engineering Reference
In-Depth Information
The magnitude of water-budget compo-
nents can be readily assessed, at least in a
preliminary sense, by using available data
sources, such as those shown in
Table 2.1
or
in the Hydrometeorlogical Networks internet
site of the National Center for Atmospheric
Research (http://www.eol.ucar.edu/projects/
hydrometnet/; accessed August 17, 2009). These
preliminary estimates may indicate that one
component is very small relative to the others
so that a large relative uncertainty in that com-
ponent will ultimately contribute little to the
overall uncertainty in recharge. For example,
groundwater recharge from a lake might
be addressed by examining the lake's water
budget. If that water budget is dominated by
surface-water inflow and outflow and lake level
is fairly constant, then initial analysis might
indicate that Δ
S
and
ET
are minor components.
The uncertainty in the estimates of Δ
S
and
ET
would have little impact on the overall water
budget and on the estimate for recharge. This
implies that there would be little value in
investing resources to improve upon the initial
estimates of Δ
S
and
ET
. Resources are generally
best directed at accurately quantifying the lar-
gest components in the water-budget equation,
in this case surface-water flow into and out of
the lake, because uncertainties in these compo -
nents are generally the largest source of error
in recharge estimates.
For ease of reading, uncertainty terms are
not explicitly included in all water-budget equa-
tions in this text; however, the reader should
understand that uncertainty terms are inher-
ent to and implicitly included in every water-
budget equation. Examination of uncertainties
should be an integral part of any water-budget
analysis, even though many studies reported in
the literature neglect to do this.
components. The control volume corresponds
to a field plot or perhaps a small watershed with
a length scale typically in the range of 1 m to
1 km.
2.3.1 Precipitation
Hourly and daily precipitation data are avail-
able from tens of thousands of weather sta-
tions within the United States maintained
by the National Oceanic and Atmospheric
Administration (NOAA) (
Table 2.1
). Analogous
data are available in many other countries.
However, it may be necessary or beneficial to
directly measure precipitation in specific study
areas, especially in regions with few gauges or
where precipitation is dominated by convect-
ive thunderstorms or orographic effects. Most
readers should be familiar with techniques for
measuring precipitation; therefore, these tech-
niques are only described briefly. More details
can be obtained from the World Meteorological
Organization (
1983
).
A number of gauges can be used to measure
precipitation. The most common are the stand-
ard NOAA gauge and the tipping-bucket gauge.
Considerations when selecting a gauge include
accuracy, desired frequency of data collection
(e.g. hourly or daily), operating temperatures
(if snow and freezing conditions are a concern),
and cost of purchase and maintenance. Most
gauges have some type of analog or electronic
recording capability.
The standard NOAA precipitation gauge
consists of a 200 mm diameter cylinder with
t he top open to a l low ra i n a nd snow to acc u mu-
late within the cylinder. The height of water
in the cylinder is measured with a graduated
rod. Advantages include easy calibration, abil-
ity to measure total precipitation under any
intensity, and ability to measure snowfall.
Antifreeze must be added to the collector dur-
ing periods of freezing temperatures. During
periods of warmer temperatures a small
amount of oil is sometimes added to reduce
water loss due to evaporation. Disadvantages
of this type of gauge include limited accuracy
and poor resolution of precipitation intensity
(Strangeways,
2004
). Automatic recording is
possible with an electronic balance, a float,
2.3 Local-scale application
Working at the local scale permits use of the
most accurate methods for measuring individ-
ual components of the water-budget equation.
Discussion in this section centers on point meas-
urement techniques for each of the individual
