Environmental Engineering Reference
In-Depth Information
or a pressure transducer connected to an elec-
tronic recorder.
Tipping-bucket rain gauges are often used
in hydrologic studies. This gauge contains two
small buckets that fill with water from a col-
lector. When one bucket fills, it drops, trig-
gering a switch, emptying that bucket, and
exposing the other bucket. Advantages of tip-
ping-bucket gauges are their ability to measure
precipitation intensity and small quantities
(0.25 mm or less). Disadvantages include a limi-
tation on the maximum rate of precipitation
that can be measured (this depends on the size
of the gauge, but is usually about 250 mm/hr),
the need for calibration, and the necessity of
heating the gauge to collect data in subfreezing
temperatures (Molini
et al
.,
2005
).
Precipitation gauges should be placed at loca-
tions that are representative of the field site and
mounted so that the collectors are horizontal
and not shielded by vegetation, fences, or other
obstructions that could intercept precipitation.
Ideally, the collector would be at a height equal
to that of the vegetative canopy in the immedi-
ate surroundings. Wind shields can help miti-
gate the distortion of measurements caused by
high winds if the gauge must be located above
the canopy.
Snow pillows, or lysimeters, that can record
small changes in weight (Johnson and Schaefer,
2002
) are gradually replacing older snow boards
and snow stakes for monitoring snow depth.
Proper location and maintenance of snowfall
gauges are important because wind can redis-
tribute snow after it has fallen. In some parts
of the world, dew or condensation is an import-
ant form of precipitation; it can be measured
with a screen fog collector (Schemenauer and
Cereceda,
1994
).
relatively flat and contain uniform vegetation
(in terms of density, plant type, and height) for a
distance at least 100 times instrument height in
the predominant wind direction. Instruments
mounted at a height of 2 m above the vege-
tative canopy would provide an estimate of
evapotranspiration that was integrated over the
horizontal surface extending approximately
200 m upwind. Instrumentation is expensive
and can require frequent maintenance. The
most popular of the micrometeorological meth-
ods are the energy-balance Bowen ratio (EBBR)
and the eddy correlation methods.
The EBBR method is based on an energy-
balance equation written for a unit area of land
surface (
Figure 2.3
):
R
=++
H
G
λ
ET
(2.15)
n
where
R
n
is net radiation,
H
is sensible-heat flux,
G
is soil heat flux, λ is latent heat of vaporiza-
tion of water or the amount of energy required
to evaporate a unit mass of water, and the prod-
uct λ
ET
is referred to as the latent-heat flux and
represents the energy required to evaporate
water at the specified
ET
rate; units are usually
R
n
Net
radiation
λ
ET
Latent
heat flux
H
Sensible
heat flux
2.3.2 Evapotranspiration
Evapotranspiration can be measured or esti-
mated at the local scale by one or more of
the
micrometeorological
methods described by
Rosenberg
et al
. (
1983
). These methods can pro-
vide accurate estimates of evapotranspiration
over small areas. Requirements for proper
application of the methods can be onerous,
however. The measurement site should be
Surface Energy Budget
R
n
− G = H +
λ
ET
G
Surface
heat flux
Figure 2.3
Energy budget for the Earth's surface.
