Environmental Engineering Reference
In-Depth Information
rate must be determined by measuring the
volume of water in the collection vessel. Wick
lysimeters do not require a vacuum pump,
but the applied pressure head is constant and
cannot be adjusted. A trench is required for
installation, and sufficient depth below the
collection surface must be provided for the
hanging wick and collection vessel. Also, the
collection surface must be in good hydraulic
connection with the overlying soil.
Carefully installed and maintained lysime-
ters can provide direct measurements of drain-
age rates with excellent temporal resolution
(Fayer and Gee, 2006 ). Drainage and precipita-
tion rates can be used to infer travel times of
water moving through the unsaturated zone.
Depending on design, lysimeters can capture
both matrix and preferential flow, but their
very presence may disrupt the natural flow
system. Lysimeters with large surface areas (a
few m 2 or more) are prohibitively expensive
for most studies, but they can provide an inte-
grated drainage estimate that may account for
field heterogeneity. Small lysimeters provide
point measurement of fluxes; they are inexpen-
sive to fabricate and operate, and they can pro-
vide useful information for many studies. For
any lysimeter, it is difficult to ensure that con-
ditions inside or above it are similar to those
in the adjacent undisturbed soil. Preferential
flowpaths can be inadvertently created during
lysimeter installation.
Neutron probe access tubes
Depths (m)
0
0.3
Tensio-
meters
0.9
1.5
1.8
Loamy
fine
sand
2.2
2.8
6.0 m
Access
hole
3.9
4.8
Ceramic
cups
5.4
To
vacuum
pump
Collection
flasks
2.4 m
Figure 5.14 Schematic of Las Cruces, New Mexico,
lysimeter (after Gee et al ., 1994 ).
Water content of the lysimeter sediments
increased throughout the profile from 1983
to 1992 ( Fig ure 5.15 ). The increase initially
appeared only at shallow depths. Over time
the wetting front moved slowly downward.
A quasi-steady state appears to have been
reached in the middle portion of the lysimeter
by 1988 indicating that the rate of infiltration
to the lysimeter was being balanced by evapor-
ation and drainage. It is interesting to exam-
ine water travel times through the lysimeter.
For 1989 to 1991, the average drainage rate
was 20 mm/yr ( Table 5.2 ). If the average water
content was 0.16, then the average vertical vel-
ocity should be about 125 mm/yr. By this cal-
culation, however, it would take 48 years for
the wetting front to travel from land surface
to the lysimeter drain. A possible explanation
for this inconsistency is that the net infiltra-
tion (infiltration minus evapotranspiration) for
Example: Las Cruces, New Mexico
An experiment was initiated in 1983 in south-
central New Mexico to determine drainage rates
for unvegetated areas under natural precipita-
tion (Gee et al ., 1994 ). A 2.44-m diameter, 6-m
deep lysimeter was filled with Berino loamy
fine sand packed to a density of 1670 kg/m 3 . The
surface of the lysimeter was kept free of veg-
etation. There were two access tubes for neu-
tron probe measurements of water contents,
and tensiometers were installed at nine depths
( Fig ure 5.14 ). Drainage at the base of the lysim-
eter was collected through porous ceramic cups
attached to a vacuum pump. Water contents
and soil-water pressure heads were measured
monthly.
 
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