Environmental Engineering Reference
In-Depth Information
Table 5.2. Precipitation, change in water storage, and drainage for the Las Cruces, New Mexico, lysimeter
(after Gee
et al
.,
1994
).
Year
Precipitation (P), mm
Change in storage, mm
Drainage (D), mm
D/P, %
1984
385
172
0
0
1985
337
150
0
0
1986
320
145
0
0
1987
323
47
0
0
1988
344
6
0
0
1989
363
55
12
3.3
1990
250
-65
17
6.8
1991
278
124
30
10.8
lysimeter at the Fleam Dyke pumping station
near Fulbourn, Cambridgeshire, in eastern
England. The lysimeter encompasses a 5 m
3
block of undisturbed chalk. The chalk is highly
fractured due to jointing and faulting; typical
spacing between fractures is in the order of
0.2 m. The water table was at a depth of about
18 m. The lysimeter was constructed by initially
driving steel piling for the four walls, then
excavating to a depth of 6 m along the outside
of the walls to allow driving of additional steel
pilings that served as the base of the lysimeter.
The base was installed as two panels driven in
from opposite sides at an angle of 10°. Finally,
a tunnel was dug beneath the centerline of the
base, and a sloping trough was installed to col-
lect freely draining water and transport it to a
measuring tank. During construction in 1977,
the surface was not disturbed and retained its
original grass cover. After construction, the
excavations were backfilled, and grass was
replanted to provide a uniform cover over the
entire area.
Most drainage occurred between the months
of January and May (
Fig ure 5.16
). Winter and
early spring rainfall appears to be responsi-
ble for most of the drainage. Annual totals for
drainage were 201 mm for 1978 and 197 mm
for 1979. Average annual rainfall for these years
was 642 mm; the long-term average for the site
is 543 mm. Over the 2-year period, drainage
accounted for 31% of total rainfall.
The large size of this lysimeter is well suited
for measuring drainage through the chalk
0
1983
1
1985
2
1986
3
1988
4
1992
5
6
0
0.04
0.08
0.12
0.16
0.20
0.24
Water content (m
3
/m
3
)
Figure 5.15
Plot of water content with depth within the
Las Cruces, New Mexico, lysimeter for Aug. 1983, Sep. 1985,
Nov. 1986, Feb. 1988, and Jul. 1992 (after Gee
et al
.,
1994
).
the early years of the experiment was greater
than 20 mm/yr and that the low precipitation
in 1990 and 1991 resulted in a reduction in the
rate of water movement through the lysimeter.
In a separate study, Wierenga
et al
. (
1987
) used
neutron-probe measurements along a 2.7 km
transect near the lysimeter to examine drain-
age under vegetated conditions. Those measure-
ments indicated that in the presence of native
plants (mostly creosote bushes) there was neg-
ligible deep drainage.
Example: Chalk aquifer, Fleam Dyke,
England
Kitching and Shearer (
1982
) reported on the
construction and operation of a large-scale
