Environmental Engineering Reference
In-Depth Information
could translate into large uncertainties in
hydraulic conductivities.
800
700
600
Example: agricultural field, France
As part of an intensive experiment to deter-
mine an optimal fertilizer application scheme
for irrigated maize near Grenoble, France, eight
plots were instrumented with tensiometers and
neutron probe access tubes (Kengni
et al
.,
1994
).
Detailed water budgets were developed for the
period April 1991 to February 1992, with drain-
age calculated by the Darcy method. Soil was a
highly permeable sand. A layer of coarse gravel
at a depth of 1 m restricted plant root growth.
Two of the sites were bare soil; the other six
were planted with maize under different fer-
tilization methods. Each site had tensiometers
at depths of 150, 300, 500, 700, and 900 mm,
and a neutron probe access tube. Tensiometer
readings were taken daily during the grow-
ing season and weekly thereafter. Water con-
tents were measured with a neutron probe at
100 mm intervals to a depth of 900 mm on a
weekly basis. Unsaturated hydraulic conductiv-
ity was determined with a modified instantane-
ous profile method. When a zero-flux plane was
present, the ZFP method was used to determine
downward fluxes. These fluxes were used with
measured water contents and hydraulic gradi-
ents to construct the unsaturated hydraulic
conductivity curve.
Drainage rates for the bare and cropped sites
were similar in the spring (
Fig ure 5.12
), but by
Julian date 180, evapotranspiration from the
crops was much greater than that from bare soil.
As a result, drainage from the cropped sites was
essentially 0 for Julian date 180 to 280, whereas
drainage from the bare soil sites for that period
exceeded 200 mm. After day 280, drainage rates
for all sites were again similar. Average drainage
was 551 mm for the two bare sites for the study
period (which extended beyond the time shown
in
Fig ure 5.12
) or 63% of the total rainfall and
irrigation of 871 mm. For the six cropped sites,
average drainage was 364 mm or 42% of rainfall
and irrigation. The Darcy method appears to be
an appropriate choice for estimating drainage
in this rather unique study area. The fact that
the rooting depth is restricted to the top 1 m by
Rain
500
400
Bare
300
200
Crop
100
0
100
150
200
250
300
350
Julian date, 1991
Figure 5.12
Average cumulative rainfall and drainage
for bare and cropped soil sites near Grenoble, France,
April through December 1991 (reprinted from
Journal of
Hydrology
, v. 162, Kengni
et al
. (
1994
), Figure 5, copyright
(1994), with permission from Elsevier).
the underlying gravel layer limits the number
of depths that must be instrumented. Because of
the permeable nature of the soil and, at times,
high application rate of rainfall and irrigation,
wetting fronts moved rapidly through the soil.
Thus, a high frequency of data collection was
required.
5.5 Lysimetry
Many different types of lysimeters have been
used in hydrologic studies. Perhaps the most
common types are those whose upper boundary
is the land surface; these are containers filled
with soil and placed in a field so as to mimic as
closely as possible surrounding soils and vege-
tation. They are used to study atmosphere, soil,
water, and plant interactions under natural
or artificial conditions. Originally designed to
measure leaching and percolation of solutes
through soil, these lysimeters can provide dir-
ect measurements of evapotranspiration as
well as drainage. Brutsaert (
1982
) traces their
use back to the early 1700s. In the last several
decades lysimeters have been used primarily
to measure evapotranspiration rates, although
they also have been used for estimating rates
of water drainage through the unsaturated
