Agriculture Reference
In-Depth Information
Box 7.5
The Water Table and Capillary Rise
Soil water above the water table is drawn upward in continuous pores in the
same way that water is drawn up a capillary tube inserted into free water (see fig.
6.1b). This rise of groundwater, called
capillary rise
, depends very much on the
soil's pore-size distribution, and is usually not greater than 1 m in sandy soils but
can be up to 2 m in some silt loams. A water table within 1-2 m of the soil
surface can lead to high soil evaporation losses. As the water evaporates, salts from
the groundwater accumulate at the surface; this encourages capillary rise because
the gradient in
m
gradient (section 6.1.1). This can result in a
significant potential gradient upward even in moist soils, when the
K
value is high
and appreciable water movement can therefore occur. If groundwater is saline, the
problem of soil salinity is exacerbated.
s
adds to the
7.2.4.1
Draining Clay Soils and Subsoils of Low
K
Value
The usual method of draining clay soils is to install continuous perforated PVC
pipe at depths of 1.5-2 m. Older systems have short lengths of porous clay pipes
called
tiles
, usually 75 mm in diameter, laid end to end. Because water enters the
pipe from all sides, the drain's performance is much improved if it is embedded
in gravel or crushed stone (size range 5-50 mm) or in well-structured topsoil.
Such “permeable fill” filters out small soil particles that may otherwise block the
drain and reduces the loss in head caused by the restricted number of entry points
for water. The pipes drain into ditches that must be deep enough to allow ade-
quate outfall from the pipes.
Because there is resistance to water flow through the soil to the drains, the
water table “mounds” to a height
h
between pipes, as shown in figure 7.6. There
is a trade-off between drain spacing
L
and drain depth
d
to ensure that the water
table at the midpoint between drains does not rise above a critical depth (
d
h
).
The drain spacing can be determined by using the approximate formula
L
2
h
K
s
/
J
w
(7.3)
Soil surface
d- h
d
Water table
h
L
Pipe drain
Diagram of pipe drains and the “mounding” of the water table between drains.
Figure 7.6
