Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 362

quality, and the water table position. Field research and model studies suggest that
shallower placement of drainage laterals and reduced depth to mid-point water table
will result in reductions in drainage volumes and salt loads.
9.6.5.2 Interceptor Drain
Where it is not practical to drain water out of a pocket point by other means (or
if constraints exists), a collector system should be provided to drain water from
the drainage layer. Collector systems may include plastic pipe slotted at the edge, or
drain pipe installed in a longitudinal collector trench. This will limit the longitudinal
seepage distance in the drainage layer, minimizing the drainage time and preventing
the buildup of a hydrostatic head under the surface layer (Fig. 9.17 ) .
Fig. 9.17 Schematic
representation of interceptor
drain
An interceptor drain may be defined as the drain that is constructed for the pur-
pose of intercepting or cutting off groundwater which is moving down-slope. Its
source may be of different origin. Interceptor drain may be either open or close
construction.
Donnan (1959) raised several issues regarding interceptor drain: (1) How deep
should the interceptor drain be placed? (2) How far upslope will the drawdown
be effective? (3) Where does the post-installation water table become asymptotic
coincide with the undisturbed (original) water table? (4) What is the shape of the
drawdown curve on the down slope side? (5) How much of the total flow is inter-
cepted? (6) What is the required capacity of the drain? (7) Should the drain be open
or closed? He concluded in the way: drain should be placed as deep as it is practical
(open ditch at the bottom of impervious layer), drawdown upslope will depend on
the initial slope of the water table, down slope will be dependent on the water level
in the drain device, the quantity of flow will vary directly with the depth of flow
intercepted, and both open and tile drain are equally efficient.
9.6.6 Sample Workout Problems
Example 9.5
In an agricultural command area, the long-term average of daily maximum rainfall
and evapotranspiration are 70 and 6 mm, respectively. The surface runoff on catch-
ment basis can be considered as 40% of the rainfall. If the proposed crop does allow