Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 361

9.6.5 Controlled Drainage System and Interceptor Drain

9.6.5.1 Controlled Drainage

Introduction

In the past, subsurface drainage systems were typically designed to discharge water
continuously, without regard to the environmental consequences and the effects on
crop production. This philosophy has changed in humid areas of the world, as the
environmental consequences and crop production impacts have been researched.
Recently, controlled drainage has been identified as a potential management method
in humid areas to reduce nitrate loading to surface water (Ayars et al., 2006 ;
Hornbuckle et al., 2005; Lalonde et al., 1996; Doty and Parsons 1979).

Principle of the Method

In this drainage system, the water table is maintained at a shallower depth by a
control structure which reduces deep percolation below the root zone by reducing
hydraulic gradients and increases potential capillary upflow, as evapotranspiration
depletes soil-water in the root zone (Fig. 9.16 ) .

Fig. 9.16 Schematic of
control drainage system

Root zone

Capillary
rise

The flow lines are shallower than in the uncontrolled system and are more con-
centrated when close to the soil surface. In soil profiles with zones of lower soil
salinity at the soil surface, this system results in decreased drain water salinity com-
pared to the uncontrolled system. The reduced drain flows and lower salinity result
in much reduced salt loads.

Management of Control Drainage System

Traditional drainage does not need much management aspects, but simply letting
the system run continuously. In control drainage, active management measures are
needed to regulate flow and reduce the impact of saline drainage water on the envi-
ronment. The goal may be to reduce total drainage flow, reduce contaminant load,
improve irrigation efficiency, or some combination of these outcomes. The effec-
tiveness of drainage system management will depend on the crop, the ground water