Background

A schematic of the type of water management system considered is given

in Figure 2-1. The soil is nearly flat and has an impermeable layer at a

relatively shallow depth. Subsurface drainage is provided by drain tubes or

parallel ditches at a distance d, above the impermeable layer and spaced a

distance, L, apart. When rainfall occurs, water infiltrates at the surface

and percolates through the profile raising the water table and increasing

the subsurface drainage rate. If the rainfall rate is greater than the

capacity of the soil to infiltrate, water begins to collect on the surface.

When good surface drainage is provided so that the surface is smooth and on

grade, and outlets are available, most of the surface water will be

available for runoff. However, if surface drainage is poor, a certain

amount of water must be stored in depressions before runoff can begin.

After rainfall ceases, infiltration continues until the water stored in

surface depressions is infiltrated into the soil. Thus, poor surface

drainage effectively lengthens the infiltration event for a given storm

permitting more water to infiltrate and a larger rise in the water table

than would occur if depression storage did not exist.

 

 

 

 

Figure 2-1.  Schematic of water management system with subsurface drains that may be used for drainage or subirrigation.