Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 340

soil, runoff component is greatly influenced due to change in air/water filled pores.
After draining the water, the soil has more pore volume available for water infiltra-
tion during the next rain because of the larger volume of empty pores. Consequently,
more infiltration and less runoff may occur with an artificially drained soil compared
to a poorly drained (or with no artificial drained) soil.
The amount of infiltration in drained soil depends on many factors such as the
nature and timing of the next rain, soil texture (pore space and its distribution),
hydraulic conductivity, and depth of drain and spacing. The amount will be greater
when the difference between the shallow (initial) and deep water level (final) is
greater, and soil texture is coarser. Smaller rains of low intensity will reduce the
total runoff rate, because proportionally more water will have an opportunity to infil-
trate and pass through the drainage system. Smaller rains may cause surface runoff
on the undrained soil and no surface runoff at all on the drained soil. However,
if one or more rains occur before the drained soil has had time to drain the pre-
vious water adequately, water balance differences between the two soils will be
lessened.

9.2.4 Sample Workout Problem

Examples 9.1

An agricultural soil contains 47% pore space, and the moisture content after gravity
drainage is 39% (by volume). Find the void ratio, drainable porosity, and drainable
water volume from a 20 m

× 15 m plot having 1.0 m root zone depth.

Solution

We know, void ratio = (volume of void/volume of solid)
= vol. of void

/ −

(100 vol. of void)

=47 / (100 − 47)

= 0.886

Drainable porosity = total porosity − water content after gravity drainage

=47 − 39%

=8%

Drainable water volume = drainable porosity × drainable soil volume

1m 3 ) =(8 / 100) × (20 × 15 ×

=24m 3

Ans.: Void ratio = 0.886, drainable porosity = 8%, drainable water volume =

24 m 3 .