Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 356

9.5.3 Determination of Drain Pipe Size

The pipe size should be 25-50% higher than the maximum design discharge, to
compensate possible reduction in the net capacity of pipe due to deposition of silt.
Discharge carrying capacity of the pipe can be calculated using Manning's equation
(which is described in Chapter 1, this volume).

9.6 Design of Subsurface Drainage System

9.6.1 Factors Affecting Spacing and Depth of Subsurface Drain

Factors which influence drain spacing and depth include

• root zone depth of the proposed crop

• sensitivity of the crop to water logging or salinity

• soil texture (coarse or fine)

• salinity level of soil and/or groundwater

• depth of groundwater table

• root zone depth at saline sensitive growth stage

• depth of impervious soil layer

• hydraulic conductivity of the soil (horizontal and vertical direction)

9.6.1.1 Soil Salinity

Salinity distribution data of soil profile should be considered when selecting the
drain depth.

9.6.1.2 Impact of Soil Texture on Drain Depth

Upflow or capillary rise of water through capillary tube (resembled to soil pore) can
be expressed as

2 τ

h cr = cos θ (9.24)

r ρ

where

h cr = capillary rise of water (cm)

r = radius of tube (cm)

density of water (g/cm 3 )

ρ =

τ = surface tension (g/cm)

θ = angle of contact between meniscus and wall of tube (deg.)

1gm/cm 3 , 0.074 g/cm (for water of 20 ◦ C), and Taking density of water, ρ = τ =

neglecting the angle (i.e., θ = 0), we get