Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 360

Under natural rainfall condition, water balance can be expressed as

P = ET + R + D

or

D = P − ET − R (9.26)

where

=

P rainfall rate (mm/d)

=

ET evapotranspiration rate (mm/d)

R = surface runoff amount, mm/d (if surface runoff is feasible)

D = deep percolation or subsurface drainage amount (mm/d)

The “ D ” value will indicate the drainage coefficient or drainage requirement.
Instead of average rainfall, peak rainfall of 10-20 years recurrence interval may be
used. Procedure for such determination has been described in Chapter 3 ( Weather ),
Vo l um e 1 .

Drainage Requirement for Salinity Control

For salinity control purpose, leaching requirement (LR) should be calculated.
Procedure for calculating LR has been described in Chapter 8 ( Management of
Salt-Affected Soils ), this volume.

9.6.4.4 Criteria and Considerations

The USBR (United States Bureau of reclamation) recommends a minimum water-
table depth from 1.1 to 1.5 m below land surface midway between lateral drains,
depending on the crop rooting depth. From past records, this should result in achiev-
ing at least 90% of maximum crop production (US Department of Interior, 1993 ) .
The midpoint water-table recommendation ensures that the soil oxygen status is
maintained in the root zone and reduces capillary transport of water and salt from
shallow groundwater into the root zone and to the soil surface due to evaporative
demand.
Deep placement of the drains generally results in a wide drain spacing that lowers
the system cost relative to shallow one. However, in many cases deep placement has
been shown to result in an excessive salt load being discharged with the drainage
water (Christen and Skehan, 2001 ; Ayars et al., 1997 ) . This is because the soil is nor-
mally more saline than the irrigation water salinity in most cases (Ali and Rahman,
2009 ; Ayars et al., 1997 ) .