Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 363

ponding water more than 24 h, determine the subsurface drainage requirement for
survival of the crop.

Solution

Considering water balance on daily basis,

P D

or, subsurface drainage, D

= ET + R +

= P − ET − R

Given,
P = 70 mm

=

ET 6mm

= × =

R 28 mm

Putting the values, D

70 0.4

= 70 − 6 − 28 = 36 mm

That is, daily drainage requirement is 36 mm, or 1.5 mm/h (Ans.)

Example 9.6

In a saline agriculture, estimate the drainage requirement for successful crop
production from the following information:

Crop: Wheat
EC e = EC dw = 8.0 dS/m

EC iw = 1.5 dS/m

ET = 5 mm/d (peak rate)

Solution

D dw
D iw =

EC iw
5EC e −

1.5

LR =

EC iw = 1.5 =

0.038

5 × 8 −

ET 5

D iw =

LR = 0.0389 =

5.202

1 − 1 −

D dw = 5.202 − 5.0 = 0.202 mm/d

Considering irrigation application efficiency, E a = 0.8, that is 20% is lost to deep

percolation; the excess application is

5
0.8 −

= 5 = 1.25 mm/d, which is greater than 0.202mm/d

Thus, the drainage coefficient, or drainage requirement = 1.25 mm/d (Ans.)