Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 354

Van Beer Equation

Van Beers equation for equivalent depth is (ILRI, 1973):

D S

d e = (9.21)

8 D
π

8 D S
π

1 +

S ×

2 r

where, r is radius of drain pipe (m), D is drain depth (m), D s is thickness of the
aquifer below drain level.
Moustafa (1997) suggested from field investigation that a 5m depth instead of
infinity for the impermeable layer in Nile Delta should be used in design purpose.

Important Parameters in Hooghout Drain Spacing Equation (and also in Others)
and Their Interpretation

Form the equations of (9.15), (9.16) and (9.17), it is revealed that drain spacing is
directly related to the hydraulic conductivity of the soil (K), and inversely related to
the drainage discharge/outflow (q) from the field. So, these two parameters (input
values) should be determined/estimated accurately.

K of Soil

If the estimated K value is less than the actual one, calculated drain spacing will be
lower, and hence more financial involvement compared to actual need (i.e. financial
loss). On the other hand, if the estimated K value is higher than the actual field value,
the drain spacing will be higher than the actual need. Thus, there is a possibility of
prolonged standing water in the root zone, and hence chance of crop loss.

q Value

The reverse will be true in case of q value.

9.5.1.2 Donnan's Formula

Donnan proposed the following formula for parallel drain spacing:

( D

D 2

4 k
q

S 2 h ) 2 = + − (9.22)

Solving for algebraic functions, the above equation reduces to

4 kh 2
q

8 kDh
q

S 2 = +

which is similar to Hooghoudt's equation.