Irrigation System Designing - Practices of Irrigation and On-farm Water Management - page 71

3.2.5 Design Approaches and Procedures for Border

3.2.5.1 Approaches

Design parameters of border irrigation system can be determined using either of the
following two approaches:

(a) Design discharge ( Q ) for a predetermined border size and slope ( L , W , S )
(b) Design L , W , S for a given Q

In designing surface irrigation system, several difficulties are encountered. This
is because the output requirements such as application efficiency, storage efficiency,
and distribution uniformity (DU) have interaction with the input parameters. The
storage efficiency is expected to be above 95% and DU is above 90%.

3.2.5.2 Empirical Models for Designing Border Irrigation System

SCS Method

To ensure adequate spread of water over the entire border, a minimum allow-
able inflow rate q min must be used. The following equation was proposed by SCS
(USDA, 1974 ) to estimate q min :

S 0.5
0

L ×

10 − 6 )

q min = × ×

(5.95 (3.1)

n

where

discharge per unit width, m 3 /s/m

q min =

L border length, m

S 0 =

=

border slope, m/m

n = roughness coefficient (0.15-0.25, the higher the rougher, the higher the n

value)

When the soil erodibility causes restrictions on q , the maximum allowable inflow
rate q max can be obtained using the empirical method proposed by SCS (USDA,
1974 ) , where q max is expressed as a function of field slope S 0 and type of crop, sod,
and nonsod, by

CS − 0.75
0

q max = (3.2)

where

q max is in m 3 /s/m
S 0 =

field slope in m/m

10 -4 for sod and 1.7 10 -4 for nonsod. C = empirical coefficient equal to 3.5 × ×

When the dike height causes the restrictions on q , the maximum allowable inflow
rate can be obtained using Manning's equation: