Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 5

where

S is the seepage rate, m 3 /m 2 /day
L is the length of the canal reach (test section), m
W is the average top width of the canal cross-section, m
P is the average wetted perimeter of the canal section, m [Average of the initial
and final perimeter (P i +P f )/2]

d 1 is the initial water depth, m
d 2 is the final water depth, m
t is the duration of ponding, h
The percentage of seepage losses in small canals and farm ditches is normally
greater than in large conveyance canals.

=

Limitations

Major limitations of this method are as follows:

(i) it cannot be used while canals are operating
(ii) it does not reflect the velocities and sediment loads of operating conditions

Merits
(i) the method is simple to understand
(ii) no special equipment is needed to perform the measurement
(iii) does not need too long a channel section as that of inflow-outflow method
(iv) more accurate result can be obtained than the inflow-outflow method, espe-
cially where the seepage rates are fairly small

1.1.4.2 Inflow-Outflow Method

In this method, seepage is determined through measuring the inflow and outflow
of a canal test reach. Flow rate can be measured by current meter or by other flow
measuring structures such as flumes, weirs.
The water balance for the reach of the canal used in an inflow-outflow test, in
the general case where there are off-taking channels that are flowing, is

S = Q 1 − Q 2 − Q f − R − F − U − E (1.2)

Each term of the above equation is a discharge, e.g., m 3 /s,
where

S rate of water loss due to canal seepage

Q 1 =

=

inflow at upstream end of reach

Q 2 = outflow at downstream end of reach

Q f = flow in off-takes which are noted and gauged at their measuring points