Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 6

R = rainfall

F = water losses at off-takes between the parent canal and off-take measuring

points

U = the water losses through unmeasured orifices in the canal side (e.g., animal

burrows, unauthorized outlets, other sorts of water abstraction)

E = the evaporation from the reach

Steady flow condition is necessary during the conduct of the test. In a small
irrigation channel, where the terms Q f , F , and U are nil, the above equation takes
the simplified form as

=

Q 1 − Q 2 −

−

S R E (1.3)

Merits
This method can reflect actual operating (dynamic) conditions.

Limitations
(i) Sufficiently long test reaches are not available in some cases, which may
prevent accurate measurement over short stretches of special interest.
(ii) Steady flow condition is necessary.
(iii) Accurate result cannot be obtained where the seepage rate is fairly small.

1.1.4.3 Seepage Meter Method

Various types of seepage meters have been developed. Here, a seepage meter with
submerged flexible water bag is discussed. It is the simplest device in construction
as well as in operation. It consists of a water-tight seepage cup connected by a hose
to a flexible (plastic) water bag floating on the water surface (Fig. 1.2 ) .
During measurement, the seepage meter is set under water. Water flows from the
bag into the cup, where it seeps through the canal subgrade area isolated by the cup.
By keeping the water bag submerged, it will adapt itself to the shrinking volume so

Plastic
bag

Handle

Seepage
cylinder

Fig. 1.2 Schematic view of
measuring seepage by
seepage meter