Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 2

The conveyance efficiency in irrigation projects is poor due to seepage, percola-
tion, cracking, and damaging of the earth channel. Seepage loss in irrigation water
conveyance system is very significant, as it forms the major portion of the water
loss in the irrigation system. Irrigation conveyance losses controlled through lin-
ing may reduce the drainage requirement and also enhance irrigation efficiency. As
such, reliable estimates of quantities and extent of seepage losses from canals under
pre- and post-lining conditions become important. Various methods are used to esti-
mate the canal seepage rate. The loss in conveyance is unavoidable unless the canal
is lined. Lining may be done with a large variety of materials. Selection of a suit-
able one depends mainly on cost, performance, durability, and availability of lining
materials.
Irrigation efficiency is greatly dependent on the type and design of water con-
veyance and distribution systems. Designing of economic cross-sections of various
types of irrigation channels is important to minimize cost, water loss, and land
requirement. This chapter illustrates these issues with sample design problems.
1.1 Water Conveyance Loss
1.1.1 Definition of Seepage
Seepage may be defined as the infiltration downward and lateral movements of water
into soil or substrata from a source of supply such as reservoir or irrigation channel.
Such water may reappear at the surface as wet spots or seeps or may percolate to
join the groundwater, or may join the subsurface flow to springs or streams.
1.1.2 Factors Affecting Seepage
Many factors are known to have a definite effect on seepage rate. The major
factors are
(i) the characteristics of the soil or strata through which the channels are laid
(e.g., texture, bulk density, porosity, permeability)
(ii) bulk density, porosity, and permeability of the side soil
(iii) top width and wetted perimeter of the channel
(iv) depth of water in the channel
(v) amount of sediment in the water
(vi) viscosity or salinity of canal water
(vii) aquatic plants
(viii) velocity of water in the channel
(ix) pump discharge
(x) length of time the channel has been in operation (canal age)
(xi) nature of channel like dug or raised (topography)
(xii) channel geometry
(xiii) presence of cracks or holes or piping through the subgrades of the section