Irrigation System Designing - Practices of Irrigation and On-farm Water Management - page 69

former is taken up to equal or is larger than the advance time. Cutoff, however,
has an influence on recession. The most important effect of cutoff is reflected on
the amount of losses, deep percolation and surface runoff, and hence efficiency
as well as adequacy of irrigation.
Advance ratio : Advance ratio quantifies the relative proportion of advance time to
that of cutoff time.
Cutback ratio : The cutback ratio represents the ratio of post-advance flow rate to
advance flow rate.
Tailwater recovery ratio : Tailwater recovery ratio ( R TWR ) represents the ratio of the
volume of runoff that can be recovered for use in subsequent sets to that of the
total volume of surface runoff resulting from a single application, that is,

runoff · recovered

R TWR =

total · runoff

Return flow : Water that reaches a surface water source after release from the point
of use, and becomes available for use again.
Irrigation efficiency : It is the ratio of average depth of irrigation water beneficially
used to the average depth applied, and normally expressed as percentage.
Application efficiency : It is the ratio of the average depth of irrigation water stored
in the crop root zone to the average depth applied, expressed as percentage.
Farm irrigation efficiency : It is the percentage of the water applied at the farm inlet
which is stored in the root zone for crop use.
Distribution uniformity : It is a measure of evenness (or unevenness) of application
and has a significant effect on application efficiency.
Recession flow : After the inflow stream is cut off, the tail water recedes from the
plot downward. This flow of water is termed as recession flow.

3.2.2 General Overview and Considerations

Borders generally are prepared with zero side slope and a small but uniform longi-
tudinal slope not exceeding 1%. The borders are divided by levees running down the
slope at uniform spacing. The lower end of the border is opened to a drainage ditch
or closed with a levee to create ponding on the end of the border. Levees are pulled
across the end on steeper borders. Border irrigation is best adapted to grain and
forage crops where there are large areas of flat topography, and water supplies are
large. The major investment is land preparation, and border irrigation is relatively
inexpensive to operate after installation.
The main design considerations for border irrigation include flow rate, width,
length, slope, and outlet conditions. Water requirements range from 15 to 45 gal
per min per foot of width depending on length, slope, and soil type of the border.
Sometimes crops are planted across borders or planted parallel to the border or
parallel to the border with shallow furrows or corrugations to help guide the water
down the border, especially if there is a small side slope.