Irrigation System Designing - Practices of Irrigation and On-farm Water Management - page 81

They are significant in basin irrigation because they cause local stagnation of water
and irregular advance of the waterfront.
3.3.1.6 Number of Check Bank Outlets
The amount of drainage between basins depends on the number of outlets installed
in the check bank. Drainage of the basin is very important for good water and crop
management. It is a common practice that in a multiple-basin operation, runoff from
the upstream basin drains into the downstream basin. Typically, designers use either
one or two outlets in their basin designs.
3.3.1.7 Elevation Difference (Vertical Interval) Between Adjacent Basins
The vertical interval between adjacent basins is an important parameter in the design
of contour layouts because it affects ponding of water and drainage of excess water
from the upstream basin. The vertical difference in elevation between contour basins
is primarily dictated by the natural land topography. Through land forming prac-
tices, a designer can alter the existing elevation intervals between adjacent basins to
better suit other features of the design including the elevation of the water source,
supply channel, and reuse pond.
3.3.1.8 Irrigation Depth
The duration of irrigation is dependent on the depth to be applied.
3.3.2 Hydraulics in Basin Irrigation System
Overland flow in surface irrigation systems is commonly described using a one-
dimensional analysis, in which the pattern of water flow over and into the soil
surface is assumed to be repeated across the width of the field (Clemmens and
Strelkoff, 1979 ) . This assumption produces good results in cases when the flow
can be considered linear such as in furrow and border irrigation. However, in a
basin irrigation configuration, as well as in contour basin layouts, a one-dimensional
approach is difficult to justify especially if the field geometry is irregular or if water
does not enter the field uniformly along one of its sides. Hence, it is more appropri-
ate to simulate the hydraulic processes in contour layouts using a two-dimensional
flow simulation approach.
There are two main processes involved in flow over porous media. One is
the surface flow and the other is the vertical movement (infiltration) of water
into the soil. A typical advance process in basin irrigation layouts is shown in
Fig. 3.3 The figure shows the waterfront lines during the advance phase in a
basin.