Irrigation System Designing - Practices of Irrigation and On-farm Water Management - page 80

3.3.1.1 Flow Rate
Supply channel discharge is an important parameter in the design and manage-
ment of contour basin layouts as it determines the boundary inflow depth, irrigation
uniformity and efficiency, and ultimately it is the key controllable design and man-
agement parameter that determines how fast a basin can be irrigated to the target
depth.
An increase in unit inflow rate can reduce deep percolation losses and improve
application efficiency. The time of advance decreases with the increase in flow
rate. Advance time in contour layouts is a very important factor for crops that do
not require or tolerate ponding or when the target depth of irrigation is relatively
small. The objective in this situation is to irrigate and drain the basin as quickly as
possible.
In summary, it can be said that higher inflows lead to higher efficiencies and
uniformity but should be used judiciously to avoid an excessive application depth.
3.3.1.2 Soil Type
The efficiency and uniformity of basin irrigation depend on the relative magnitude
of the soil infiltration rate and flow rate. A soil with a relatively high infiltration
rate will require a substantially higher flow rate to achieve the same uniformity and
efficiency as for a heavier soil.
3.3.1.3 Basin Longitudinal Slope
By definition, basins are irrigation units graded to zero slope in both directions.
However, it is a common practice among designers to provide some slope in the
longitudinal direction to facilitate advance. But the research results (Khanna et al.,
2003a ) suggests that while some longitudinal slope might aid in achieving better
uniformity and efficiency, the selection of the best slope for a basin requires careful
analysis for each case.
3.3.1.4 Aspect Ratio
Aspect ratio is the ratio of width to length of the basin. With the increase in aspect
ratio, the application efficiency and irrigation uniformity decrease, meaning a reduc-
tion in irrigation performance. The reason is that increase in aspect ratio leads to
greater deep percolation losses as the time of advance also increases. The indicators
are particularly sensitive at low aspect ratio, whereas these two parameters remain
largely unchanged for greater aspect ratios.
3.3.1.5 Local Surface Micro-topography
The local undulations are commonly termed as micro-topography. Local undula-
tions on the basin's surface are important factors affecting advance and recession.