4.1 When to Use Border Irrigation
4.2 Border Layout
4.3 Irrigating Borders
4.4 Maintenance of Borders
Borders are usually long, uniformly graded strips of land, separated by earth bunds. In Contrast to basin irrigation these bunds are not to contain the water for ponding but to guide it as it flows down the field (Figures 47 and 48).
Figure 48 Border irrigation, field not properly levelled
Border irrigation is generally best suited to the larger mechanized farms as it is designed to produce long uninterrupted field lengths for ease of machine operations. Borders can be up to 800 m or more in length and 3-30 m wide depending on a variety of factors. It is less suited to small-scale farms involving hand labour or animal-powered cultivation methods.
Suitable slopes: Border slopes should be uniform, with a minimum slope of 0.05% to provide adequate drainage and a maximum slope of 2% to limit problems of soil erosion.
Suitable soils: Deep homogenous loam or clay soils with medium infiltration rates are preferred. Heavy, clay soils can be difficult to irrigate with border irrigation because of the time needed to infiltrate sufficient water into the soil. Basin irrigation is preferable in such circumstances.
Suitable crops: Close growing crops such as pasture or alfalfa are preferred.
The dimensions and shape of borders are influenced in much the same way as basins and furrows by the soil type, stream size, slope, irrigation depth and other factors such as farming practices and field or farm size.
Many of the comments made about basins and furrows are generally applicable to borders also and so do not require repetition here. Table 4 provides a guideline to determine maximum border dimensions. It must, however, be stressed that this table is for general guidance only as the values are based on field experience and not on any scientific relationships.
Table 4 SUGGESTED MAXIMUM BORDER LENGTHS AND WIDTHS
|
Soil type |
Border Slope (%) |
Unit flow per metre width (l/sec) |
Border Width (m) |
Border Length (m) |
|
SAND |
0.2-0.4 |
10-15 |
12-30 |
60-90 |
|
Infiltration rate greater than 25 mm/h |
0.4-0.6 |
8-10 |
9-12 |
60-90 |
|
0.6-1.0 |
5-8 |
6-9 |
75 |
|
|
LOAM |
0.2-0.4 |
5-7 |
12-30 |
90-250 |
|
Infiltration rate of 10 to 25 mm/h |
0.4-0.6 |
4-6 |
6-12 |
90-180 |
|
0.6-1.0 |
2-4 |
6 |
90 |
|
|
CLAY |
0.2-0.4 |
3-4 |
12-30 |
180-300 |
|
Infiltration rate less than 10 mm/h |
0.4-0.6 |
2-3 |
6-12 |
90-180 |
|
0.6-1.0 |
1-2 |
6 |
90 |
Note: The flow is given per metre width of the border. Thus the total flow into a border is equal to the unit flow multiplied by border width (in metres).
Borders are irrigated by diverting a stream of water from the channel to the upper end of the border. The water flows down the slope. When the desired amount of water has been delivered to the border, the stream is turned off. This may occur before the water has reached the end of the border. There are no specific rules controlling this decision. However, if the flow is stopped too soon there may not be enough water in the border to complete the irrigation at the far end. If it is left running for too long, then water may run off the end of the border and be lost in the drainage system.
As a guideline, the inflow to the border can be stopped as follows:
- On clay soils, the inflow is stopped when the irrigation water covers 60% of the border. If, for example, the border is 100 m long a stick is placed 60 m from the farm channel. When the water front reaches the stick, the inflow is stopped.- On loamy soils it is stopped when 70 to 80% of the border is covered with water.
- On sandy soils the irrigation water must cover the entire border before the flow is stopped.
However, these are only guidelines. Realistic rules can only be established locally when testing the system.
As with the other irrigation methods it is important to ensure that adequate irrigation water is supplied to the borders so that it fills the root zone uniformly. However, there are many common problems which result in poor water distribution. These include:
- poor land grading;
- wrong stream size;
- stopping the inflow at the wrong time.
i. Poor land grading
If the land is not graded properly and there is a cross-slope, the irrigation water will not spread evenly over the field. It will flow down the slope always seeking the lowest side of the border (Figure 49). This can be corrected by regrading the border to eliminate the cross-slope or by constructing guide bunds in the border to prevent the cross flow of water.
Figure 49 Effect of a cross-slope on the water movement in a border
ii. Wrong stream size
A stream size which is too small will result in deep percolation losses near the field channel (Figure 50), especially on sandy soils.
Figure 50. Stream size too small
If the stream size is too large the water will flow too quickly down the border and the point where the flow should be stopped is reached before sufficient water has been applied to fill the root zone (Figure 51). In this situation the flow will need to be left running until the root zone has been adequately filled and this results in considerable losses from surface runoff. Large stream sizes may also cause soil erosion.
Figure 51 Stream size too large
iii. Inflow stopped at the wrong time
If the inflow is stopped too soon, the water may not even reach the end of the border. In contrast, if the flow is left running too long, water will run off the border at the downstream end and be lost in the drainage system.
Maintenance of borders consists of keeping the border free from weeds and uniformly sloping. Whatever damage occurs to the bunds must be repaired and the field channel and drains are to be weeded regularly. By checking frequently and carrying out immediate repairs where necessary, further damage is prevented.
