FIELD INTAKES
Irrigation water can be supplied from the farm channel into the field through:
a breach in the channel bank
siphons
spiles.
A breach (Figure 68) is a cut in the channel embankment to allow water to flow into the field.
Figure 68 Breaches
After completing the irrigation, the farmer closes the embankment again. This has to be done carefully to prevent leakage. This is the most common method of releasing water from a channel, but it can also be the most damaging. Not only is it difficult to control the discharge, but there can be serious erosion of the channel embankment which is difficult to repair. If other more controllable methods are available then these should be used in preference to this. Breaches can be most easily controlled on clay soils which do not erode easily.
On sandy and loamy soils cutting a breach may cause serious erosion and leakage problems. In this case it is better to use siphons or spiles.
Siphons are small diameter pipes used to convey water over the channel embankment (Figure 69).
Figure 69 Siphons
Spiles are small pipes buried in the ditch bank (Figure 70).
Figure 70 Spiles
For siphons and spiles to work properly, the water level in the farm channel must be higher than in the field. When the water level in the farm channel is much higher than in the field, the outlet from the siphon or spile may be above the water level in the field. This is known as free discharge (Figure 70). When the water level in the farm channel is lower, then the outlet may be below the field water level. This is known as drowned discharge (Figure 69). Both modes of operation are acceptable.
The discharge through siphons and spiles depends on the diameter of the pipe and the head. For free discharge, the head is the difference between the water level in the farm channel and the outlet from the pipe (Figure 71b). For drowned or submerged discharge, the head is the difference between the water level in the farm channel and in the field (Figure 71a). Discharge can be changed by a change in pipe diameter or a change in the head (see Table 6).
Table 6 DISCHARGES FOR SIPHONS AND SPILES (l/s)
|
Diameter |
Head (cm) | |||
|
(cm) |
5 |
10 |
15 |
20 |
|
2 |
0.19 |
0.26 |
0.32 |
0.73 |
|
3 |
0.42 |
0.59 |
0.73 |
0.84 |
|
4 |
0.75 |
1.06 |
1.29 |
1.49 |
|
5 |
1.17 |
1.65 |
2.02 |
2.33 |
MEASURING SIPHON OR SPILE DISCHARGE
The most common method used to measure siphon or spile discharge is the volume method.
Figure 71 Determining the head (a) Submerged discharge
Figure 71 Determining the head (b) Free discharge
Equipment
Bucket of a known volume, a siphon or a spile, a (stop) watch, two people.
Set-up
Just behind the bund of the field channel a hole is made in the soil, in which a bucket is placed (Figure 72).
Figure 72 Measuring the siphon discharge (a) Free discharge
Figure 72 Measuring the siphon discharge (b) Drowned or submerged discharge
If the siphon or spile is freely discharging then the pipe can discharge directly into the bucket (Figure 72a). However, if the siphon discharge is drowned, then the bucket must be held firmly with the bucket lip at the same level as the normal water level in the field (Figure 72b). Water is discharged into the hole alongside the bucket, where the water level rises and overflows into the bucket. This procedure is important in order to measure the discharge under the normal operating head. If the siphon was allowed to discharge directly into the bucket the head would be changing as the bucket fills and this would affect the siphon discharge.
Procedure
The siphon is first filled with water to take out all the air. This is called priming (Figure 73). One end is kept under water and the other end is covered with the hand to prevent air from re-entering (Figure 73). The siphon is placed over the embankment with one end in the channel and the other in the hole beside the bucket.
Figure 73 "Priming" of a siphon
As the water level rises it flows over into the bucket. The time taken to fill the bucket is then recorded.
For free siphon flow the water can be discharged directly from the siphon into the bucket.
The siphon discharge is then calculated as follows:
Example:
A 5-litre bucket is filled by a siphon according to the volume method.The time to fill the bucket is 12 seconds. Hence 5 litres/12 seconds is the discharge of the siphon, i.e. 0.42 l/s.
