QUARTER TIME RULE
In surface irrigation water is supplied to the field from the supply channel. From the side of the supply channel the water flows to the opposite side of the field; this is called the advance of the water front (Figure 77).
Figure 77 Advance of the water front
When the water supply is stopped, the water on the field gradually infiltrates into the soil and moves away from the field channel; this is called the recession of the water front (Figure 78).
Figure 78 Recession of the water front
Ideally the advance of the water front should be the same as the recession; this would result in a uniform infiltration of water over the entire field. Usually, however, the advance and recession are not the same: the advance is often slower than the recession. The result is that the side of the field near the supply channel receives more water than the opposite side of the field. This is especially true if the water supply to the field is too small.
When for example on a sandy soil a small stream size is applied to a large field, it will take a long time before the water reaches the far end of the field; the water infiltrates rapidly into the sandy soil. The side of the field near the supply channel receives too much water and the opposite side of the field receives too little water. This is shown in Figure 79.
Figure 79 Water distribution with too small stream size
When the stream size is increased, the distribution of the water will improve. The water, of course, infiltrates at the same rate, but the water front will reach the opposite side of the field sooner. So also this side will receive a fair share of the water, albeit always less than the side near the supply channel (see Figure 80).
Figure 80 Water distribution with adequate stream size
In order to choose an appropriate stream size, the following "rule of thumb" called quarter time rule is used. The quarter time rule says that the stream size should be large enough for the water to reach the end of the field (furrow irrigation) or for the water to cover the entire field (basin irrigation) in a quarter of the time needed to fill the root zone with sufficient water (the contact time). The contact time is the time needed to infiltrate the required amount of water. The contact time can be determined from the infiltration curve, as explained in Annex 2.
Example:In this example the infiltration curve of Annex 2 is used (Figure 76). Suppose it has been determined that 70 mm of water has to be supplied to a basin. From Figure 76 it can be observed that to Infiltrate 70 mm would take approximately 74 minutes. This means that when applying the quarter time rule, the basin must be covered with water in 74/4 - 18 to 19 minutes. So the stream size must be chosen in such a way that indeed the field is covered with water within some 18 or 19 minutes. If it takes longer, the distribution of water in the root zone is poor. If it is for some reason not possible to Increase the stream size and it takes longer than 18 or 19 minutes to cover the field, then it will be necessary to reduce the size of the basin such that it is possible to cover the field within 18 or 19 minutes.
IRRIGATION TIME
The irrigation time (in minutes or hours) is the time needed to supply the required irrigation depth (in mm). The irrigation time depends on: the stream size (l/sec), the required irrigation depth (mm) and the size of the field to be Irrigated (ha). The following formula is used to determine the irrigation time:
Example:
If for example the required irrigation depth is 50 mm, the available stream size is 20 l/sec and the size of the field is 75 x 50 m, the irrigation time is calculated as follows:
|
Step 1: |
Determine the field size in hectares. |
|
|
The size is 75 m x 50 m = 3 750 m2 = 3750/10 000 = 0.375 ha |
|
Step 2: |
Determine the irrigation time |
|
|
|
|
|
|
|
|
Irrigation time (hours) = 2.6 hours = 156 minutes |
Applying the quarter time rule it would mean that the water has to reach the end of the furrow or cover the basin in 156/4 39 minutes. If it takes longer the stream size per furrow or basin has to be increased or the furrow length or basin size reduced.
