This water can be supplied to the crops by rainfall (also called precipitation), by irrigation or by a combination of rainfall and irrigation. If the rainfall is sufficient to cover the water needs of the crops, irrigation is not required. If there is no rainfall, all the water that the crops need has to be supplied by irrigation.

If there is some rainfall, but not enough to cover the water needs of the crops, irrigation water has to supplement the rain water in such a way that the rain water and the irrigation water together cover the water needs of the crop. This is often called supplemental irrigation: the irrigation water supplements or adds to the rain water.

As has already been explained in Volume 1, section 4.1.4, not all rain water which falls on the soil surface can indeed by used by the plants.

Part of the rain water percolates below the root zone of the plants and part of the rain water flows away over the soil surface as run-off (Fig. 9). This deep percolation water and run-off water cannot be used by the plants. In other words, part of the rainfall is not effective. The remaining part is stored in the root zone and can be used by the plants. This remaining part is the so-called effective rainfall. The factors which influence which part is effective and which part is not effective include the climate, the soil texture, the soil structure and the depth of the root zone. These factors have been discussed in some detail in Volume 1, section 4.1.4.

Fig. 9 Part of the rain water is lost through deep percolation and run-off

If the rainfall is high, a relatively large part of the water is lost through deep percolation and run-off.

Deep percolation: If the soil is still wet when the next rain occurs, the soil will simply not be able to store more water, and the rain water will thus percolate below the root zone and eventually reach the groundwater. Heavy rainfall may cause the groundwater table to rise temporarily.

Run-off: Especially in sloping areas, heavy rainfall will result in a large percentage of the rainwater being lost by surface run-off.

Another factor which needs to be taken into account when estimating the effective rainfall is the variation of the rainfall over the years. Especially in low rainfall climates, the little rain that falls is often unreliable; one year may be relatively dry and another year may be relatively wet.

In many countries, formulae have been developed locally to determine the effective precipitation. Such formulae take into account factors like rainfall reliability, topography, prevailing soil type etc. If such formulae or other local data are available, they should be used.

If such data are not available. Table 6 could be used to obtain a rough estimate of the effective rainfall.

Table 6 RAINFALL OR PRECIPITATION (P) AND EFFECTIVE RAINFALL OR EFFECTIVE PRECIPITATION (Pe) in mm/month

P (mm/month)	Pe (mm/month)	P (mm/month)	Pe (mm/month)
0	0	130	79
10	0	140	87
20	2	150	95
30	8	160	103
40	14	170	111
50	20	180	119
60	26	190	127
70	32	200	135
80	39	210	143
90	47	220	151
100	55	230	159
110	63	240	167
120	71	250	175

EXAMPLE

Estimate the effective rainfall in mm/month if the rainfall is 60 mm/month. From Table 6 it can be seen that the effective rainfall is 26 mm/month. This means that out of 60 mm/month, some 26 mm can be used by the plants; and it is estimated that the remaining (60 – 26 =) 34 mm is lost through deep percolation and run-off.

QUESTION

Determine the effective rainfall for the following monthly rainfall figures: 65, 210, 175 and 5 mm.

ANSWER (see Table 6)

P (mm/month)	Pe (mm/month)
65	29
210	143
175	115
5	0