CHAPTER 2: THE INFLUENCE OF WATER SHORTAGES ON YIELDS

2.1 Which Crops are Sensitive to Water Shortages
2.2 Which Growth Stages are Sensitive to Water Shortages

In irrigation schemes, crops, ideally, do not suffer from water shortages: irrigation water is applied before the crops are under drought stress.

However, it may not be possible to apply the irrigation water exactly when it would be best; for example, in a dry year the river may not have enough water to irrigate all the fields on time; the farmers may be badly organized and lose too much water at the upstream end of the scheme, thus causing problems downstream; the scheme management may decide to spread the available water over a large area, thus allowing more farmers to irrigate, although less than the optimal amount.

In such cases of unexpected - or sometimes even planned - water shortages, it is good to know:

· the crops which suffer most from water shortages; i.e. crops that will have severe yield reductions when the water is in short supply;

· growth stages during which the various crops suffer most from water shortages.

Of course, also other factors, such as, for example, the economic value of the crops, may influence the decision on how best to divide the scarce water. Here only water-related factors will be discussed.

2.1 Which Crops are Sensitive to Water Shortages

In general, crops grown for their fresh leaves or fruits are more sensitive to water shortages than those grown for their dry seeds or fruits. Table 1 shows four categories of crops; the categories are based on the sensitivity of the specific crops to drought.

Table 1. SENSITIVITY OF VARIOUS FIELD CROPS TO WATER SHORTAGES

Sensitivity	Low	Low-Medium	Medium-High	High
Crops	cassava	alfalfa	beans	banana
	cotton	citrus	cabbage	fresh green
	millet	grape	maize	vegetables
	pigeon pea	groundnuts	onion	paddy rice
	sorghum	soybean	peas	potato
		sugarbeet	pepper	sugarcane
		sunflower	tomato
		wheat	(water) melon

As can be seen from the above table, crops like paddy rice, banana, potato and sugarcane are very sensitive to water shortages. This means that if they suffer - even little - water shortages, their yields will be reduced considerably; such water shortages must be avoided.

Crops like millet and sorghum, on the other hand, are only slightly sensitive to drought; they are drought resistant. If the water shortage does not last too long, the effect on the yield will be minimal.

If various crops are grown on an irrigation scheme, e.g. groundnuts and lettuce (fresh green vegetables), and water is short in supply, it is advisable to give priority to irrigating the most drought sensitive crop; in this case lettuce.

2.2 Which Growth Stages are Sensitive to Water Shortages

The total growing season of an annual crop can be divided into four growth stages (see also Volume 3):

· the initial stage; from sowing to 10% ground cover
· the crop development stage; from 10% to 70% ground cover
· the mid-season stage; including flowering and grain setting or yield formation
· the late season stage; including ripening and harvest.

In general it can be stated that of the four growth stages, the mid-season stage is most sensitive to water shortages. This is mainly because it is the period of the highest crop water needs. If water shortages occur during the mid-season stage, the negative effect on the yield will be pronounced.

The least sensitive to water shortages is the late season stage. This stage includes ripening and harvest. Water shortages in this stage have - especially if the crop is harvested dry - only a slight effect on the yield. Care should, however, be taken even during this stage with crops which are harvested fresh, such as lettuce. Fresh harvested crops are also sensitive to water shortages during the late season stage.

The initial and crop development stages are between the mid-season and late season stages with respect to sensitivity to water shortages. Some crops react favourably to water shortage during the crop development stage: they react by developing a deeper root system, which is helpful during the later stages. Table 2 indicates the growth stages most sensitive to water shortages for various important field crops.

On an irrigation project, if only one crop is grown, but not all fields have been planted at the same time (staggered planting), and water is in short supply, it is advisable to give priority to irrigating those fields on which the crop has reached the mid-season stage (flowering and yield formation).

Figure 6. Irrigation water should not be withheld during those stages sensitive to water shortages

Table 2. PERIODS SENSITIVE TO WATER SHORTAGES

Crop	Sensitive period
Alfalfa	just after cutting
Alfalfa (for seed prod.)	flowering
Banana	throughout
Bean	flowering and pod filling
Cabbage	head enlargement and ripening
Citrus	flowering and fruit setting more than fruit enlargement
Cotton	flowering and boll formation
Grape	vegetative period and flowering more than fruit filling
Groundnut	flowering and pod setting
Maize	flowering and grain filling
Olive	just prior to flowering and yield formation
Onion	bulb enlargement
Onion (for seed prod.)	flowering
Pea/fresh	flowering and yield formation
Pea/dry	ripening
Pepper	throughout
Pineapple	vegetative period
Potato	stolonization and tuber initiation
Rice	head development and flowering
Sorghum	flowering and yield formation
Soybean	flowering and yield formation
Sugarbeet	first month after emergence
Sugarcane	vegetative period (tillering and stem elongation)
Sunflower	flowering more than yield formation
Tobacco	period of rapid growth
Tomato	flowering more than yield formation
Watermelon	flowering and fruit filling
Wheat	flowering more than yield formation

Figure 7. Potatoes are highly sensitive to water shortages, in particular during stolonization and tuber initiation