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.
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.
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 |