The major climatic factor which influences crop growth is rainfall. In areas with little rainfall the natural vegetation is limited, while in areas with high annual rainfall dense tropical rain forests are found.
Based on the annual rainfall, a distinction can be made between 6 major climatic zones as is shown in Table 1. Figure 2 indicates, as an example, the major climatic zones of Africa.
Table 1 MAJOR CLIMATIC ZONES
|
Climatic Zone |
Annual Rainfall (mm) |
Wet period (months) |
Vegetation |
|
Desert |
less than 100 |
0-1 |
Little or no vegetation |
|
Arid |
100-400 |
1-3 |
Some scrubs, some grassland |
|
Semi-arid |
400-600 |
3-4 |
Scrubs & bushes, grassland |
|
Sub - humid |
600-1200 |
4-6 |
Bushes to woodland, grassland |
|
Moist sub-humid |
1200-1500 |
6-9 |
Forest and woodland |
|
Humid |
more than 1500 |
9-12 |
Tropical rain forest |
The wet period (see Table 1) refers to the period during which the rainfall is higher than the evapotranspiration. Evapotranspiration is the sum of the evaporation from the soil surface and the transpiration by the plants (see Volume 1, Chapter 4).
Fig. 2 Major climatic zones of Africa
Desert and arid areas
In desert and arid areas irrigation is absolutely essential for crop growth. In these areas the wet period is maximum 3 months. Even when some water is stored in the root zone during the wet period and used later by the plants, it is still not enough to obtain a good harvest. In addition the rainfall is unreliable in desert and arid regions: one year there is a lot of rainfall, the next year there may be only a little. On top of that, the rainfall usually comes in heavy showers resulting in high water losses due to runoff.
Fig. 3a Rainfall and irrigation in desert and arid areas
Semi arid areas
Even in the semi-arid areas irrigation is indispensable for a reliable and good harvest. Some drought resistant crops such as sorghum and millet may give reasonable yields, but also here is a risk of unreliable rainfall and subsequent crop failure.
Fig. 3b Rainfall and irrigation in semi-arid areas
Sub-humid and moist sub-humid areas
In the sub-humid and moist sub-humid areas, irrigation is required only during the dry season. In principle, on a yearly basis there is a rainfall excess: the yearly rainfall is higher than the yearly evapotranspiration. During the rainy season it may even be too wet for crops like sorghum and millet.
Fig. 3c Rainfall and irrigation in sub-humid and moist sub-humid areas
Humid areas
In the humid areas irrigation is usually not required, except maybe for paddy rice (see section 4.4). During more than 9 months per year there is excess rainfall and even in the remaining drier months the plants may use water which has been stored in the root zone during the rainy season.
Fig. 3d Rainfall In humid areas
Crop growth depends not only on rainfall, but also on other climatic factors (most notably sunshine and temperature) and non-climatic factors such as the availability of suitable soils.
During some periods of the year (the cool season) or at high altitudes (i.e. in mountainous areas) crop growth may be prohibited by the low temperature. Other areas may not be suitable for certain crops due to high temperatures.
Table 2 provides, as an example, data on land use, i.e. the most important crops grown in the 6 major climatic zones of Africa.
Table 2 MAJOR CLIMATIC ZONES AND LAND USE IN AFRICA
|
Climatic Zone |
Land Use (Major Crops) |
|
Desert |
Hunters and gatherers, nomadic pastoralists, sedentary irrigators around oases, no rainfed agriculture. |
|
Arid |
Extensive grazing (nomadic pastoralists), some millet and sorghum under flood irrigation in moist depressions. |
|
Semi-arid |
Both nomadic pastoralists and cultivators. Mainly millet and sorghum, also short cycle cowpea, phaseolus beans and groundnuts. No fodder or sown pasture. In cooler parts maize. |
|
Sub-humid |
Traditional nomadic pastoralists in dry season and drought years. Crops grown by settlers: millet, sorghum, maize, groundnuts; also cassava, cowpeas, cotton, sweet potatoes, tobacco, rainfed rice, soybean, mango, cashewnuts. Fodder and sown pasture possible. |
|
Moist sub-humid |
Transition zone for agriculture: too wet for seasonal crops, too dry for tree crops. Tropics: maize, cassava; also yams, bananas, pineapple, sugarcane and rice. Winter rainfall areas and East African highlands: wheat and barley. |
|
Humid |
Tree crops: oilpalm, rubber, cacao; shifting cultivation based on root crops (yams, cassava, etc.). Also some sorghum, maize, banana, sugarcane, rice. Some tropical hard woods. |
