6.1 Introduction
6.2 Crops
6.3 Trees
6.4 Rangeland and fodder
The production aspects of rainwater harvesting systems for crops and trees are outlined in the sections which follow. A brief note is also included about grassland and range. Water harvesting improves growth by increasing the availability of water to plants in dry areas. However there is little point investing in rainwater harvesting structures unless attention is also given to other aspects of husbandry. Plants which are well nourished, cleanly weeded and protected from pests and diseases will respond best to the extra water.
6.2.1 General
6.2.2 Crop choice
6.2.3 Fertility
6.2.4 Other husbandry factors
Water harvesting helps crops by providing extra moisture at different stages of growth - although timing cannot be controlled. Periods when the extra moisture can make a significant difference are:
- around sowing time when germination and establishment can be improved;
- during a mid-season dry spell when a crop can be supported until the next rains;
- while the crop is at the vital stages of flowering and grainfill.
The most common cereal crops grown under water harvesting are:
- Sorghum (Sorghum bicolor) is the most common grain crop under water harvesting systems. It is a crop of the dry areas, and in addition to its drought adaption, it also tolerates temporary waterlogging - which is a common occurrence in some water harvesting systems.- Pearl Millet (Pennisetum typhoides) is grown in the drier areas of West Africa and India, and apart from being drought tolerant, it matures rapidly.
- Maize (Zea mays) is occasionally grown under water harvesting but is neither drought adapted nor waterlogging tolerant - but in parts of East and Southern Africa it is the preferred food grain, and farmers are often reluctant to plant millet or sorghum instead.
Legumes are less frequently grown under water harvesting but should be encouraged because of their ability to fix nitrogen and improve the performance of other crops. Suitable legumes are Cowpeas (Vigna unguiculata), green grams (Vigna radiana), lablab (Lablab purpureus), and groundnut (Arachis hypogea). All are relatively tolerant of drought and are fast maturing.
In dry areas soil fertility is usually the second most limiting production factor after moisture stress. The improvement in the supply of water available to plants under water harvesting can lead to depletion of soil nutrients. Therefore it is very important to maintain levels of organic matter by adding animal manure or compost to the soil. Inorganic fertilizers are seldom economic for subsistence crops grown under water harvesting.
Crop rotation helps maintain the fertility status. Legumes should be alternated with cereals as often as possible. Intercropping of cereals with legumes - sorghum with cowpeas for example - can also lead to higher overall yields as well as soil fertility maintenance.
Some water harvesting systems actually harvest organic matter from the catchment and therefore build up fertility. This can most clearly be seen with stone bunding techniques which filter out soil and other organic particles, thereby building up fertile deposits.
- Weeds are a problem where water harvesting is used, due to the favourable growing conditions where water is concentrated. Weeds are especially a problem at the start of the season and therefore early weeding is extremely important.- Early planting makes the best use of limited rainfall. In some areas it may be best to plant seeds before the rains arrive. This technique is known as "dry planting".
- "Opportunistic" or take-a-chance planting of a quick legume crop like cowpeas can make use of late season or out of season rainstorms.
- Low plant populations in themselves can improve yields in low rainfall zones, and therefore spacing of crops is another important consideration.
Rainwater harvesting is used to help tree seedlings become established in dry areas. The microcatchment technique concentrates water around the seedlings, and make a considerable difference to growing conditions at this vital early stage. In semi-arid areas, tree seedlings in the natural state usually germinate and grow only in years of above average rainfall - water harvesting imitates these conditions.
Table 27 summarizes the most important characteristics of the most commonly planted trees in semi-arid areas of Africa and India.
Table 27. CHARACTERISTICS OF COMMONLY PLANTED TREE SPECIES
|
Species |
Characteristics |
|
Acacia albida |
"The" agroforestry tree of West Africa. Genuinely multi- purpose. Pods for fodder. Needs water table. Slow at first. |
|
Acacia nilotica |
Widespread in India and Africa. Likes deep soils and water table. Good fuel/fodder. Quite quick growing. |
|
Acacia saligna |
Introduced species from Australia. For dune fixation/fodder/windbreaks. Hardy. Fast growing. |
|
Acacia Senegal |
"Gum arabic" tree producing commercial gum. Good also for fuelwood/fodder. Direct seeding possible. Slow. |
|
Acacia seyal |
Likes low-lying areas with heavy soils which flood. Good forage/fuelwood. Quite fast early growth. |
|
Acacia tortilis |
"Umbrella thorn". Good fuelwood and charcoal. Branches for fencing. Pods good fodder. Fast once established. |
|
Albizia lebek |
From India. Small shade/amenity tree in Sahel. Needs high water table. Foliage for mulch. Rapid growth. |
|
Azadirachta indica |
Neem tree: from India/Burma. Grown for shade mainly but also good fodder/fuel. Fast growing. |
|
Balanites aegyptiaca |
"Desert date" widespread and ecologically "flexible". Fodder/edible fruit. Direct seeding possible. Slow. |
|
Cassia siamea |
Grown for shade, amenity, fuelwood and poles. Better with higher rainfall. Direct seeding possible. Quick. |
|
Casuarina equisetifolia |
Good on deep sands (also at coasts) so used for dune stabilization. Also fuelwood. Fast growing. |
|
Colophosperum mopane |
Indigenous to Southern Africa. Poles for construction and leaves for fodder. Firewood. Wood very hard. |
|
Eucalyptus camaldulensis |
From Australia. Best eucalypt for dry areas. Coppices well. Windbreak/fuelwood. Very quick growing. |
|
Prosopis chilensis |
Similar to, and often confused with, P. juliflora, see below. |
|
Prosopis cineraria |
Indigenous to NW India where grown as agroforestry tree. Fodder/fuel/building materials. Slow. |
|
Prosopis juliflora |
Very drought resistant and establishes naturally. May invade better areas. Coppices well. Good fuel supply. Pods for fodder. Quick growth. |
|
Ziziphus mauritiana |
"Jujube". Produces edible fruit. Can be grafted. Small tree. Branches for fencing. Slow growth. |
Planting of more than one seedling at a planting station is usually a good idea. The reason is that the additional cost is small compared with the cost of the water harvesting structures and there is extra assurance of at least one seedling establishing. It is advisable to plant one seedling at the bottom of the pit/furrow and one by the side. The former may establish better in a dry year, and the second if conditions are wetter.
Direct seeding is a technique which saves all the nursery costs of producing seedlings. However it does mean that the trees will be slower growing in the early stages. Tree species which can successfully be direct seeded include Balanites aegyptiaca and Prosopis juliflora.
Weeding is only necessary close to the seedling. The area between the rows of trees should be allowed to grow grass (or even planted with an annual crop) so that some economic benefit can be gained in the first few years, before the trees mature.
Although the use of water harvesting for range and fodder is relatively rare, there are some important points to make about husbandry. Often the most important factors are initial fencing and protection, followed by long-term grazing management.
Natural revegetation often gives satisfactory results without reseeding. However, where reseeding is used, it is usually best to collect seeds from local species known to do well in the area.
One advantage of range/grassland systems is that the roots of these perennial species tend to protect structures, and therefore maintenance requirements are reduced.
