- HYDROLOGICAL CYCLE
The hydrological cycle is the process whereby rain or snow precipitates and travels through various different routes to join the ocean or lakes, where it is evaporated to form atmospheric vapour which, in turn, is precipitated again as rain. The cycle has no beginning and no end. The diagramme below depicts in a very simplified form the hydrological cycle as it operates in Central and Southern Africa.
Figure 1: A simplified Hydrological cycle
Some of the rainfall never reaches the ground because the raindrops are intercepted by trees and other vegetation. It is then evaporated back into the atmosphere when the rain ceases. This process can be seen in Figure 2 on the next page. Usually, only a small part of the rain is intercepted and the bulk reaches the ground where a large part infiltrates into the soil. Most soils have a porous nature and can hold a considerable amount of water. This is especially evident at the beginning of the rains when the soil is very dry and can absorb much of the rain before the surface runoff starts.
Transpiration is the process by which water is moved from the
soil into the atmosphere by plants. This movement of water within plants is an essential function of their normal growth because it transports nutrients to the upper parts of the plant, and equally important keeps the plant cool in hot and sunny weather. The plant retains only a very small portion of the water it absorbs.
Figure 2: The Hydrological Cycle
Therefore, although there is about 5 to 10kgs of water in a plant for each kg of dry matter, the plant requires between 300kg and 1 000kgs of water to produce 1kg of dry matter. These 300kgs ‐ 1 000kgs of water are transpired by the plant as it grows and produces the dry matter.
Large quantities of water are returned to the atmosphere by transpiration which forms an important part of the hydrological cycle.
Evaporation is the movement of water vapour from water, soil and vegetative surfaces to the atmosphere. The evaporation from a free water surface, such as a dam or lake is about 2 metres/annum (5mm/day) in central Africa, and can rise to 7mm/day in the hotter, low‐lying areas. Evaporation from the soil is considerably less than this. For practical purposes transpiration is usually considered together with evaporation and the sum total is called evapotranspiration. For example, in an area where the mean annual rainfall is 900mm, the evapotranspiration can amount to about 83%, meaning that 750mm of rainfall is returned directly to the atmosphere by evaporation and transpiration.
The water that has infiltrated into the soil can sink deeper until it reaches rock where it can percolate through the cracks and fissures to replenish the ground‐water reservoir. This is the water that is tapped artificially by wells and bore‐holes. If it is not drawn off artificially it may build up towards the surface where it can be drawn upon by vegetation.
It can happen that the soil, into which rain water has infiltrated, is underlain by an impervious layer of solid rock or clay and in this case the water will flow along this surface in the direction of its slope. This water may join the ground‐water if the impervious layer dips into the ground or may seep out as stream flow if the layer comes to the ground surface.
The water that appears in streams and rivers is the final phase of the hydrological cycle, in which water rejoins the ocean or lakes and dams to be evaporated into the atmosphere again.
Stream‐flow is made up of the following classes of flow:
· Surface Runoff:
This is the part of the runoff which has travelled over the ground surface to reach the stream. It usually occurs when the rainfall rate is greater than the rate of infiltration into the soil. The excess water appears as surface runoff and flood‐flow. This runoff, therefore, appears immediately after rain except in cases where it is artificially retarded in dams and weirs.
· Interflow or sub‐surface stream‐flow:
This is that part of the rainfall which infiltrates into the soil to form a temporary ground‐ water reservoir above the main water‐table, then moves laterally through the soil to seep out into the stream. A small part of this flow enters the stream immediately after the rain but the bulk takes a long time before it appears in the stream.
· Ground‐water flow:
This is the part of the flow which has percolated into the ground‐water table and then been discharged into the stream. Rain which forms ground‐water takes a long time to appear as stream‐flow. At the tail‐end of the very severe 1968 drought when surface and subsurface water supplies were greatly depleted, it appeared from analysis of the tritium content of the stream‐flow in the Umwindsi River, near Harare, that the bulk of the water in the river at this time was more than 10 years’ old. In this case all the surface run‐off and interflow from the previous rains had been used and the stream‐flow was drawing almost entirely on the ground‐water supplies. The rate of flow was, of course, much less than in normal years.
For the purpose of run‐off analysis, the total stream‐flow is divided into storm‐flow and normal flow or base flow. Storm‐flow means all flow of water directly caused by floods due to rainfall and is made up of surface runoff and that part of the subsurface runoff that enters the stream soon after the rain. Normal flow is the sustained base flow from ground‐water runoff and delayed sub‐surface runoff.
The following table shows the figures which have been compiled in respect of a typical high‐rainfall area, a medium rainfall area and an average low‐rainfall zone.
Table 1: The average for each rainfall zone
|High Rainfall||Medium Rainfall||Low Rainfall|
|Mean annual rainfall||1 400mm||900mm||500mm|
|Total surface runoff||500mm||130mm||30mm|
|Evapotranspiration and minor losses||900 mm||770mm||470mm|
Table 2: Expressed as a Percentage of the Annual Rainfall, these figures are set out below:
|Total surface runoff||36%||14%||6%|
- ADVISABILITY OF IRRIGATION
Almost any irrigation scheme may be shown on paper to make a profit, but to produce that profit in practice requires careful planning and attention to detail by the farmer. Most of central Africa is marginal for arable farming with a growing season of 4‐5 months and periodic droughts which last from 2 ‐ 4 weeks. About one in every four seasons, the rainfall is less than half the average and droughts can occur even in the higher rainfall areas.
ADVANTAGE OF IRRIGATION
- With irrigation, there is much less risk of crops failing in a drought year. Because of this, more money can be spent on fertiliser, seed and general crop management. The risk factor in cropping is greatly reduced;
- The level of crop yields can be maintained much higher over a number of years, and this increases profitability. This can be seen from the diagram below;
Figure 3: A comparison of Dry‐land and Irrigated Maize Yields
- Healthier and more vigorous crops increase the soil cover and reduce erosion. This is controlling soil erosion by biological rather than mechanical means;
- The farmer can grow alternative or even additional crops. The growing season becomes controlled (e.g. shorter and more regular) and allows for more effective crop rotations and the intensification of farming systems. Much better use is made of the land;
- With irrigation, cash cropping is possible in all areas;
- Irrigation can be used either partly or wholly to produce more feed for stock so that stock enterprises will benefit;
- Because the risks are reduced, businessmen are encouraged to invest in farming and loans become easier to obtain;
- Land values appreciate faster and farms with irrigation schemes fetch higher prices than dry‐ land farms and;
- Both the local area and national economy benefit from more effective work, more cash and greater stability.
DISADVANTAGES OF IRRIGATION
When considering the introduction of an irrigation scheme onto a farm, the following factors should be considered very carefully.
- The greater need for capital, cash and technical know‐how. Irrigation is very expensive and requires a heavy capital outlay and management expertise;
- Because of the large financial investment in an irrigation scheme the farmer must have the business ability to make the most of his investment. Long term planning and day to day management is required. It can happen that a good dry‐land farmer does not have the ability to make the best possible use of irrigation on his farm;
- The farmer must be able to forecast prices and secure markets for his produce so that best profit from his increased cropping is made;
- There must be a plentiful and reliable supply of water for all stages of the crops grown. The greatest demand for water is usually in October when the daytime temperatures are high and before the rains have started;
- The day to day management of an irrigation scheme must be efficient. Pipes must be moved at the right time and the correct place and equipment must be constantly checked for leaks. A crop reaching maturity is carrying a large investment in seed, fertilizer and labour and must not be damaged through the inefficient application of water;
- Before planning the cropping programme for irrigation, the farmer must take into account the capability of the soil and land. He must consider the natural region in which his farm is situated; for example, temperatures are too high in the lowveld of South Africa and Zimbabwe for maize production even under irrigation and;
- In any irrigation scheme great care must be taken to avoid soil erosion, damage to the soil structure and problems of soil salinity. Drainage of the soil is another aspect that must be considered.
3. USE OF PUBLIC WATER
Primary Use: Every owner, tenant or occupier of land riparian to a public stream has a right to use such water for ‘Primary Purposes’, i.e. without application to a water court.
- Human use (drinking, cooking, washing) or use in or about a dwelling (sewage, garden,
washing a car) in some dry areas limited to 225 litres per day per person residing in the dwelling.
- Support of animal life. 45 litres per day per head of cattle is used as a general yard stick.
Abstraction of water for primary purposes can only be from that portion of a public stream to which one is riparian (unless special rights are granted).
Water for primary use may be impounded (i.e. in a dam) where stream flow is not always available. In determining the quantity that can be stored for essential needs, provision may be made to cover the evaporation losses and to cover a period of use up to about 18 months.
In most countries, the government must be informed of the details of any storage work of capacity in excess of five million litres. If it is desired to impound water in excess of that required for primary use, an application must be made for water rights.
Agricultural Use: This means the use of water for:
- The irrigation of land;
- The purposes of fish farming and and;
- The operation of a feedlot in which 12 or more cubic metres of water per day are used for a period of more than 6 months in any 1 year. A feedlot is
any enclosure in which animals or poultry are confined and fed mainly by means other than natural grazing.
Institutional Use: Water used for:
- Boarding houses, guest farms, hotels or clubs;
- Missions or boarding schools and;
- A permanent labor force which excluding the dependents is more than 100 workers.
Miscellaneous Uses means the use of water for any purposes other than agricultural, electrical, institutional, local authority, mining, primary, railways, roads or township purposes.