viii) Run-offs induced by surface treatments have given good results. Classification of surface treatments can be made as follows:
- Clearing from sloping surfaces the unwanted vegetation and loose material;
- Improving vegetation management y planting with different species or by cropping;
- Providing most desirable mechanical treatments to the soils, including smoothing and compacting surfaces, on contour steps in micro-catchments as workable units.
- Making hard surfaces using traditional soil stabilization techniques (as used on Botswana threshing floor catchments);
- Reducing soil permeability by te application of chemicals (e.g. sodium salts);
- Applying chemical binders such as asphalt to seal the surface.
- Covering the catchments with conventional paving materials:
- Covering the catchments with other rigid materials;
- Covering the catchments with flexible/ plastic materials.
Sheet metal, butyl rubber, asphalt roofing, bentonite paraffin, fiber glass reinforced asphalt, plastic sheeting with gravel cove etc. have been used to cover the contributing areas and thus induce larger volumes of run-off.
As regards chemical treatment of the soil, a lot of research has been carried out on substances which can be mixed with, spread or sprayed over the soil. Experiments have been made with sodium salts (which encourage surface crusting on soil containing clay) with silicones (which are water repellent) and with oil paraffin wax, bitumen or asphalt (which bind soil particles together). The sodium treatment is cheap but may not last for more tan a year leading to increased soil erosion subsequently.
The desirable attributes of a satisfactory ground surface treatment are:
- The run-off must be non-toxic.
- The surface should be smooth and impermeable or at least have a low infiltration rate.
- The surface should have a high resistance to weathering and should prevent internal chemical or physical deterioration.
- The surface need not have great physical strength, but should be able to withstand hail, intense rain, wind, occasional animals, moderate water flow, plant growth insects, birds, burrowing animals and maintenance vehicles.
- The treatment should be inexpensive on an annual cost basis. This necessitates low maintenance costs and generally low capital cost.
- Construction and maintenance should not require special machinery or skills.
However, it is recognized that it is usually necessary to forego one desirable characteristic in order to achieve another, particularly when the overriding objective is lower cost.
ix) Large quantities of water are lost through evaporation from tanks, lakes and talabs. 5 M.ha.m form the total storage of 15 M.ha.m evaporates in the reservoirs, spread all over the country (33% loss).
A number of approaches/techniques have been developed to reduce evaporation from water bodies. Some of these are:
- Keeping the area/volume ratio of water body lower.
- Minimizing exposed surface through reservoirs regulation.
- Constructing artificial aquifers.
- Achieving application of mono-molecular film.
- Reducing the energy available for evaporation
- Installing wind breaks.
x) The technique of rainwater collection for domestic supply or run-off farming, is essentially small in scale. Where large concentrations of population have to be supported, intensive form of development is inescapable and must sometimes include, large dams and river basin projects.
Traditional systems, in technological terms, are good and have stood the test of time. There is not need for immediate upgradation through research and development, unless the systems are well documented and understood. Modern water managers have shown little interst in traditional ystems due to lack of clear cut policy and encouragement.
The Ghala tank in Kenya is mad eby modifying a traditional granacyh basket or Ghala and plastering it inside and out with a 2:1 sand / cement mixture (see Fig.3.4). Cisterns built in Indonesi and Thailand using bamboo reinforcement instead of metal, have been highly significant in enabling to make low-cost tanks.
Excavated cisterns, suitably lined with cement plaster or other impermeable material, can be cheaper and may collect large volumes of water from ground surfaces. Galvanized corrugated iron tanks also have merits when looked at, in terms of social benefits and livelihoods.
Significant improvements have been made in design and layout of gutters and pipes for hygienic and economical collection of rain water. Technologies for manufacture and laying of system, appropriate to the volumes of water required to be stored, are available. Flexible piping, single tank/multi-tank system, provisions for first flush diversion and /or inlet filter mould are other developments.
Proper maintenance of roofs, tanks, gutters and pipings to ensure hygienic conditions is very essential.
xi) With regard to run-off farming as opposed to the exploitation of flood water, the inundation method used in Sudan, Pakistan and India has been developed to its highest level of sophistication. Techniques differ in details, but the principle is to allow run-off to collect behind a bund and leave he water standing until the planting date for the crop approaches. Then the land is drained and the crop is sown. The area behind the bund is known as a submergence tank, or an ahar (in Bihar), or a khadin (in Rajasthan), and may cover may hectares.
The specific systems are:
b. Submergence bunds
c. Conservation ditches
e. Farm ponds, and percolation tanks
xii) Whenever surface or ground water irrigation is developed in isolation form each other, the experience has been constantly bad with regard to resource conservation.
Exclusive reliance on canal irrigations tends to raise the water table, leading ultimately to water logging which in turn produces soil salination in case of arid and semi arid tracts. Exclusive reliance on ground water as a source of irrigation creates the problem in a reverse manner. That is, over exploitation of ground water may occur, leading to a permanent lowering of the ground water table.
xiii) Aquifers can provide a viable solution for storage, if certain topographical, hydrological, geological and socio-economic conditions exist and appropriate recharging technologies are followed. Aquifers have been used as reservoirs for thousands of years and warrant increased attention.
Several method of artificial recharging are in vogue, the choice being dictated by local conditions. The methods may be classified as follows:
a. Spreading method
b. Pit method
c. Induced – recharge method
d. Well method
For artificial recharging, there is a need to identify whether the quality of ground water and that of surface water are compatible. With temperature variations and with reversible flow direction due to ion exchanges, the soluble salts have a tendency to precipitate which results in clogging of pore spaces in spreading basins and clogging of slots in injection wells and reduces artificial recharging rates. Back