Water harvesting inducement techniques

Many researchers in the field of hydrology promoted and suggested the use of water harvesting techniques for their low cost and simple implementation in addressing water crisis and other water-related emergencies (Buraihi et al., 2015). Rainwater harvesting is one of the artificial recharge techniques used in water resources management around the world. There is not even a single technique or method that which is best in all situations. The use of best and suitable techniques vary depending on topography, soil condition, storage devices, labor, availability of soil surface dressing materials, and projected use of the water harvested. The cost of other available water sources and the importance of water supply system determine the costs which can be justified. The overall cost for preparing a catchment area is composed of two main elements: the cost of materials and cost of labor. Some of the water harvesting enhancing materials and application techniques are labor-intensive but have comparatively low overall costs. However, this type of techniques may be suitable for areas such as Afghanistan where labor cost is cheap. Other approaches may have high capital costs but require a minimum of labor, e.g. mechanized compaction. Such techniques may be appropriate in areas with high labor costs. Generally, water harvesting enhancement material applied in water harvesting those materials which are cheap, locally available, and easily handled. Modifying the topography or soil surface, modifying the soil itself and covering the surface with an impermeable layer are methods usually used to induce runoff (Oweis et al., 2012).

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Rainwater harvesting technologies are best suited and most relevant to areas with arid and semi-arid climatic conditions where the problems of environmental degradation, drought, and population pressures are most evident. It is a major and indispensable part of the treatment package for these problem zones, and there is no doubt that implementation of water harvesting techniques will become even larger (Thompson, 1999). Effective rainfall produces the highest amount of runoff, but, by modifying the soil surface, it can also reduce the threshold required to get runoff.

Runoff in the watershed and catchment can be improved by removing plants and its components from the soil surface, and by treating the soil surface with infiltration resistant compounds or by applying an impervious film. Materials and techniques used to maximize surface runoff include an application of sodium salts, melting point wax, soil crusts, synthetic membranes, asphalt and concrete and compaction (Matthew and Bainbridge, 1994).

1.3.4.1 Modifying the topography or soil surface

Clearing rocks, small stones and all kind of vegetation usually increases runoff.

Surface runoff volume can even be induced if some of the rocks, gravel, stones, and vegetation be removed from the soil surface, or with little modification to the topography or surface structure. Clearing or removing obstructing elements (decrease runoff) from the surface catchment can be on of the most economical ways to enhance water harvesting in arid land if low-cost hillside land is available. The soil surface may also be smoothed by flattening small obstructions such as ridges and furrows across the contour of the land. In this method, small amount of topographic modifications are required. This may require considerable amounts of labor or the use of machinery, depending on the topography and soil conditions.

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Compaction of the soil surface can also increase the surface runoff volume. In this method the slopes are graded and compacted manually or mechanically. Various soil compacting techniques such as hand hammer, a tractor and rubber-tired roller is used for mechanical compaction. In Western Australia compaction method and surface smoothing methods have been used successfully in roaded catchments. Tractors and rubber-tired rollers are used for compaction and soil surface smoothing of the steep road catchments (Oweis et al., 2012). The porosity of the soil is decreased by compaction which make it difficult for water to go into the soil (Brutsaert, 2005). The compacted soils can decrease infiltration to a level where even a low-intensity rainfalls and little rainfall can produce surface runoff. Li and Gong (2002), reported that, efficiency of compacted soil surface is way higher from total rainfall compared to that of uncompacted soils. Thus, soils with a certain amount of compaction had low infiltration rate, and require minimum precipitation to produce runoff. Soils in compacted catchments have a greater potential for rainfall water harvesting in the semiarid regions.

1.3.4.2 Surface sealing

Surface sealing involves chemical and physical treatments either sprayed or applied as dressing onto the catchment area or mixed with the surface soil to decrease or stop water infiltration into the soil. Many types of materials have been tested. Sodium chloride, sodium bicarbonate, spraying molten, refined, low-melting-point paraffin wax onto the soil surface are the most common surface sealing treatments (Singh, 2016).

Soil dispersants such sodium salts cause clay particles in the soil to disperse, swell, or break down into small particles, resulting in increasing runoff. They show promise as a soil sealant because of their low cost, ready availability (Singh, 2016). The sodium chloride (NaCl) salt disperses the clay which clogs the soil pores and reduces the rate of

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water permeability (Dutt, 1981; Frasiar et al, 1987). Though, the application of sodium dispersants onto the soil surface might increase the risk of soil erosion. Another water-repellent treatment consists of waxing the soil surface. Applied wax is initially deposited as a thin layer on the surface, As the sun heats up the soil surface, the applied wax will gradually meltdown and will move downward into the soil, covering each discrete soil particle with a thin wax layer and acts as a soil water repellent. This treatment is appropriate and applicable to the soils which contain less than 20% clay content and catchment sites where the soil temperature goes beyond the melting point of the wax during part of the year (Frasier, 1980).

1.3.4.3 Impermeable coverings

Rather than making the soil itself the water-detaching surface, as a replacement it may be better in some situations to cover the soil surface with a waterproof layer such plastic sheet. Most types of plastic and other thin sheeting materials have been studied as potential soil coverings for water harvesting catchments, including thin plastic films, butyl rubber, asphalt membranes, and highway surfaces. Bitumen emulsion or use of asphalt are best suited to fine sandy soils but have an effective life of only 2–5 years (Singh, 2016; Laing, 1981). The simplest technique for utilizing cheap plastic sheets or is to put a shallow layer of clean gravel on the sheeting after it has been positioned on the catchment surface. The sheets of plastic and the tar paper provides the waterproof membrane and the gravel protects it from wind and sun. A good catchment surface can also be made by covering asphalt with a better quality plastic film with a gravel layer on top. The application of asphalt onto soil surface bonds the plastic film to the catchment surface, whereas the plastic film keeps the applied asphalt from being oxidized. The treatment mentioned above needs regular care to ensure the sheeting remains covered with gravel. Runoff is virtually 100% of all precipitation more than 2 mm. The

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treatment is relatively inexpensive, if clean gravel is readily available (Singh, 2016;

Cluff, 1975). Also, many kinds of construction materials such as concrete, sheet metal, or artificial rubber sheeting can be used on water harvesting catchments. However, most soil treatments have a limited lifespan and must be maintained and renewed periodically (Singh, 2016).