DESIGN AND PLANNING OF WATER HARVESTING STRUCTURES.
Water harvesting is the collection of water for production purposes. It is majorly practiced where rainfall is normally inadequate to meet soil and crop water requirements. It involves transfer of runoff from catchment area that is not cropped to supplement the rainfall received directly on the area that is cultivated.
Factors to consider when designing water harvesting structures.
1. Soil and crop water requirements.
For the effective design of water harvesting structures, it is good to identify and assess the water requirement by the crop that is to be grown. Soil and crop water requirements are determine by the following:
the important factors are sunshine, temperature, humidity and wind speed. When the weather is hot, sunny and dry, the crops require more water that when the weather is cold and humid. If the water is not available from soil reserves, rainfall or irrigation, the crops will suffer from water stress with consequent reduction in yields and if the situation is not corrected the crop will wilt and eventually die.
Physical characteristics of soil such as leaf structure, root system and growth pattern determines the crop water requirements. Crops with longer growing season consume more water that that short seasoned crops. Perennial cops require more water that annual crops.
The physical, biological and chemical soil properties affect the yields response to availability to soil water. The soil in the catchment area should have high runoff coefficient while the soil the cultivated area should be deep, fertile with high infiltration rate. Soil properties like structure and texture also determine the water requirements by crops.
2. Rainfall-runoff analysis.
The amount of runoff depends on the characteristics of rainfall such as intensity, duration, distribution and of the catchment. On designing of water harvesting structures, it is beneficial to estimate from the local rainfall data, how much rain can be expected on the average during the growing season.
3. Catchment characteristics.
Catchment characteristics determines how much of the rain in a given storm will fall will become rainfall. This depends on soil type, vegetation, slope of the land and size of the catchment.as estimate is to be made on the proportion of rainfall that is to become run off during the growing season. This is called the rainfall coefficient, and it is varies from 0.1 to 1.0.
4. Water harvesting systems.
A distinction is made between runoff harvesting and which uses water from ground surfaces and floodwater which uses floodwater which involves diversion of floodwater from watercourses. Water spreading is the distribution of floodwater over the land surface for crops and pasture. Water harvesting systems are affected by:
Ratio of catchment to cultivated area: Each harvesting system consists of catchment which is the collection area and concentration area which is the cultivated area. The two coefficients are very much important in designing of the water harvesting structures. The recommended ratio is 1:1 to 3:1 for micro catchment and above that ratio for macro catchments structures.
Catchment slope length: Catchment with slope length of 1-10 is suitable for micro catchment while those of longer lengths are for macro catchments.
Catchment covers: Bare catchment is best suit for water harvesting due to absence of vegetation but then it requires more maintenance to controls it. Where less water is required, catchment is covered with vegetation.
Source of run off: runoff could either be within the field or external through the hills and roads.
Characteristics of the runoff: Runoff could originate from shallow overland flow, turbulent, channel flow or spate flow systems.
5. Site selection.
Selection of the site for installation of water harvesting structures depends on the following factors such as: Technological factors which include cops and soils or soil – economic factors such as preference of intended beneficiaries, quantities of the earthwork or stonework, cost of labour and construction and land availability.
Types of catchment areas for rainwater harvesting.
A catchment area is the part of runoff water harvesting system that receives rainfall and drains the water into a storage or harvesting structure through a conveyance system. The size of a catchment area varies from simple roof tops to bigger systems where large catchments drain water to dammed reservoirs from which water is either gravitated or pumped to treatment plants. The desirable characteristics of a good catchment includes impermeability in order to be able to generate adequate runoff and some slope to direct flow to the storage structure, and also they should not contaminate water seriously with dangerous chemicals or microorganisms.
Ground catchment system is describes all systems which use the ground surface as a catchment area. These can be natural, treated and covered surfaces such as roads, runways, pavements, car parks and courtyards. Ground catchment systems are cheaper than roof catchments and are normally employed where suitable roof surfaces are not available. The ground catchment surface can be collect water from a larger area, which is very advantageous in areas of low rainfall. The main disadvantage is that the water supply can easily become contaminated and since it can only be stored below the surface it is generally less convenient to withdraw.
Surface catchment areas
surface catchment includes roads, paths, railway lines and other paved surfaces provide catchments which can also be used for rainwater harvesting. The runoff coefficient for such surfaces is quite high and the water can be diverted and stored using simple diversion structures directing surface water into underground tanks, pans, ponds, check-dams and other storages. The water trapped can be used for livestock watering or supplemental irrigation of crops. There is huge potential for rainwater harvesting from road surfaces since the catchments can be relatively large.
Man -made catchment
Catchments for water harvesting can also be created artificially, such as roof water harvesting, paved areas, concrete surfaces, plastic sheet coverings or treated ground surfaces. The material used for paving should not contaminate the water.
Precipitation catchment techniques
rainfall, dew, snow and fog collection systems. Rainwater probably accounts for more than 99% of all precipitation harvested directly for domestic use. Moreover, dew fog and snow are also harvested and in certain arid localities provide essential sources of water.
Micro catchment water harvesting techniques.
1. Zay pits or zai basins.
Zay pits are shallow pits dug into the soil, usually about 30-40 cm in diameter and 10 -20 cm deep. Soil fertility and structure are enhanced by placing organic matter, usually grass or manure, mixed with earth into the pits. Pests and parasites activities commonly reduce the organic material to a state whereby it can be readily exploited by crops and an improvement in infiltration may be achieved due to their burrowing activity. The remaining earth is used to construct a small bund around the pit on the down slope side. Zay pits are designed at a spacing of about 1m to 2m apart. This is a revived technique practiced in degraded and low lying land that experiences low rainfall pattern and is used in conjunction with stone bunds which reduce runoff velocities. The structure installation is labour intensive.
2. Contour bunds or ridges.
Contour bunds are used to prevent runoff and soil erosion and to supplement soil moisture for crops. The structures are mainly applies in high slope areas, where water retention is of primary importance. They are used to prevent any loss of water by lateral flow. The ridges are constructed with a slight gradient usually about 0.5 to 1.0% to allow controlled drainage and render runoff velocities non-erosive. Ridges can be broken to provide drainage and thereby, rudimentary water control.
contour earth ridges.
The design size and spacing of the contour bunds is dependent upon land slope, the bund height and the desired area of control. They are constructed by manual y or mechanically and the soil is excavated up slope of the bund which is under construction. Excessive depth of extraction must be avoided as this may lead to the loss of top soil. Water naturally accumulates adjacent to the bund, where the top soil is removed. In areas of inundation landscape, the crop is planted on the side of the ridges to overcome temporary waterlogging.
3. Crescent shaped bunds/hoop/semi-circular bunds.
The crescent shaped system could be considered as a macro catchment technique that is of off-field measure. Mostly an external runoff may enter it but most water is captured from local runoff. They are mainly used for land rehabilitation and fodder production. Their construction may be undertaken with the dug furrow that provides the bund material excavated downslope, thereby retaining all topsoil within the hoops. In some conditions, the furrow is dug on the inside of the hoop, thus increasing water storage. Material moved is in the order of 40 – 50 m³ for each hoop depending on size and slope. The semicircular bunds about 30 cm high, are between 4 to 10 meters across and may be placed in lines or staggered to manipulate the catchment to crop area ratio.
semi- circular bunds.
4. Trapezoidal bunds.
Trapezoidal bunds design is more or less the same as that of the semi-circular bund s except the shape that brings a distinction. They are larger in size and construction is so much involving. The design scale depends largely upon land slopes. The main trapezoidal bunds may be 60 cm high and 6 m in width, the tapered arm tips 120 m apart and 40 from the main basal bund. The main bund has a freeboard of about 30 – 40 cm with the enclosed area filled with runoff when sited on low slopes. Construction is estimated to involve the movement of about 400 m³ of soil on a 1% slope.
5. Contour stone lines/bunds.
The stone bunds are discontinuous lines of stones, piled to extend perhaps 10 – 20 m, with a height of about 30 cm and laid in a trench to aid stability. Their action is to reduce runoff velocity by means of their permeability rather than to block the flow of runoff. This reduction in velocity encourages infiltration, reduces erosion and increases the deposition of suspended material. They allow the passage of water and are not so easily washed away. Given suitable material they are easy to construct and need no special equipment but a simple leveling tube. Farmers may depend on mechanized transportation of the material to site. Their location is usually on low slope areas in cultivated fields or on highly degraded land under rehabilitation.
contour stone bunds.
sectional view of contour stone bund.
5. Retention ditches.
Retention ditches are large ditches designed to catch and retain off incoming runoff and hold it till it infiltrates into the ground. They are used as an alternative to diversion ditches if there is no place to discharge runoff or if there is a need as in semi-arid areas to harvest water. Retention ditches are constructed at zero gradients with close ends. They are useful on low slopes in dry areas for harvesting water, and are mainly constructed where soils are permeable, deep and stable and are not prone to landslides. Retention ditches should be sited very close to the contributing catchment area. They should have water impounded, and infiltrated within 48hours to avoid erosion of the surrounding area. They should not be constructed in areas that are prone to landslides and mudslides. They are best suitable for banana and tree plantation farm where constant water retention capacity is required at the plant roots.
cross -sectional view of retention ditch.
The sizes vary from 0.4 – 0.6 m deep and 0.5 – 2m wide. They are usually used on relatively large land where they may be spaced at 20 m or on gentle slopes where the spacing can be decreased to I0 – I5m.When constructing the ditches, the soil is thrown to the lower side to form an embankment.
6. Roof water harvesting.
The main catchment is the house roof. The roofing should be of galvanized iron sheets, aluminum, clay tiles, asbestos or concrete. For the case of grass thatched-roof, it may be covered with waterproof sheeting. For collection of water, gutter is used along the edge of the roof. It is fixed with a gentle slope towards down pipe, which is meant for free flow of water to the storage tank. This may be made up of iron sheet, wood, bamboo or any other locally available material. The down pipe should be at least 100 mm diameter and be provided with a 20 mesh wire screen at the inlet to prevent dry leaves and other debris from entering it. During the period of no rain, dust, bird droppings etc. accumulate on the roof. These are washed off with the first rains and enter the storage tank to contaminate the water. This can be prevented by two methods which includes simple diversion of foul water or by installation of foul flush system.
Roof water harvesting layout.
Common problems associated with water storage systems.
The development of cracks in any form of water retaining structure can have serious repercussions. This happens in concrete structures and may be caused by poor concrete mixes, bad workmanship and incompatibilities between the phases of cement paste, sand and reinforcement, during construction. It may also be due to fatigue caused by repeated loading, or induced stress caused by shrinkage.
Water stored in structures may leak or may be subject to seepage. Seepage through the wall increases the risk of breaking and failure of the wall. Seepage can be reduced by compacting the ground prior to filling with heavy equipment. This is achieved by covering the reservoir bed with a 10 to 20 cm thick clay layer then compacting it either manually, with machinery or using animals. Seepage can also be reduced by including an impervious core in the structure. The reservoir can be lined with concrete, thick plastic film, clay grouting or other impervious material to reduce seepage.
Large quantities of water are lost from storage structures through evaporation. Evaporation can be reduced by various methods which include good design or site selection for surface reservoirs, dividing the reservoir into two or more compartments, covering the structure with a roofing material or shaded to protect the water surface from wind and direct sun thus reducing evaporation. Furthermore, it is also possible to reduce the rate of evaporation by 50% through a combination of wind breaking and shading using bush rafts built using logs, tree branches and twigs are good covers. These are easy to make and will cost appreciably less as compared to other means such as netting or plastic films.
Siltation is caused by various factors such as cultivation and poor land use in the catchment. The design of the structure can also contribute to siltation. It is important to make a good estimate of the sediment load in the water to be harvested. A factor called sediment trap efficiency of the reservoir can then be used to predict siltation rate.
Rainwater storages usually hold stagnant water which can attract various pathogen, disease vectors and pollution. For instance, mosquitoes breed in rainwater storages and they are vectors of serious diseases such as malaria, yellow fever, dengue fever and filariasis. Careful use of the water is also necessary. For instance, reservoirs constructed for storing domestic water should not be used by livestock unless off-take facilities are provided.
Maintenance of rainwater harvesting structures.
Maintenance of rainwater harvesting structures is very important aspect as it can increase the life span of the systems. The following are some of the maintenance practices that need to be put in place:
Regular cleaning and minor repair of the whole system from the catchment, the conveyance, the tank and the various tank components such as tap.
Frequent removal of branches of trees over hanging on roofs, also the droppings of birds and small animals contaminate rainwater. Dust and other such dirt also need to be cleaned regularly from the catchment.
Regular cleaning and minor repair of the conveyance system such as gutters and downpipes/gutters.
Regular inspection of water quality in the tank, testing and disinfecting regularly.
There should be no opening that allows small animals to enter into the storage structure; it is therefore necessary to inspect, clean and repair/replace screens and filters. Screens and filters unless cleaned regularly can themselves be a source of water contamination.
Cleaning of accumulated sediment and sludge when necessary.
There should be no tree growing within 10 m from the tank to protect the foundation from damage/crack by roots searching for moisture underneath.
Dispose of safely runoff and/or ponding water around the tank as this may damage the tank or bring health risks.
Regular inspection of the amount of water in the tank and compare with demand and abstraction rates.