PLANNING WATER HARVESTING AND STORAGE SYSTEMS

1.1 Basic components of a RWH and storage system

Rainwater harvesting systems are orderly schemes in which organized components and techniques harness and make rainwater available for human consumption and environmental conservation. Thus, the planning and design of rainwater harvesting and storage in various structures should be based on water supply for multiple uses. This is because communities in the target areas may use the water not only to grow crops, but also for domestic use, livestock watering, as well as industrial and commercial purposes. Thus, a good quality RWH system is planned to provide communities with access to an on-site water supply, if possible at home, or near their homesteads, or at locality that is easy to access. Ideally, the RWH collection system should involve basic construction techniques, be inexpensive to maintain, and have a long functional life span. If the system is designed well, it should provide a good safe source of water at a relatively low cost when compared to the conventional methods such as river diversion or ground water pumping. In order to determine whether or not rainwater catchment systems are an appropriate choice of water supply in any given situation it is necessary to estimate the potential rainwater yield to check that it can meet the required demand. The reliability of the systems along with technical, socio-economic and environmental considerations must be compared with all alternative forms of supply.

Rainwater catchment system consists of a number of components. These include: Rainfall potential (in amounts and intensities to generate runoff) A catchment surface where the rainwater runoff is collected A storage reservoir where the rainwater is stored until required A delivery system for transporting the water from the catchment to the storage reservoir

e.g. delivery pipes, gutters or drains, down pipes Extraction device to take the water from the reservoir e.g. piping, a tap, bucket, or pump. Other auxiliary structures such as filters, sedimentation basins, foul flush systems, covers,

spillways and safety features. 1.1.1 Rainwater harvesting potential

The total amount of water that is received in the form of rainfall over an area is called the rainwater endowment of that area. Out of this, the amount that can be effectively harvested is called the water harvesting potential. Among the several factors that influence the rainwater harvesting potential of a site, climatic conditions specially rainfall and the catchment characteristics are considered to be the most important. Rainfall Characteristics The amount of water that can be harvested depends on the rainfall amounts, seasonal patterns and intensities. Thus rainwater can be harvested in both wet and dry zones, and actually, it should be more cost-effective in the wet areas where structures can be made smaller. In the arid, semi-arid

or sub-humid zones rainfall is characterized by low amounts of up to 700mm per annum. Further, it is erratic with periodic droughts and unreliable patterns. Inter-annual rainfall varies from 50-100% in the arid zones with averages of up to 350 mm. In the semi-arid zones, inter-annual rainfall varies from 20-50% with averages of up to 700 mm. Thus it is necessary to gather rainfall data and its characteristics. Rainfall data In order to determine the potential rainwater supply for a given catchment, reliable rainfall data are required preferably for a given period of at least 10 years. Ideally if accurate local historic rainfall data for the past few decades are available a 20 or 30-year rainfall series is preferable especially in drought prone climates. Water Demand Water demand is the volume of water requested by users to satisfy their needs. A simplistic interpretation considers that water demand equals water consumption. However, conceptually, the two terms cannot be equated because, in some cases, especially in rural parts of Africa, the theoretical water demand considerably exceeds actual consumptive water use. 1.1.2 Types of catchment surfaces A catchment surface is the part of a RWH system that receives rainfall and drains the water into a storage facility through a conveyance system. The size of a catchment surface 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. Generally, 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. Another important requirement of catchments is that they should not contaminate water seriously with dangerous chemicals or micro organisms. There are many types of catchment surfaces, such as roofs, rocky areas, hillsides, roads, home compounds, built up areas, open grasslands and valleys. Generally, water harvesting at household level makes use of roof catchments, with the storage being a tank located within the home compound. Ground catchment system is a general term describing all systems, which use the ground surface as a catchment area. These include natural, treated and covered surfaces. Cement or tarmac 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 main advantage of the ground catchment surface is that water can be collected from a larger area. This is particularly 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. Roads, paths, railway lines and other paved surfaces provide catchments which can also be used for RWH. 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 so stored can be used for livestock watering or supplemental irrigation of crops. There is huge potential for RWH from road surfaces since the catchments can be relatively large. Catchments for water harvesting can also be created artificially, such as paved areas, concrete surfaces, plastic sheet coverings or treated ground surfaces. The material used for paving should not contaminate the water.

Figure 1.1 (a) A rock catchment with earth dam (Source: Seifu, 2011)

(b) Paved artificial catchment with underground tank

Dew, snow and fog collection systems: Rainwater probably accounts for more than 99% of all precipitation harvested directly for domestic use. Nevertheless, dew fog and snow are also harvested and in certain arid localities provide essential sources of water. 1.2 Determining water storage volume The volume of rainwater that can be harvested over a given period depends upon the amount of rainfall in that period, the catchment area and the runoff coefficient (Figure 1.2). The characteristics of the catchment area determine the storage conditions. Rainwater yield varies with the size and texture of the catchment area. All calculations relating to the performance of rainwater catchment systems involve the use of a runoff coefficient to account for losses due to spillage, leakage, infiltration, catchment surface wetting and evaporation, which will all, contribute to reducing the amount of runoff. This is determined as follows:

Water harvesting potential = Rainfall (mm) x Area of catchment x Runoff coefficient or

Water harvesting potential = Rainfall (mm) x Collection efficiency

The collection efficiency accounts for the fact that all the rainwater falling over an area cannot be effectively harvested, because of evaporation, spillage etc. Factors like runoff coefficient.

Figure 1.2 Illustration of water requirement in rainwater harvesting (Source: MoANR-2011) Runoff Coefficient Runoff coefficient is the factor which accounts for the fact that all the rainfall falling on a catchment cannot be collected. Some rainfall will be lost from the catchment by evaporation and retention on the surface itself. The Runoff coefficient (Cr), for any catchment is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface. It is calculated as follows:

Runoff coefficient (Cr) = Volume of runoff/volume of rainfall

The Runoff coefficient accounts for losses associated with leakage, evaporation and overflow for a roof catchment system. It is normally taken to be 0.8 for metal roofs, but can have higher values if the roofs and gutters are well constructed. It has lower values for most other types of roofing material. For natural ground catchments, it is less than 0.3 and actual figures depend on various characteristics of the catchment. Some typical values are given in table 1.1 Table-1.1 Runoff Coefficients for Various Catchment Surfaces

Type of Catchment Runoff Coefficient (Cr) Roof Catchments Corrugated metal sheets Tiles

0.7-0.9 0.8-0.9

Ground Surface Covering Concrete Brick pavement

0.6-0.8 0.5-0.6

Untreated (Natural) Ground Catchments Soil on slope less than 10% Rocky natural catchments Green area

0.0-0.3 0.2-0.5 0.05-0.1

Source: Source: (Pacey and Cullis 1989)

1.3 Techniques for determining storage size Water storage capacity is required to balance out the differences between rainwater supply and household demand. If rainwater supply exceeds demand in any given month, storage is needed to allow this water to be carried over and use in a future month when demand exceeds supply. Dry-season demand versus supply This is the simplest approach to system design but is relevant only in areas where distinct dry seasons exist. In this approach the tank is designed to accommodate the necessary water demand throughout the dry season. The dry season is taken as the period during which there is no rainfall. Thus if the daily household water demand is 100 litres and the dry season lasts for 120 days a storage structure with a capacity of at least 12,000 litres would be required. This method is easy to calculate and provides a rough estimate of storage volume requirements. However, it does not take into account:

(i) Variations between different years, (ii) Rainfall input, or (iii) Capacity of the catchment to deliver the runoff necessary to fill the storage structure.

This technique has some advantages. It can be used in the absence of any rainfall data and is easily understandable by the layperson. These points are especially relevant when designing systems in remote areas where obtaining reliable rainfall data may be unavailable. Graphical method

In this approach, the basic steps that have to be followed are: Plot a bar graph of mean monthly roof runoff Plot a cumulative roof-runoff graph by summing the monthly runoff totals Add a dotted line showing cumulative water use.

The storage volume needed is equivalent to the greatest differences between the available rainfall supply and consumption/water demand. Statistical methods

A number of statistical methods have been developed which can be used in combination with other methods such as mass curve analysis to determine the reliability of supply or in other words the probability of system failure. By applying standard statistical techniques the minimum rainfall with a given probability can be determined for the various time periods. If the cumulative minimum rainfall values are plotted against time a mass curve can be derived and mass curve analysis conducted Computer based methods

The use of computer-based models allows great flexibility when producing output for system design since the model can be tailored to any particular system under given rainfall conditions. The format of the output can also be customized to requirements and the performance of specific designs simulated under various demand scenarios.

Rainwater Conveyance Systems

Rain water conveyance systems are components of a RWH system that collect water from catchment surfaces and transport it to the inlets of storage facilities. Each type of rainwater catchment has a conveyance system that is appropriate to the specific type of RWH system.

1.4 Prevention/ control of common problems in storage structures

Water-borne diseases

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. The livestock can contaminate the water with zoonotic diseases and dip chemicals. In addition, if high levels of nitrates e.g. as fertilizer effluents should not be allowed into reservoirs as the cause pollution. Several approaches to mosquito control have been tried with some success. These include the addition of small amounts (5ml per 1000 litres) of domestic kerosene, and various forms of biological control such as using fish and dragonfly larvae to consume mosquito larvae. Although insecticides are sometimes sprayed on open water breeding sites, these should not be applied to rainwater stored for consumption. Cracking

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. Evaporation

Large quantities of water are lost from storage structures through evaporation. However, there are methods for controlling evaporation. They include a good design or site selection for surface reservoirs, whereby the ratio of storage volume to surface area is optimized. An alternative is to divide the reservoir into two or more compartments. If the storage is small, it can be covered with a roofing material or shaded to protect the water surface from wind and direct sun thus reducing evaporation. It is possible to reduce the rate of evaporation by 50% through a combination of wind breaking and shading. Bush rafts built using logs, tree branches and twigs are good covers. They are easy to make and will cost appreciably less as compared to other means such as netting or plastic films.

Seepage

Water stored in either surface or sub-surface structures may leak or be subject to seepage. Seepage through the wall increases the risk of breaking and failure of the wall. The main factors contributing to this problem are the soil type and the amount of compaction of the embankment. This can be prevented by careful selection of the site for the structure. Sites with sand or gravel should be avoided. 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. Siltation

Siltation is caused by various factors including 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. This data is necessary for designing dead storage and for estimating the life span of the reservoir. Silt accumulation in a reservoir can be reduced by careful site selection to avoid highly erodible areas. Also, sediment traps should be constructed at the entry point of runoff into the structure.

1.5 Hygiene in Rainwater storages and uses

1.5.1 Rainwater- quality standards It is normally recommended that international standards e.g. WHO standards, should be achievable for harvested rain water, but the rules are usually made less stringent in tropical regions and developing countries. Faecal coliform counts are the most appropriate indicator of cistern water quality and the following three-tier classification is suggested as a useful guide to cistern water quality originating from rooftop runoff:

Class I 0: faecal coliforms/100ml Class II 1-10: faecal coliforms/100ml Class III > 10: faecal coliforms/100ml

In this classification, Class I represents the highest and ideal water quality, Class II represents water unacceptable for drinking purposes It has been postulated that another possible cause of adverse health conditions associated with the exclusive use of rainwater may result from its low mineral content. Although there is some debate over whether or not all mineral requirements can be met from dietary sources alone. Minerals in drinking water may make a significant contribution. Russian studies have shown that drinking distilled or weakly mineralized, desalinated water may result in adverse health conditions. Such as increased urinary excretion. The possibility of similar effects from prolonged exclusive consumption of rainwater deserve further investigation.

1.5.2 Treatment of stored rainwater While rainwater quality will not always match WHO or national drinking water standards when compared with most unprotected, traditional water sources rainwater from well-maintained roof catchments usually represents a considerable improvement and is generally safe to drink without treatment. Rainwater from ground catchment systems is not recommended for drinking unless first boiled

or treated. Except in heavily urbanized and industrialized areas or regions adjacent to active volcanoes,

atmospheric rainwater is very pure and any contamination of the water usually occurs after contact with the catchment.

The chemical and physical quality of stored rainwater is normally high. Care should be taken to avoid any possible sources of lead or other heavy metals e.g. from lead-based roof paints.

A degree of chemical and microbiological contamination of roof rainwater runoff is inevitable, but this will not generally cause a problem if the roof gutters and storage tank are properly maintained and regularly cleaned and inspected.

Reports of disease outbreaks linked to roof water sources are rare. A few cases of gastrointestinal illness linked to large quantities of bird or animal droppings on the roof have been reported and appropriate measures should be taken to reduce any risks.

Rainwater tanks can provide breeding sites for mosquitoes, which in some areas act as vectors for diseases such as dengue fever, yellow fever and malaria. It is therefore essential that any openings to the tank are fully screened.

To protect water quality good system design operation and maintenance are essential. Water quality will generally improve during storage provided light and living organisms are excluded from the tank, and fresh inflows do not stir up any sediment.

The use of filters and foul flush diverts can further improve the rainwater quality. Further treatment through boiling exposure to sunlight or ultraviolet radiation and chlorination can be undertaken if there are concerns over the water quality.

1.6 Operation and maintenance of RWH systems Proper operation and maintenance of RWH infrastructure is an important part for the success of the system. It affects the efficiency, effectiveness and durability of the structures and ensures water is available and utilized as planned. Proper maintenance is an important aspect in the management of RWH systems and needs to include, among others, the following activities.

Inspection, regular cleaning and minor repair of the whole RWH system: the catchment, the conveyance, the tank and the various tank components such as tap.

Removal of branches of trees over hanging on roofs. Not only leaves and debris, but also the droppings of birds and small animals contaminate rainwater. Dust and other such dirt also need to be cleaned regularly from the catchment/roof.

Cleaning and minor repair of the conveyance system (gutters and downpipes/gutters) at least once a year;

Inspection of water quality in the tank, testing from time to time and treating/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.

Clean/wash-out accumulated sediment and sludge when necessary; take the opportunity to clean the tank when it is empty.

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.

Inspect regularly the amount of water in the tank, and compare with demand and abstraction rates.

Inspect and maintain/repair/replace water taps. 1.7 Management of rainwater harvesting systems Once a rainwater harvesting system is planned and designed properly, and built with good construction material and workmanship, it is ready to provide the services expected and aimed at meeting the objectives set for it in a sustainable manner. This however requires certain arrangement in terms of putting in place a management system/mechanism that is capable of ensuring the proper operation, maintenance and repair of the RWH system. This is necessary not only for RWH systems, but for any development work and infrastructure. Whenever possible, the management of RWH systems is done by the users themselves, unlike design and construction which in many cases are initiated and implemented by qualified professionals. The users of RWH systems could be individual households, institutions such as schools, or communities for whom the RWH systems are built for communal use. In case where a household individually owns and uses a RWH system, the management is straight forward; the household itself assumes the direct individual responsibility. In cases where a system is owned by an institution, usually a work unit that provides general services or a person/committee is entrusted with the task. In RWH systems that are built for communal use by a community, a water users committee is elected for the management with a trained technician/tap attendant assigned for the day to day work. The communal management of RWH systems is generally more difficult and complicated owing to the communal nature of ownership. It is therefore important that bylaws are developed for such systems with clear guidelines for their management. It would also be useful if the management have links with organizations that are capable and willing to extend support in situations where external assistance needed. The guidelines/bylaws to be prepared for the management of RWH systems need to lay out clear duties and responsibilities in respect of the following items.

The management arrangement/system and responsibilities, Physical safety and protection of the RWH system, Maintenance and control of water quality, Regulation of water abstraction rates, time and appropriate use, Operation, maintenance and repair of the system; and

Allocation/collection of water fees/budget for operation maintenance and repair; and the management of finance and other properties.

Water storage systems operate at a larger scale than runoff farming systems, often on a watershed scale, and thereby necessitate addressing issues like ownership, local institutions and land tenure. They require relatively high capital and labor investments (often too high for individual households) and are relatively complicated systems to design. Service-giving institutions, generally, have very little capacity to disseminate and assist in design of storage water harvesting systems. As with any other technology, it is vital when planning and implementation of rainwater harvesting systems is viewed holistically beyond the technical issues. It is necessary to consider the broader aspects in terms of economic environmental, health and social factors. A key factor in project success is community involvement at every stage from inception to long-term maintenance and operation. Involvement in planning and construction phases will not only help to build skills and a sense of self reliance within communities but also prepare the community better for any future maintenance or repair work.