Water harvesting

Water harvesting refers to methods used to collect water (1) from sources where the water is widely dispersed and quickly changes location or form and becomes unavailable or (2) that is occurring in quantities and at locations where it is unusable unless some intervention

is practiced to gather the water to locations where it can provide benefits.

There is a number of water harvesting techniques that are practiced in many of the water scarce areas of the world. However there are many other techniques that have developed from local necessity in sympathy with the local physical conditions and customs that could be adapted to benefit populations in other water scarce locations. A large number of such practices have been described in a number of papers in Pereira et al. (1996) and by Khouri et al. (1995) and included in the bibliography list. Other complementary information is provided in Chapter 7. A number of the widely practiced techniques will now be discussed.

5.4.1. Rainwater collection

Rainwater can provide a considerable water resource, not only in humid regions but also in semi arid and arid regions. Large volumes of water flow from roofs. In many regions roof water has not been collected because traditional roofing materials did not permit easy collection, and storage of collected water was difficult and expensive. However in recent years the ready availability of some form of roof sheeting and innovative ideas for water storage have made roof water a serious water resource consideration. There is a great need to encourage its use and to teach simple, low-cost means of collecting water from roofs, and constructing suitable storage facilities. For example in the dry north east of Thailand storage containers (jars) were constructed by householders from concrete reinforced with wire netting to hold volumes ranging from 100 up to 3 000 litres. Roofs were modified slightly to facilitate collection of all but the first few litres from each rainfall event. These innovations radically changed the lifestyles of the majority of inhabitants of the region, by providing enough water for in-house uses over the dry season. Measures may be needed to keep insects (mosquitoes and flies) away from the stored water to avoid increases in diseases carried by these creatures (malaria, dengue fever). This can be achieved by covering the opening of storage jars with mosquito netting or by carefully disinfecting the stored water (with bleach).

In many regions of the world, rural populations continue to use some traditional methods for rainwater collection for their households and for animals. However, there is a need to pay better attention to these systems and support those populations to improve their primitive systems, to make them more efficient and safe with respect to human health. In such rural areas it is very likely that the villagers are willing to adopt some kind of innovation. Existing systems usually have the potential to be improved and to continue in use for a long time, constituting a valuable alternative to more sophisticated systems, which are generally more difficult to implement and maintain.

5.4.2. Terracing

Terracing can be used to collect water for two purposes. Firstly a horizontal surface reduces runoff and maximises the infiltration of water into the soil. If the soil surface is kept tilled and free of vegetation except for the desirable crop, almost all rain falling on the terrace will be used for crop growth. In regions of low rainfall, soil water can be stored over long periods provided the soil surface is kept tilled or mulched and vegetation free. Thus it may be possible to store up to 2 years rainfall to obtain one cereal crop. If there are several

terraces down a hillslope it may be possible to grow a crop on alternate terraces each year.

Infiltration terraces can support soil water storage to make a crop viable each year where the rainfall total is sufficient. It can also provide for perennial tree crops when these are able to develop extensive root systems, as typically occurs for olive trees in semi-arid Mediterranean areas. In some cases, the cropped terraces are downslope of a runoff area and may infiltrate more water than that provided directly by rainfall, as occurs in the meskats, which are traditional in Tunisia (Missaoui, 1996).

Secondly terraces can be used as temporary water storages to reduce flow velocities of surface water and prevent erosion. The slow flowing water on the terrace may then be directed into a storage facility (excavated hole or small dam) for storage and subsequent use. Without the terraces and their associated flat slopes small storage facilities usually become filled with silt in a very short time. Terracing helps keep the soil where it is needed and useful and provides silt free water for storage.

5.4.3. Small dams

In regions of high evaporation, small and shallow dams are usually a cause of high evaporation losses. However in groundwater recharge areas, small dams, or even low embankments across floodways, can be used to increase aquifer recharge. On grasslands used for grazing the encouragement of infiltration by low embankments which reduce flow velocities and hold water for a few hours after rain events can increase soil water and greatly increase the time after rain that water is available to vegetation roots. By this means it may be possible to double annual biomass production in semi arid regions.

A major difficulty in management of small water supply dams is siltation. Small dams will usually be constructed on small headwater streams. These are often steep and can carry large amounts of silt. If the land surface upstream of the dam is in its natural state the silt and suspended solids load may be small, but usually the upstream area will already have been heavily affected by human activity. Most human activity increases erosion and sediment loads in watercourses. Exceptions occur where large efforts are made to keep erosion to a minimum.

If no attention is given to sediment control most small headwater dams will fill with sediment within a few years, with almost total loss of the water source and the effort and/or capital invested in the construction. The most effective erosion prevention is complete protection of the upstream land from human activity (including grazing of their livestock animals). This is usually unrealistic. Next best is to take intensive measures to prevent surface and stream channel erosion. Third best is to provide a sediment trap upstream of the reservoir. This will usually take the form of a region where flow velocity will be reduced almost to zero so that sediment will be deposited before the water flows into the main storage. Sediment traps need to be quite large to reduce velocities sufficiently for suspended materials to be deposited. It may be possible to achieve this with a wide horizontal section in the approach channel. The sediment trap must be constructed to allow easy maintenance. The sediment will need to be removed periodically (perhaps as often as annually) and so the trap must be arranged so that locally available removal equipment can be used to clear the basin with minimum effort and cost. It is obvious that a sizeable proportion of the total capital cost of the small dam must be invested in the sediment trap (perhaps 20% of the total cost) and that a large proportion of the ongoing maintenance cost

(perhaps 80%) will be used for clearing the sediment trap.

An alternative to an active sediment trap is to allow the sediment basin to fill and then to plant a hardy crop on it, usually olive and palm trees. Provided this cropped surface is large, it can continue to remove sediment from flows into the reservoir. However it must have a large surface area, otherwise sediment will pass over it and enter the reservoir. Good examples of this technique are the tabias and the jessours, which are traditional in North Africa (Missaoui, 1996; Prinz, 1996). These systems not only provide for life support of the rural populations but also create beautiful landscapes in quite arid environments.

Animal and human access to the edges of reservoirs behind small dams is another serious cause of damage to the storage capacity and to the water quality. While animals wading and wallowing at the edge of the reservoir may be thought to be kind to the animals, this activity will quickly destroy the water resource. Continual stirring up of edge sediment by the animals reduces the water storage capacity, causes the stored water to be continually turbid, and introduces dangerous bacteria and nutrients. Small reservoirs should be securely fenced and water should be supplied to both humans and animals at valved collection points and drinking troughs outside the fence. When required, power to lift water to these points can be supplied by small hand pumps, animals or a simple wind mill.

Without careful consideration of sediment management and provision of resources for this activity most small headwater dams will be condemned to early failure. In most cases combinations of the above measures will be needed to prevent loss of the reservoir by siltation.

5.4.4. Runoff enhancement

In places of water scarcity where the availability of water needs to be increased it is possible to enhance runoff from rainfall events by partially sealing the soil surface. This can be done by applying surface sealing materials or by compacting the soil surface.

Application of surface sealants (such as bitumen) is expensive and provides increased runoff for only a limited time. The sealing materials usually weather rapidly and are penetrated (from below) by vegetation so that their integrity as a surface seal is usually lost quite quickly.

Compaction of the surface requires labour and perhaps some machinery but compaction tends to be longer lasting than adding surface covering materials. In some cases, it may be necessary to import aggregates to mix with the surface layer (perhaps to a depth of 20 mm) to provide a high density material at the surface, and to add some cementing agent to prevent the surface layer being removed by wind and water. The surface can be compacted by one or two passes of machinery, by using hand tamping or by the passage of many feet, particularly hoofs of sheep and goats. Compacted natural soils, or modified soils can have very low infiltration characteristics and provide up to 80% of all rainfall from a storm as runoff. Even small areas can deliver significant quantities of water.

For example a 12.5 mm rainfall event could produce 1000 litres of water from a 100 m² compacted area.

Prepared runoff enhancement surfaces need protection from damage and general maintenance to preserve their impermeable characteristics. There is also a need for collection of the runoff in a suitable container from which it will not be lost by seepage or

evaporation. The modified surface and any delivery channels need to have flat, but definite gradients to move water to the collection point without causing erosion of any of the surfaces. Care is needed to choose enhancement materials which are non toxic and do not produce toxins as they break down over time.

Runoff enhancement is often practiced to grow trees and vegetables. Water can be gathered from a semi sealed surface to flow towards a tree or small vegetable plot to infiltrate the soil in the vicinity of the plant roots. In this way water availability for crops can be increased many fold over the natural rainfall to provide sufficient for growing essential life-support or high value crops in regions of water scarcity. Missaoui (1996) and Prinz (1996) describe many of these runoff enhancement arrangements, generally called micro-catchment and medium-sized catchment water harvesting systems.

5.4.5. Runoff collection

In undulating to flat terrain where runoff collects into large numbers of very small stream channels, the available water resource may be quite large, but its diversity makes it difficult to use. Under these circumstances it is often possible to capture part or all of the flow in a small channel and divert it to another channel. Diversion channels which run almost parallel to the contours can be used to carry this diverted flow to a different location. If flows from a few of these channels can be gathered there may be enough water to fill an excavated hole or small dam which is deep enough to prevent loss of all the water by evaporation. The small diversion channels, following the contours, with a slight slope, can often be constructed using hand tools, or animal power, or with simple earth moving equipment such as a road grader blade. In this way it may be possible to capture flow from several stream channels, each draining a few tens of hectares, to gather water from two or three hundred hectares to a single location. Five millimetres runoff from 200 ha represents 10 Ml of water. If 75% is removed by evaporation and seepage, this is still enough to supply the in-home needs of 100 people for 1½ years. There are many landscapes where there is small relief where many small streams flow. However when these streams reach the almost horizontal surrounding land the water is dispersed and quickly infiltrates. If excavated holes were dug in the undulating terrain above the flat area, and water was diverted via near horizontal catch drains to these holes, a relatively large water resource could be available.

It is also possible to use the water that is temporarily held in horizontal catch drains and infiltrates to grow a row or two of a crop, in the bed. Such techniques are described by Khouri et al. (1995).

5.4.6. Flood spreading

Flood flows are a feature of all landscapes, including regions of water scarcity. A very large part of the annual flows may occur in one or two floods, but the flow is often so large that the water passes through the region and can not be used where rain fell. Some advantage can be gained from these large flows by encouraging them to spread across flat areas. If water can be retained on flat surfaces for a day or two the upper soil layers may be saturated or water may percolate downwards to replenish the local aquifer. In both these circumstances the water thus “harvested” is available for later use, in the first case for

growth of crops or to support grazing and in the second case for whatever purpose groundwater is used (Prinz, 1996; Missaoui, 1996).

Where the terrain is suitable it may be possible to restrict flow in the river channel and force water to flow over the floodplain or into an old floodway. Flood spreading by these means can most easily be achieved where the upstream-downstream gradient of the river valley is quite small. With small longitudinal gradients water forced onto the floodplain will tend to flow in the downstream direction very slowly, allowing maximum time for infiltration to occur.

Channel flow can be restricted by placement of large rocks, dams and erodible dikes in the bed or by securing a log in the stream bed. Overbank flow velocities can be reduced by construction of low banks (perhaps 30 cm high) across the direction of flow. Low, wide banks will become stabilised by natural vegetation and are likely to need minimal maintenance. However large maintenance is required after a large flood destroys the constructed infrastructure every few years. The benefits of flood spreading, where the terrain is suitable, can be quite large in terms of increased biomass production and increased groundwater resources.

5.4.7. Water holes and ponds

Natural water holes and ponds can be exploited for water supply purposes. There is a need to ascertain the source of the water – is the hole fed by groundwater or by small surface water flows after each rainfall event? If the supply is from groundwater it may be possible to treat the pond as a well and increase the extractable water by making the pond deeper and increasing the hydraulic gradient towards the pond. If the water is supplied by surface flows in a stream bed there are several possible ways to increase the water availability. One means could be to raise an embankment across the stream bed downstream of the water hole to increase the volume of water captured during each flow event. This is a kind of small dam. The embankment will need to be carefully designed and constructed to prevent its destruction in the next significant flow in the stream.

It is possible that the water hole exists because there are water tight strata below.

Raising the water level may not increase the volume stored, but may supply water to the permeable strata above the natural water level of the hole. In such a case increasing the available resource may only be possible by excavating a larger hole in the water tight stratum (very expensive) or by pumping the excess water to another storage location during the brief period the water is above its natural level.

The water supply obtainable from a natural pond can often be increased by raising an embankment around the pond, or by increasing the depth of the pond. Increasing the surface area of the pond usually only increases the removal of water by evaporation, but increasing the depth of a pond can often provide significant benefits. If an embankment is to be raised, excavating materials from the pond bed and thus deepening the pond can provide the added benefit of increasing storage capacity without increasing evaporation loss. It must always be remembered that in areas of high potential evaporation, depth of water is the key to effective water storage.

5.4.8. Tanks

The word “tank” has several different meanings. In some countries, particularly in south Asia, a tank is a water hole. It is often an enhanced natural pond or it may be an excavated hole. In South India and Sri Lanka it is a small surface reservoir. These types of tanks have been dealt with in previous sections.

In other countries a tank means a constructed water container. It is usually fabricated from sheet steel, cement concrete or plastic. Such containers are expensive, relative to the volume of water they store, but they have an important set of advantages over larger, landscape-type storages. They are of particular importance where the resource is very limited and needs to be protected from evaporation and/or contamination.

As discussed elsewhere it is not possible to prevent large evaporation losses and some seepage from ponds and lakes. However small tanks, with capacity anywhere from 1000 l to 10 Ml can be totally enclosed, almost completely preventing losses. Such storage containers are worthwhile where limited amounts of roofwater are collected or where water is desalted at considerable expense. Constructed containers also offer easy prevention of contamination since access to the stored water can be controlled. This includes prevention

As discussed elsewhere it is not possible to prevent large evaporation losses and some seepage from ponds and lakes. However small tanks, with capacity anywhere from 1000 l to 10 Ml can be totally enclosed, almost completely preventing losses. Such storage containers are worthwhile where limited amounts of roofwater are collected or where water is desalted at considerable expense. Constructed containers also offer easy prevention of contamination since access to the stored water can be controlled. This includes prevention