RUN OFF RAINWATER HARVESTING
THE PATH TO ENHANCED LIVELIHOODS Introduction
Even though agriculture is an integral part of rural livelihoods of communities all over the world, there are many instances when farmers in water scarce areas are unable to su cceed in their agricultural ventures due to the unavailability of water at correct time in required quantities. The aim of this Technical Brief is to create awareness of a successful method of “run off rain water harvesting” which can be effectively used by resource poor farming households, to overcome the hardships of nature. This method has been developed over a long period of time by various institutions experts and community members, and people working with men and women who live in drought situations.
Rainwater for Agriculture
Rain fed agriculture is not a novel idea to drought affected areas. As a counter measure to the unpredictability of rain, communities developed various agricultural practices to suit ‘low water intensive’ agriculture. Rainwater harvesting is one such method. In Sri Lanka, the first evidences of rainwater harvesting can be traced back at least 2500 years to ancient civilizations. The use of rainwater harvesting for agriculture has become common as a result of the intermittent nature of rains in many Asian counties and is widely practiced in areas where there is a failure of the seasonal rains, or drier climatic conditions persist for most part of the year. Rainwater harvesting is a viable option for areas with two or three scattered rainy seasons. In Sri Lanka, even the driest area would get an average annual rainfall of 1250mm. In Sri Lanka, rain occurs only during four months of the year and during the remaining eight months the Monthly Rainfall-SURIYAWEWA rainfall is Represents Andarawewa 1988 minimal. 1989 Therefore, 500 1990 rainwater 450 1991 harvesting 400 1992 technology 350 1993 helps to store 300 1994 water in rainy 250 1995 seasons, for 200 1996 usage, during 150 1997 the drought 100 1998 season.
mm
50 0
1999
F
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Figure 1: Rainfall pattern in Suriyawewa - Hambantota District
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Run off rainwater harvesting
Practical Action
As an example, the rain fall pattern in Suriyawewa divisional secretariat, in Hambantota district is shown in figure 1, which clearly highlights that rain is mainly received during the four months; March - April and October - November.
“Not even a single raindrop should be allowed to flow into the sea without having made use of it for the benefit of the people” - King Parakramabahu the Great (1153-1186AD),
Pollonnaruwa Era
Sri Lankan ancestors were successful in harvesting rainwater through a cascade system of massive tanks. The following data (table 1) indicates the scale of ancient irrigation tanks in which rainwater harvesting was an integral part of their irrigation management. Name of the Tank Parakrama Samudraya Kalawewa Minneriya wewa Kantale wewa Nuwara Wewa Extent (acres) 6614 6380 4670 3263 2960
Table 1: Major constructed tanks in the Dry Zone Sri Lanka tanks These tanks were built to irrigate vast areas and support a large population. The rainwater tanks described in this brief are on a domestic scale, that can store about 10,000 litres of rainwater at a time, and sufficient to irrigate a quarter or half an acre of land, to successfully grow 35-40 perennial crops such as – `Jak` (Artocarpus nobilis), coconut (Cocos nucifera), pomegranate (Punica granatum), orange (Citrus spp) and mango (Mangifera indica).
What is rainwater harvesting?
Managing, controlling and making use of rainwater in-situ or within the vicinity of rainfall is termed as rainwater harvesting.
Run off rainwater harvesting
In this method of collecting rainwater for irrigation, water flowing along the ground during the rains will be collected to a tank below the surface of the ground. The tank is constructed using bricks, which are coated with cement. During storage, it is important to incorporate efficient and effective water conservation methods – by reducing evaporation and also by adopting efficient irrigation techniques. It is a very ‘easy to adopt’ technology proven with many communities in the country that if used properly can be very profitable.
Step-by-step procedure of run off rainwater harvest ing
Selecting a location for the construction of a rainwater harvesting tank Observe the direction of the surface flow of rainwater in the land. Even though some believe that such tanks should be constructed in the lowest lying area of the land, this is not essentially so. Due to the seasonal patterns of rainfall and the high intensity of rains received in Sri Lanka, it is possible to fill a 12,000 litre capacity tank without much difficulty. The tank may be subjected to cracks due to the root zone activities (i.e. ramification), therefore, it is advisable not to construct the tank in close proximity to large trees. The tank should be close to the area of cultivation to ensure ease of irrigation.
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Run off rainwater harvesting
Practical Action
The tank should not be in close proximity to the house or to paths /roadways as it is possible for children and even negligent adults to fall in. As an additional security measure, construct a fence around the tank. The opening of the tank should be to the direction of the flow of rainfall. It is not advisable to obstruct patterns of natural flow of water as there is a possibility of mud and other waste getting into the tank. (The mud filters function only when the water flows directly through them). Things to be bare in mind If a very strong current of water is flowing it could place the tank in jeopardy. If by construction of the tank, the natural water flow is obstructed, soil erosion can occur and crops can consequently be destroyed. Constructing the rainwater harvesting tank Clear the selected land thoroughly. Flattening the land is important for ease of taking measurements. It is advisable to construct a circular tank as it will withstand greater pressures. Determine the quantity of water required for irrigation purposes. In such instances, the following factors should be considered; a.) The rainfall pattern of the area. (If the area experiences regular rainfall throughout the year, a small tank of 4000-5000 litres would suffice, whereas in particularly dry areas which experience dry spells for about 6 months of the year, it would be beneficial to store as much water as possible.) b) The extent of land, which is proposed to be cultivated c) The amount of investment that can be made. The tank should not be more than 1.75 m in depth in order to withstand the pressure of the water. Low depth makes cleaning and use of the tank easier. The chart given below can be used as a guide to determine the radius of the tank. Radius of tank (Meters) 0.9 1 1.125 1.2 1.275 1.35 1.425 1.5
Capacity of tank (Litres)
5000 6000 7000 8000 9000 10000 11000
Table 2: Capacity according to the radius of the tank Take a length of rope as long as the radius and tie it to a wedge. Plant the wedge at the place you want to be the centre of the tank and draw a circle.
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Run off rainwater harvesting
Practical Action
Figure 2: Measurement taken for radius of the tank Now dig-out the soil within this circle.
Figure 3: Pit for the tank construction
The tank should have a slope of about one foot from the periphery to the middle of the tank. Figure 4 will give a side view of the tank.
Walls of the tank
1.75 m
0.3
m
Figure 4: Cross section of a tank After the soil has been removed, a 10 cm slab of concrete has to be laid at the bottom of the tank. The ratio of sand, cement and metal stone in the concrete mixture should be 1: 2: 4 After the slab of concrete is hardened and has completely dried, construct th e walls one foot in height from the inlet with a width of one brick. It is important to use bricks with dimensions of: 5cm x 10cm x 23cm for this purpose. The cement mixture should have a ratio of cement to sand of 4:1.
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Run off rainwater harvesting
Practical Action
Figure 5: Wall construction of the tank on the concrete base As the water inlet is connected to the tank at the ground level, hence, the water inlet wall should not be raised above the surface level.
Figure 6: Measurement taken for the inlet construction The mud filters are attached to the water inlet. Hence the door has to be sturdy. As depicted in the picture below, a concrete slab measuring in metres 0.75m X 1m (height and length) should be laid near the door.
Figure 7: Concrete base for the inlet construction When constructing the water inlet, it is necessary to face it in the direction of the natural water-flow of the garden. As the mud filters should be placed around this door, a drain should be constructed close to the inlet of 0.5 meters (near the door) and 1 metre width. The total length of this drain should be 1 metre.
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