Line 49: | Line 49: | ||
| Kantale wewa|| 3263 | | Kantale wewa|| 3263 | ||
|- | |- | ||
| Nuwara Wewa || 2960 } | | Nuwara Wewa || 2960 | ||
| } | |||
Revision as of 21:07, 29 January 2008
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 succeed 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 rainfall is minimal. Therefore, rainwater harvesting technology helps to store water in rainy seasons, for usage, during the drought season.
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.
Table 1: Major Constructed Tanks in the Dry Zone Sri Lanka tanks
Name of Tank | Extent (acres) | |
Parakrama Samudraya | 6614 | |
Kalawewa | 6380 | |
Minneriya wewa | 4670 | |
Kantale wewa | 3263 | |
Nuwara Wewa | 2960 | }
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?
Step-by-step procedure of run off rainwater harvestingSelecting a location for the construction of a rainwater harvesting tank
Things to be bare in mind
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. • 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. • 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 • 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. • 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. • 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. Mud filters
• Now plaster the tank completely with cement. In this case you should plaster the outside of the tank about 15 cm above the ground level. • You would have observed an empty space of several inches, which has been left around the tank during the construction, to facilitate the process of construction. This empty space should be filled tightly with sand. Sand is used for the filling of this space as it can be packed tightly and is not easily subjected to decomposition. In the instance of repairs being needed for the tank, this sand layer will allow access to the tank. Best practices of operation and maintenance
• A small thatched hut and fence should be constructed around the tank to reduce the evaporation of water and for the security of children & domestic pets.
How to use water efficiently for irrigation
References and further readingMicro irrigation Practical Action Technical Brief Wisdom of traditional collection of rainwater for domestic use by Ariyabandu R D E S,
Lanka Rain Water Harvesting Forum, 1998
Practical Action South Asia |