Scientific Water Harvesting
What is Water Harvesting
- It means capturing rain where it falls or capturing the run off in your own village or town. And taking measures to keep that water clean by not allowing polluting activities to take place in the catchment.
- Therefore, water harvesting can be undertaken through a variety of ways
- Capturing runoff from rooftops
- Capturing runoff from local catchments
- Capturing seasonal floodwaters from local streams
- Conserving water through watershed management
These techniques can serve the following the following purposes:
- Provide drinking water
- Provide irrigation water
- Increase groundwater recharge
- Reduce stormwater discharges, urban floods and overloading of sewage treatment plants
- Reduce seawater ingress in coastal areas.
- In general, water harvesting is the activity of direct collection of rainwater. The rainwater collected can be stored for direct use or can be recharged into the groundwater.
- Rain is the first form of water that we know in the hydrological cycle, hence is a primary source of water for us. Rivers, lakes and groundwater are all secondary sources of water. In present times, we depend entirely on such secondary sources of water.
- In the process, it is forgotten that rain is the ultimate source that feeds all these secondary sources and remain ignorant of its value.
- Water harvesting means to understand the value of rain, and to make optimum use of the rainwater at the place where it falls.
WATER CONSERVATION THROUGH RAIN HARVESTING
- In spite of astonishing achievements in the field of science and technology, nature remains a mystery for human beings. Though water is being obtained through desalination and artificial rain by cloud seeding. The shortage of water even for drinking purpose is a perpetual phenomenon throughout the world, especially in the developing and underdeveloped countries.
- In most of the cities, the water supply sector is facing a number of problems and constraints. The pace of urban development and the increase in population in the urban areas have resulted in exploitation of water resources to the extremes. Fresh water sources are being heavily exploited to meet the demands of the urban populace.
- Failure of monsoon makes the situation worse. As surface water sources fail to meet the ever- increasing demands, ground water reserves are tapped, often to unsustainable levels.
- Also, the fast rate of urbanisation reduces the availability of open spaces for natural re-change of rain water. Unplanned and uncontrolled extraction of ground water would disturb the hydrological balance along the coastal areas which results in possible sea water intrusion.
- Hence, it is necessary to take up measures to conserve and increase the renewable water resources in all possible ways. Ground water recharge by rain water harvesting (RWH) is a simple and cost effective way.
- Water crisis occurs in the absence of effective collection and storage of rain water. If only we save every drop of water and recharge the underground aquifer, we can rescue ourselves from this perpetual problem of water scarcity.
- The potential of rain to meet water demand is tremendous. Unless people are involved in conserving rain water from individual households to industrialists, it would be very difficult to meet the looming water crisis.
Water-harvesting Techniques
- Four main groups of water harvesting techniques can generally be distinguished: micro and macro-catchments, floodwater harvesting and storage reservoirs.
- Typical micro catchment techniques involve the delineation of natural depressions, the construction of contour and stone bunds, systems for inter-row water harvesting, terracing, construction of semicircular (half moon) and triangular (V-shaped) bunds, eyebrow terraces, Vallerani-type micro-catchments, pits, meskats and negarim.
- Macro-catchments include large semi-circular and trapezoidal bunds and hillside conduits.
- Floodwater can be harvested within the stream bed or diverted to the cropping fields.
- Storage media include underground storage reservoirs such as soil and sediment, and cisterns, and surface storage media like tanks, jars, ponds and reservoirs.
- The catchment or water-collection area can be a roof top, a small land surface, a slope or a larger catchment area feeding seasonal water courses.
Various Techniques
- Jessour – This is an old runoff water harvesting technique widely adopted in arid highlands, which occupies the runoff watercourses. The hydraulic unit of a jessour is the jessr consisting of three components: the impluvium, the terrace and the dyke. The impluvium or the catchment area is used for collecting (harvesting) the runoff water. The terrace or the cropping zone is the area where crops or trees are grown and where the runoff water is caught. The dyke is a barrier established to block the sediments and runoff water. Its body is made of earth equipped with a central and/or lateral spillway and one or two abutments. This should assure the evacuation of excess water.
- Tabias – This is a replica of the jessour system constructed in the foothill and piedmont areas. It is a relative new technique constructed by mountain dwellers.
- Meskat – It is a traditional system consisting of two compartments, a catchment area and a downslope cropping area, both delineated by low bunds. Catchment and cropping area are connected by a spillway.
- Floodwater harvesting system – It is on old technique which diverts the total or a portion of the floodwater carried by wadis – dry riverbeds that only contain water during and immediately after heavy rains – to the neighboring cultivated fields, in the form of natural irrigation. It has three components including a diversion dam, a distribution network and the cropping fields. The diversion dam is generally made of earth acting as a fuse by breaking down in case of very intense floods. Recently, gabions and reinforced concrete are becoming most popular. The distribution network is made of open trapezoidal canals (primary, secondary, tertiary, …) with gentle slopes except in partition points to avoid silting up. The cropping fields are generally flat with a rectangular shape and delineated by earthen embankments to retain up to 1 m of water.
- Gabion units – These units are made of wire mesh cages filled with rock, and are constructed in wadi beds in order to divert water directly to the neighboring fields, often located within the wadi itself.
- Recharge wells – These are casting tubes drilled into the underlying bedrock – when of very low permeability – enhancing the infiltration of runoff water to the ground water table. This technique was adopted only recently in Southeastern Tunisia.
- Terraces – This very old technique is constructed on steep slopes and consists of small retaining walls made of rocks to slow down the flow rate of the water and to control erosion.
- Cisterns – These were traditionally used to provide drinking water. A unit consists of three components, an impluvium, a sediment settling basin and a storage reservoir. The impluvium is a sloping piece of land delineated by a diversion channel or a paved runoff area. The settling basin ahead of the entrance of the cistern allows sedimentation of runoff loads. The storage compartment or cistern consists of a hole dug in the ground, coated with gypsum or concrete, and which is leading to a stone-faced underground small or large-size tank.
World’s largest rainwater harvesting project in Karnataka
- Municipal water supply in most Indian cities is unreliable. Many villages in India do not have potable water supply. Hence Rainwater Harvesting (RWH) has been proposed as an ideal sustainable solution.
- The Karnataka State Council for Science and Technology (KSCST) has undertaken steps to promote, implement and create awareness on sustainable water harvesting and groundwater recharge concepts.
- Rainwater harvesting entails the collection of rain where it falls in a scientific and controlled manner for future use.
- RWH consists of rooftop water harvesting, water from open areas such as paved ways, parks, roads, fields and in lakes and ponds.
- Among the three projects initiated by the council, ‘Rainwater harvesting in rural Karnataka’ funded by Rural Development and Panchayat Raj Department (RDPR), Government of Karnataka is probably the largest rainwater harvesting project in the world.
- All the schools of Karnataka were carefully studied for the water availability, quality (fluoride contamination) and proximity to the secondary water source. Under this programme 23,683 schools were identified in the first phase to provide drinking water through rooftop rainwater harvesting using a very simple but rugged system.
- The RWH system for schools consists of collection of rooftop rainwater and channelizing through PVC pipes or gutters (PVC pipes slit into two halves) and stored in a fully enclosed surface tank.
- The rainwater is filtered through a sand bed filter built over the surface tank before storing. A first flush separator is part of the pipeline which allows the first rain to be flushed out with the contaminants on the roof and subsequent, relatively cleaner water is allowed to pass through sand bed filter to be stored in the tank.
- In the design of the tank and the sand bed filter, care has been taken to maintain good quality of harvested water.
- The surface tank is deliberately built without a direct manhole for entry inside and one or more taps have been provided at the bottom. This prevents the sunlight from entering the tank (presence of sunlight and air together supports growth of bacteria, algae, etc. inside the water).
- The absence of a regular manhole prevents withdrawal of water by dipping vessels and also avoids undesirable elements (lizards, cockroaches, dust, etc.) entering the storage tank. The bottom of the sand bed filter itself is fabricated as a manhole with perforations and can be removed for cleaning the tank when the sand bed is removed for periodic cleaning.
- The size of the tank and the pipes for collection from the roof are designed using parameters like local annual rainfall, seasonal variation, number of students in the school, and roof area available.
- A systematic training and awareness programme was conducted for all the stake holders across the state at various levels (Officers at the planning level in the government to plumbers and masons in the field at a remote village).
