Submitted By
DILEEP RAJ.K

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ABSTRACT
Water is most abundant component on earth surface, constituting about 70% of the planet surface. Although some 70% of the Earth's surface is covered by water much of this is saline. However freshwater is available in almost all populated areas of the earth although it may be expensive to access the water and the supply may not always be sustainable. Only 4% of the available water is suitable for drinking purpose.
Throughout most of the world, the most common contamination of raw water sources is from human sewage and in particular human faecal pathogens and parasites. In 2006, waterborne diseases were estimated to cause 1.8 million deaths each year while about 1.1 billion people lacked proper drinking water.
Here comes the necessity of water conservation. Rainwater harvesting is one of the suitable methods for water conservation.
1) INTRODUCTION
What is rainwater harvesting?
The term rainwater harvesting is being frequently used these days; however, the concept of water harvesting is not new for India. Water harvesting techniques had been
Evolved and developed in centuries ago.
Ground water resource gets naturally recharged through percolation. But due to indiscriminate development and rapid urbanization, exposed surface for soil has been reduced drastically with resultant reduction in percolation of rainwater, thereby depleting ground water resource.
Rainwater harvesting refers to collection and storage of rainwater and also other activity such as harvesting surface water extracting ground water, prevention of loss through evaporation and seepage
Why harvest rainwater?
This is perhaps one of the most frequently asked question, as to why one should harvest rainwater. There are many reasons but following are some of the important ones.
• To arrest ground water decline and augment ground water table
• To conserve surface water runoff during monsoon
• To reduce soil erosion
• Surface water is inadequate to meet our demand and we have to depend on ground water.
• Due to rapid urbanization, infiltration of rain water into the sub-soil has decreased drastically and recharging of ground water has diminished.
• Over - exploitation of ground water resource has resulted in decline in water levels in most part of the country.
• To enhance availability of ground water at specific place and time.
• To arrest sea water ingress.
• To improve the water quality in aquifers.
• To improve the vegetation cover.
• To raise the water levels in wells & bore wells those are drying up.
• To reduce power consumption.
Purpose of Rainwater harvesting
• Stored for ready use containers in ground or below ground
• Charged into the ground for withdrawal later
Benefits of Rainwater harvesting
• Rainwater harvesting prevents flooding of low lying areas
• Rain water harvesting replenishes the ground water table and enables our dug wells and bore wells to yield in a sustained manner
• It helps in the availability of clean water by reducing the salinity and the presence of iron salts
Rainwater harvesting Techniques
• Storage of Rainwater on surface for future use
• Recharge to ground water
2. STORAGE OF RAINWATER ON SURFACE FOR FUTURE USE
The storage of rain water on surface is a traditional technique and structures used were underground tanks, ponds, check dams, weirs, etc. Rainwater flows away as surface runoff. This runoff could be caught and used for recharging aquifers by adopting appropriate methods. Some of the methods are
• Check Dams
• Subsurface Dams
• Farm Ponds
• Rainwater harvesting from rooftop catchments
2.1 Check Dams
Check dam is a small dam, which can be either temporary or permanent, built across a minor channel, swale, bios wale, or drainage ditch. Similar to drop structures in purpose, they reduce erosion and gulling in the channel and allow sediments and pollutants to settle. They also lower the speed of water flow during storm events. Check dams can be built with logs, stone, or sandbags. Of these, the former two are usually permanent or semi-permanent; and the sandbag check dam is usually for temporary purposes. Also, there are check dams that are constructed with rock fill or wooden boards. These dams are usually used only in small, open channels that drain 10 acres (0.040 km2) or less; and usually do not exceed 2 feet (0.61 m) high.
2.2 Subsurface dams
Subsurface Dams are groundwater dams, which are structures that intercept or obstruct the natural flow of groundwater and provide storage for water underground.
They have been used in several parts of the world, notably India, Africa and Brazil. They are used in areas where flows of groundwater vary considerably during the course of the year, from very high flows following rain to negligible flows during the dry season. Groundwater dams can be divided in two types: subsurface dams and sand storage dams.
A subsurface dam intercepts or obstructs the flow of an aquifer and reduces the variation of the level of the groundwater table upstream of the dam. It is built entirely under the ground
2.3 Farm ponds
Farm ponds are man-made bodies of shallow water constructed for several uses. Most landowners build farm ponds for leisure, agricultural, or aesthetic purposes. Fertilizing a farm pond is quite popular for people who would like to fill the pond with different fish or adapt it for business. Conceptualizing a farm pond design, on the other hand, is going to be a major concern of anybody who wishes to make the farm pond as a recreational area. Both the case, building a farm pond had been quite a popular endeavor nowadays and a number of them were constructed on the countryside where water sources are more than adequate.
Farm pond building follows certain principles in order for it to be done correctly. This is important especially if the farm ponds are going to be used commercially and revenue is expected. Here are the things you should consider when building a farm pond:
2.3.1 The Basic Principles of a Farm Pond
Adequate Water Source
To ensure functionality, farm ponds are supposed to maintain a certain water level. It is then essential that the precise spot where the pond is going to be built have bountiful water supply. There are four types of water sources - overland drainage, groundwater, in-stream flowing water, and diverted water flow. Either one of these sources should be available for the pond.
Proper Drainage Area
This is required if the farm pond is going to be used agriculturally or if the pond will contain schools of fish. Fertilizing a farm pond is only possible with the installation of a drainage area having the right measure and runoff volume. The evaporation rate from a farm pond should also be considered so that the right drainage area maybe constructed.
2.4 Roof top rainwater harvesting
It is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the catchments, and the rainwater is collected from the roof of the house/building. It can either be stored in a tank or diverted to artificial recharge system. This method is less expensive and very effective and if implemented properly helps in augmenting the ground water level of the area.
2.4.1 Components of the roof top rainwater harvesting system
The illustrative design of the basic components of roof top rainwater harvesting system is given in the following typical schematic diagram/
The system mainly constitutes of following sub components:
• Catchment
• Transportation
• First flush
• Filter
2.4.1.1 Catchment
The surface that receives rainfall directly is the catchment of rainwater harvesting system. It may be terrace, courtyard, or paved or unpaved open ground. The terrace may be flat RCC/stone roof or sloping roof. Therefore the catchment is the area, which actually contributes rainwater to the harvesting system.
2.4.1.2. Transportation
Rainwater from rooftop should be carried through down take water pipes or drains to storage/harvesting system. Water pipes should be UV resistant (ISI HDPE/PVC pipes) of required capacity. Water from sloping roofs could be caught through gutters and down take pipe. At terraces, mouth of the each drain should have wire mesh to restrict floating material.
2.4.1.3. First Flush
First flush is a device used to flush off the water received in first shower. The first shower of rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the probable contaminants of the atmosphere and the catchment roof. It will also help in cleaning of silt and other material deposited on roof during dry seasons Provisions of first rain separator should be made at outlet of each drainpipe.