How to harvest rain water?

Rain Water Harvesting! Seems like the ‘in’ thing. But wait…. it’s not necessary to do all the ‘in’ things in life. Sometimes you’ve to give up things if they are too tedious. RWH does look tedious, isn’t it? A normal refrain for all of us to avoid doing a thing that our ancestors had been doing for ages, and with great success. But then if RWH hasn’t reached all homes, minds and brains, the cause of this failure should also be attributed at lack of attempt by the people with knowhow in convincing the fence sitters. Blame should also be shared by the government, civic authorities who’ve started so many programmes encouraging RWH in urban centres and even rural places but with nobody on the ground who could answer the most basic of queries amongst the target audience; the common man. A part of the blame lies on each one of us as well. We have come to a tipping point where anything to do with Water, Road and Electricity is solely and wholly, a state subject. We are supposed to just wait and wait, and wait before some Sarkari Babu passes the project file and puts us out of our self induced misery. Yes, we can and we should take charge of our Water! Why wait for others? With the most simplest of techniques, and zero amount of rocket science, a Rain Water Harvesting can be successfully implemented by YOU! At this juncture, the author is inclined to believe that reader is already aware of the benefits of RWH and the need to implement the same in view of impending water crisis facing the country. Hence, ‘Why RWH’ is not being addressed via this blog. So to begin with, we need to clear a few concepts!

Concept 1

Water flows freely under gravity. A simple slope is able to transmit water. To illustrate, for water to flow in a closed or open pipe from Point A to Point B, all you need is a potential difference with point A higher than point B. Till the time, this difference of height remains, water shall continue flowing. The flow would cease if both the points are at same potential or same height. Water flows from A to B - Type 2 Water flows from A to B Water flows from A to B - Type 1

No Flow Simple… huh!

Concept 2

Water can flow by defying gravity in the reverse direction, i.e. from lower height level to higher height level with the help of a pressure generating device like a Pump. Water flows from A to B - Defy gravity This ‘wonder’ device called pump is the main driving force that keeps your bore wells working. Submersible Pump is a type of centrifugal pump that’s dropped beneath the ground for pumping out water lying at a lower potential to higher potential surface, i.e. ground surface.

Concept 3

Water pipes laid in the ground and otherwise are given a gentle slope for self sustaining flow in them. This slope is normally expressed in terms of ratio 1:1, 1:10, 1:50, 1:100 or in terms of percentages 1%, 10%, 50%, and 100%. A slope of 1:100 is the same as 1%. It means that for every 100 metres horizontal travel by the pipe, vertical distance changes by 1 metre. concept 3 Concept 4

Evaporation, Rainfall, Transpiration from plants; are all various processes of the Hydrological Cycle that water follows in nature. Following illustration shows the journey of water in our ecosystem. our ecosystem Concept 5

Water assumes whatever the shape of the container is, that it is put in. It displays minimal or almost no viscosity unlike other fluids found in nature like rubber sap, honey etc that flow very slowly due to high viscosity. Water tries to occupy all the open spaces in its vicinity. Due to its physical nature, water easily percolates into soils. The pores between soil particles give it the space to move inside the layers of soil. Rain water enters the soil and gets deposited as Ground Water. For water to infiltrate faster into the soil, favourable soil conditions are also essential. Small grain size of clay and silt make it difficult for water to infiltrate into the ground whereas coarser grains of sand make it a suitable media for percolation of water. A relative comparison of the different sizes is given

A relative comparison of the different sizes Surface Concept 6

Rainfall or Precipitation is measured in terms of height – 10mm of rainfall, 50 mm of rainfall etc. But how’s it possible that rainfall intensity is measured in a uni-dimension, why not volume units like litres, m3? Simply put it, rainfall for a few hours on a small area of few square metres would be in thousands of litres. To collect all of this water and measure it would be a practical challenge. Hence, an easier option of measuring the height of water is taken. Assume your roof to be a cuboid with the top face open. Now, 10 mm of rainfall in an hour would mean that if all the rainwater that falls over the roof for an hour is made to stand, water level would be 10 mm from the roof floor. We know the roof area, so to find the volume of water that has fallen on the roof in an hour; we multiply roof area (A) with water level. Now with some very basic concepts about water explained, let’s move to the next section where we try and see the ‘How’ aspect of Rain Water Harvesting.

How do I harvest my rooftop rain water?

For any successful Rain Water Harvesting Project, you need to identify and put in place the given points;

  • A clean catchment area
  • A transportation network to carry water from the catchment
  • A storage cum Recharge structure that accepts water from the transport network

Catchment Area

In towns and cities, 3 types of catchment areas exist. Flat roof, Sloping roof made of metal sheets and open areas around your houses like courtyards, playgrounds, paved parking etc. Tapping water from the Flat Roof or Sloping Roofs made of tiles or metal sheets is called Roof-Top Rain Water Harvesting. Whereas catching water from open spaces around your house is called Surface Run-Off Rain Water Harvesting.

Conveyance Mechanism

An urban RWH has to imperatively use a conveyance mechanism to transport water from catchment to a recharge structure. This is mostly done by putting in place pipes. These pipes can be of varying diameter, material and strength depending on the need and application. Pipes can be of Rigid PVC type, Stoneware or GI Pipes. Pipes can be laid above the ground, underground, partially above and partially below the ground depending on the site constraints. Stoneware pipes are normally not used over ground. PVC pipes can be used for over ground and underground applications. In fact, PVC Pipes can be put much faster in place by a plumber; hence they are quite popular in Urban RWH systems. These pipes come in various sizes, grades and strengths. The commonly used pipes are of 100 mm diameter (4 inches dia) and 150 mm diameter (6 inches dia). Pipes with IS:4985 standard can be used for RWH applications. This is a BIS Standard, more information about this particular standard can be had from BIS site and document.

Storage System

Water received through conveyance mechanism has to be efficiently discharged into a sump for use at a later date. In a standalone house, the volume of water collected would not be very high. So it can be easily diverted to a sump from where it can be diverted to meet the daily needs of the house. But places with a big catchment area like an apartment complex, volume of water collected would be very high. All of this water, if it is to be stored, would require a huge sump. This might not be economically viable and also there would not be enough space to construct it. Hence, a better way would be to send and store this water in the soil below and draw it at a later date using bore wells or dug wells.

Factors influencing Design of Rain Water Harvesting System

An efficient Ran Water Harvesting system is the one that integrates the above three, in the most economical and user friendly way. Once you are done with that, get ready to reap the benefits of a well designed Rain Water Harvesting system at your home. But to do that, you need to design the above three components with certain factors taken into consideration. These factors are given as below:

Annual Rainfall over your catchment area

The average rainfall data for your area can be collected from Indian Meteorological Website or other such similar sites that host this kind of data. This data will help you know the Rain Water Harvesting Potential of your catchment area. For example, a rainfall of 100 cm on a catchment area of 100 square metres would mean; 100 m2 x 1 m (100 cm = 1 metre) = 100 cubic metres or 1, 00,000 litres. This figure gives you an idea of the return on your investment.

Rainfall Pattern for the Year

Your design of storage sump would depend on the distribution of rainfall through the year. For example, in a place like Jaipur, rainfall is concentrated in about 2-3 months of the year with some sporadic rainfall during rest of the months. But in a place like Kerala, rainfall is experienced for about 7-8 months of the year. So the storage sump in Kerala need not be big as it’ll get replenished by continuous rainfall which is not the case for Jaipur.

Peak Intensity of Rainfall

Peak Intensity of Rainfall is the historical data about the maximum rainfall that has occurred over your catchment area in an hour. This figure helps you design your conveyance and storage, recharge structures for the peak rainfall that might fall over your catchment area. This way, your RWH system shall not get overwhelmed by the sudden deluge. Normally, for a recharge type RWH system with a bore well for sending water into the soil, a sump that can store 15 minutes of peak rainfall is normally good enough. The peak intensity data can be collected from the IMD website.

Water Table Data

Get hold of Water Table Data for your area. This can be procured from Ground Water Authority Office of your area. Another source would be the local bore well contractors who always have a working knowledge of the water level in different pockets of the city. This data helps you know the existing water level and can be helpful in assessing the improvement of water table, year on year. This data is also necessary to design the bore well depth. Normally, recharge bore well is installed till about 5-10 metres above the water table. This is to ensure that water filters through the soil before joining the Water Table. However this gap is variable depending upon the sub soil conditions. The water table should be measured at the Pre Monsoon Level because it’s at its lowest during the dry season.

Soil Data

Soil Data is a very important piece of information. If water is to be stored in underground aquifers, the soil there should be favourable for holding it as well. Each soil has its own properties. Clayey soil absorbs water but doesn’t let it easily transmit through it. Whereas Sandy Soil absorbs water and lets it transmit through it. Hence, this data is required to suitably space the slots in the bore casing pipe so that water is fed into favourable soil layer. This data can be had from Ground Water Authority of your area. Also, local bore well contractors would know the soil structure, they can be asked about it too.

Runoff Coefficient of the Catchment

While designing the recharge structures and conveyance mechanism, volume of water from the catchment would depend upon area, average rainfall and also the runoff coefficient of the catchment area. This coefficient is a dimensionless quantity and is simply expressed as a decimal number. Higher the number, better chances of collecting maximum rainwater from the catchment. For example, sloping roof made of metal sheets would have a coefficient of 0.90 which means that almost 90% of the water that falls over it is collectable. An open lawn with sandy soil beneath and grass cover might have a coefficient of 0.2, this would mean a collectable potential of just 20% of the rainfall over it. A coefficient of 1 is an ideal condition which is rarely achievable. Table below shows the different surfaces and their coefficients

Table below shows the different surfaces and their coefficients

Availability of Piped Water

If there is piped water available and it is sufficient to meet your daily needs, then you may opt to divert the rainwater to a recharge well or bore well. However in absence of piped water, you may opt to direct the water to a sump for your own consumption. An overflow from the sump may be connected to recharge bore well.

Quality of Ground Water

If the quality of ground water is extremely bad with high TDS, Fluoride content, then you may opt to divert the water into a sump for meeting drinking, cooking needs while the rest of the water may be directed to a recharge bore well. Sending fresh water of rainfall to the soil will progressively improve your ground water quality as the concentration of salts would be diluted. In the above few pages, RWH has been explained in a concise manner.

For more information, following websites can be of great help

This blog post has been written by Aditya Sharma, a rain water harvesting consultant from Jaipur. He can be reached at aditya206[at]gmail[dot]com.


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