Rain Water Harvesting Structure

This is something I have mentioned in this series earlier. In Kerala, we get three lakh litres of rain every year in 1,000 sq. ft, which is roughly the area of a medium-sized house. Yet we hear of water scarcity in many places There is no potable water. No water for irrigation. Water is flowing off and draining completely into the sea. The solution to this problem is Rain Water Harvesting (RWH). A few viewers wanted to know more about it. I don’t have scientific or technical knowledge regarding it. But today we have an expert in this field as our guest. He is Mohankumar sir, a hydro-geologist who used to work with the Land Use Board. For the last 25-30 years since his retirement, he has been running an NGO that is engaged in activities for  the conservation of water and forest. Let us learn from his rich experience, and find out ways by which we can conserve water in our own compounds.

Sir, namaskaram. As part of Rain Water Harvesting in Kerala, people, including myself, dug rain pits in our backyards. The belief was that only then would the government give permission for RWH. Later it was rumoured that rain pits were not necessary; a small remittance of money was enough to procure permission. I don’t know what the actual fact is. If everyone in Kerala had dug rain pits during last 15 years, all of us would have had excess water by now. But in actuality, we are unable to utilize rain water, isn’t it, sir?

What you said is right, Hari. We get 3,000 mm of rain but still suffer water scarcity. That is the paradox of the situation. We need to discuss how to address the issue. When it rains, the water should not flow speedily down but rather descend slowly. That is what we should do. The average width of Kerala is 29 km,  and that is a steep slope. So when it rains, the water quickly reaches the sea.

That is, from the high ranges to the Arabian Sea, the average distance is only 29 km.

Yes. Therefore, rain water will flow down our 44 rivers and drain into the sea. How can we stagger it? Contour bunding is a method whereby we can prevent water from flowing away, and force it to seep underground. The second method is digging rain pits.

Sir, how can we dig rain pits in a scientific manner?
We recommend that small pits be made within the compound. As for contour bunding, it is like the terraced farming we see in rubber estates.
As the water flows from the top, we block it . . .

Yes, so that water seeps and recharges ground water. This is an old method.
We used to do it here, putting up short rock parapets.
What you have done here, Hari, is also contour bunding.
There are mud walls. Vetiver has been planted to hold the soil.

That is for binding. The roots will give the additional strength. Or, we can think of geo textiles. All this is to impart greater strength to the bunding. Or, we can take vegetative measures. We can grow small plants and trees. Among the small plants, vetiver is a good option. Water will sink and get stored in the interspaces called aquifer. That is how rain water becomes ground water, and that is the source we tap for our use. So, wherever we get excess water, we should permit it to seep below the surface. And where there is acute water scarcity, we should have a rain water harvesting structure. That is the product of German technology. It makes use of a ferro-cement tank. As part of a government initiative, roughly 1,000 RWH units were put up in two panchayats in Kasaragod district. Rain water is stored in them and used during summer. It has a filtering system.

Won’t the tank be a small one, sir? How big will it be?
10,000 litre capacity. That will take care of two months of summer when water scarcity will be really acute.
Perhaps 300-350 cubic feet should be adequate.

Yes. That is, we should set apart an area of a diameter of five feet, and then bend the metal wires to form an arch. Down below, a PCC foundation is made and the wires, bent to an L-shape is fitted in it. The arch shape will give it the strength to withstand pressure. A wire-mesh, with 6 mm gauge metal wires at fixed intervals, is used and the structure is given a circular shape. At the bottom, a foundation of concrete is created for giving it strength. The wires are bent and joined at the top. This will give it a height of 10 feet. It is covered on both sides with mesh, and cement plastering is done over it. Thus both sides will get adequate strength. The arch shape will give the structure enough strength to hold 10,000 litres of water.
That is the engineering part.

What reduces the cost is the mesh. The metal wires are fixed only at certain intervals.
We use a very limited number of metal wires. So how thick will the walls be?
Four inches in total, including the mesh and the slab. We leave an opening on the top. Then, there will be a rain gutter for water to come down.
All the water from the terrace will come down through the pipe and enter the tank.
There is a layer of gravel at the bottom of the tank that will act as a filter. A tap is fixed at the bottom. So we draw water after the filtering is over. There is a proportion for the mixture of gravel and coal that is spread in layers.

Sir, is the filtering done after the water enters the tank, or is it before we take it from the tank?
We get water after it is filtered. There is a kind of strainer to hold the dry leaves that may come down from the roof. This method is adopted where water shortage is really acute.
Last time, the Co-operative Department built a number of houses. A few of my colleagues went to Alappuzha to film it. What they saw there was a house on stilts. The house was new and built well. But the area suffers from acute water shortage. People collect muddy water, strain it and use it even to make tea. So when my team went there, they were offered this tea. It was difficult to refuse their hospitality but there was a fear that the water may cause health problems. But the local residents drink it every day.
In such places, if such 10,000 litre tanks are installed, the people will be able to store water and use it during summer.

It will definitely be better than the strained muddy water.
Last year, at Kuttanad, an entire unit flowed away in the flood waters. But what we constructed there remains intact. There are no complaints. We usually give our beneficiaries a one-year warranty. We get paid only after that period.

How much will it cost to set up a 10,000-litre water tank?
Around Rs 50,000. But there is subsidy on it. The beneficiary has to pay only 10 % of the cost. The rest will be covered by the government.

That is for those in the BPL category, isn’t it?
Yes.
But even otherwise, this is cheap.
Those in the middle-income group will get a corresponding subsidy from the government.
There is a possibility for the 10,000-litre tank to get filled two or three times.

The stored water won’t be required during the rainy season but only during the three-month summer season. So the tank will be a source of potable water.
I was thinking along different lines. Usually we suffer acute water shortage from March to May. We get rain in November too. So the water stored in the tank will last till October. When it rains, that water can be used for washing clothes during the months of June, July and August. Considering the cost of water, it will be advisable to use it to wash clothes, water plants and so on. We can use the stored water optimally until the rainy season ends.

Another worthwhile idea is that of well recharge. That is, we should direct all the rain water from our roofs or terraces into the well(s) in our compound after fixing a filtering tank at the end of the rain gutter of our roof. A tap at the bottom of the tank will send the water to the well. This will lead to well recharge.
That is, sir, the strained water is directed straightaway to the bore well. Just as we use a bore well to take water from, we return the water to the well.
Water from the roof that comes down via a pipe from the gutter is collected in a tank. Filtering process takes place there. There is tap at the bottom from where this stored water goes to the well.

My question, sir, is this. After fixing a water pump we have a pipe to pull the water up. In a similar fashion, for well recharge, we have a pipe going down, isn’t it?
No. That is a costly method. Instead what we do is let the pipe jut out. Whatever flows out will get stored.
Just like overflow of overhead tanks.
Yes, the water will continue to fall into the well. That is well recharge.

Sir, well recharge is very effective, isn’t it?
Of course! The storage of the well is more. Water will seep downwards and get saturated.
Our concept of well is that it is like a water tank. We dip into it and take water.

Water comes from a special zone. When we level a forest or cut down trees, what happens is . . . only when trees stand close to one another in a formation does saturation happen. We dig wells at the low end of it. This is the storage. Earlier, there were a lot of mountains. But most of them have been levelled. So the wells have gone dry because there is no space for storage. The water we get during summer is actually stored in the inter-gravel spaces where water is saturated. That is what we are losing when we level the ground. However, when we plant trees, water will not run but trickle down and get saturated. That is the reason why it is said we should have forests and not cut them down; or we should not level mountains. Where there are mountains there is storage of water. We have wells in the valleys. When it rains the water in the mountains get recharged.
This is news to me. All of us think that when it rains water rises in the fields and gets stagnated.

But that is not the fact. Water gets saturated in the soil of the mountains and then slowly come down. When we dig a well in the valley, water will come from various points. A majority of them will come from the sides of mountains and hills.

I have a couple of doubts to clear, sir. From the articles I have read, I gathered from one that wherever there are forests, the trees suck up all the water, causing ground water depletion. From another, I learnt just the opposite, with the claim that forests do retain water but not as much as wetlands do; forests permit water to seep underground; the soil composition in forests will be wet; it is when plants and trees send out water vapour that we get the air-conditioning effect, and so on. What is your opinion about it, sir?
It is said to be true in the case of rubber plantations. The roots of rubber trees suck up a lot of water, and that is what we get as latex or rubber milk. I have heard about such a study. But in other places, such a loss does not happen. When we get 3000 mm of rain in the forests, there is a thick layer of leaves on the forest floor. So the water does not drain off. If we compare the amount of water the trees consume and what they give us, certainly the latter is higher.

So the point is, even as we protect forests, we should protect meadows too. Grass does not soak up a lot of water.
But Nature is like that. There will be a meadow close to a forest, always. We should retain that. We get more water than they soak up.
Sir, if anyone has doubts about water tanks, you must give them some guidance.
Of course!

As you just saw, we heard what Mohankumar sir had to say. He has been running an NGO for the last 30 years, with a strong social commitment. We have interacted with him often, and sought his help too. That is why we thought of introducing him to you. If anyone of you have doubts about how to set up a water tank in your compound, or store rain water, or recharge ground water, or recharge wells, please raise them. We shall try to get his replies. He will give you directions, if you set out to do it on your own. Raising the level of ground water in Kerala is my need and yours. If that does not happen, we will not be able to exist on this planet. So we can hope for his whole-hearted co-operation in this venture.