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Rain Water Harvesting

Introduction

Next to air, water is the most important substance for sustaining life on this planet. We cannot imagine a form of life that can live without water. History has recorded growth of civilisations in the world along the banks of rivers.  In India, Vedic civilisation blossomed along the banks of the now extinct river Saraswati during periods as old as 6000 B.C.  In course of time, the river Saraswati, originating from the Himalayas and flowing along Himachal Pradesh, Punjab, Rajasthan, Sind and Gujarat, vanished due to tectonic movements.  With its disappearance, the regions along which the river flowed

changed from humid to arid regions. Water sustains both life and civilisation, and its lack leads to their destruction.

 

Of recent, the scarcity of potable water has been on the rise.  There is a global shortage of potable water.  Increase in demand, over exploitation of water sources, and water pollution are the major factors contributing to this global shortage of potable water. The situation in India reflects that of the rest of the world, but in a more aggravated manner than in developed countries. Gujarat has witnessed violent water riots in the Rajkot district. Madhya Pradesh has also witnessed drinking water crises in different parts

of the State leading to water riots, death, and breach of peace.  Even the Supreme Court of India is very much seized of the disquiet on the waterfront.  The apex court has directed the Central Ground Water Authority to take steps to prevent indiscriminate drilling and over exploitation of ground water in the country.

In the state of Kerala, the people are fast losing the battle over potable water.  We are witnessing mass movements in many parts of the State due to drinking water shortage.  The recent water pollution and water shortage episode in Plachimada due to over-exploitation of ground water resources and indiscriminate discharge of wastes by a mineral water producing factory exposes the roots of the problem. The problem starts with irresponsible water management policies, grows with exploitation of natural resources and their destruction by MNCs, and ends in

deprivation of the people from their right to life, their right to have clean drinking water.  Kerala is also on the brink of water riots like those witnessed in other states of the country.

The Central Government has been allocating funds to State Governments for potable water supply systems right from the first five-year plan.  Schemes with different names like rural water supply schemes, urban water supply schemes, and special component schemes have been formulated and supposedly implemented by all Indian states at the costs of millions of rupees.  But even after all these efforts, the problem has gone from bad to worse.  Government records speak of the number of problem villages covered with potable water supply every year, but ironically, the numbers of problem villages are not decreasing.  The attempts of the Governments in solving the problem of water scarcity have miserably failed.  The jugglery with statistical figures is not at all corroborated with the reality in the field.  The technology adopted by the Government in solving the drinking water problem has miserably failed to achieve the

desired objective. The reasons are numerous and include:

Inadequate implementation of adequate plans

1. Unsustainable plans that deal with the superficial and do not address the roots of the problem with foresight
2. Mismanagement of existing resources to get quick results that just shifted the problem from one area to another
3. Over exploitation of existing resources leading to the development of new problem areas
4. Deep bore wells to tap water contained in deep Aquifers leading to deterioration of water quality

The water supply schemes proved inadequate and ultimately had to be abandoned.

There are other reasons also for the failure of government efforts in solving the problem. They are  

1. No thought was given for recharging the existing sources or creating new sources. 
2. People's participation was not ensured in solving the problem. The people, who are the beneficiaries, were not involved at any stage of solving the drinking water supply problem. They viewed it as yet another government program, with its usual red tape and antagonistic attitude towards the people.  The schemes, therefore, failed at the operation and maintenance level.

According to the norms prescribed by the National Drinking Water Mission, the    present per capita need of water in rural areas is 40 LPCD (Litres Per Capita Per Day) for human beings and an additional 30 LPCD for cattle.  For urban populations, the norms prescribe a value of 130 to 150 LPCD.  It may be interesting to note here that in the metropolitan city of Delhi, the average water consumption is 240 LPCD.  (Athavale, R.N., 2003)

It is reported that per capita availability of water at less than 1700m3 per annum will lead to water stress and water scarcity conditions will set in when this value goes down to 1000m3/capita/year (Pastel, 1997).  In some areas of Tamil Nadu, the value has gone down to 400m3/person/year (Sharma S.K, 2000).  Six of the major river basins of India are classified as having less than 1000m3 of water available per head per year.  (The Hindu, daily, issue dated Feb 27, 2001).  The states of Rajasthan, Gujarat, Andhra Pradesh, Madhya Pradesh, Maharashtra, Karnataka and Tamil Nadu are drought hit.  The above States constitute one third of the area of our country.  The situation is alarming indeed.  We can say that we are at the doorsteps of water famines.

Water Endowment of India

In this context, it is interesting to consider the water resources endowment of our country.  The predominant characteristics of the Indian climate are governed by the yearly monsoons sustained by the Himalayas.  The greatness of the Himalayas in bestowing a semi arid tropical climate is explained with poetic imagery by the great Sanskrit Poet Kalidasa in the first saga of his epic poem "Kumarasambhavam".  The semi arid tropical climate covers 60% of our country, while 10% of India, the Thar Desert of Rajasthan and the Kutch District of Gujarat - fall under an arid climate zone.  A humid tropical climate is found in North East India, while the Kashmir Valley enjoys a temperate climate.  Ladakh is classified as a cold desert.Most of the rainfall in India happens during the monsoon season. The Himalayan region receives snowfall.The snow accumulates in winter, melts, and flows

through the Himalayan Rivers during non-winter months.  The yearly snowfall in Siwaliks (Foot Hills of the Himalayas) is 3m.  In the upper regions of Himalayas, the snowfall is 15m and more in a year (Rao, 1981).  The word "Monsoon" has its origin in the Arabic "Mausam," meaning season. 

The South-West Monsoon lasting during the months of June through September is responsible for the annual rainfall over a major part of India.  However, in Tamil Nadu and in border areas of Kerala with Tamil Nadu, a major portion of rainfall is received during the period of October through December. The S.W. Monsoon has the following average annual rain fall distribution- 

1. About 75% during June through September.
2. 10 to 11% during October through December.
3. 3 to 4% during January through February.
4. 10 to 11% during March through May. (Sarkar, 1988)

The average annual rainfall in India is 1170mm against the world average of 700mm. Although it would appear that we get a good amount of water as rain every year, the skewed pattern of rainfall distribution creates the problem. A major portion (about 75%) of the rain fall occurs during June to September. The spatial variation is also large. Cherapunji in Meghalaya receives a rain fall of 11 metres an year. But most of it runs off. On the other extreme, we have Jaisalmer and Barmier in the Thar Desert of Rajasthan, which receive only 0.1 to 0.2m of rain annually.

A good percentage of the rainfall is lost as run off in India. This is because, 66% of our country is covered by hard rock. These rocks have low permeability and about 90 to 92% of the rain falling in rocky areas flows away as surface run off. Thus, only a small portion of the rainfall is available for ground water recharging. Another factor to be considered is the variation in the annual rainfall. The ratio of standard deviation of average rainfall to the mean annual rainfall taken over a large number of years is termed as the coefficient of variability (CV) of rainfall. It is found that in regions with lesser annual rainfall and higher value of CV, the average value of rainfall is much less reliable. Lastly, the intensity of rainfall and raindrop size also affects run-off. High intensity of rainfall and large raindrop size contribute to high run off and soil erosion.

Hence, although our rainfall endowment is high, due to the factors explained above, availability of water remains limited. Now let us study how the precipitation received at a place in the form of rainfall or snowfall gets dissipated.

Some of the precipitation enters the soil until it becomes saturated. Once the soil is saturated, any precipitation entering the soil percolates deep into the soil and is stored in deep Aquifers. This is called natural recharge. A considerable amount of rainfall is sent back to the atmosphere as evaporation from the soil or as evapotranspiration by the plant life. Some amount of precipitation gets collected in natural depressions and valleys to form ponds and lakes. The rest of the precipitation flows into the sea through rivers. In certain cases, the ground water percolates down and travels directly to the sea as a sub marine discharge. The factors, which determine the fraction of precipitation constituting each of the above components, are 1) Soil property 2) Topography 3) Vegetation 4) Temperature 5) Humidity and 6) Hydro geological Conditions.

The water balance of India is reproduced below as given by Chaturvedi (1985).

Average annual Precipitation = 1.194m.

Total Area Covered = 328 Million Hectares

Sl. No.	Category	Quantity in mhm(million Hectare Meter)	Percentage of total
1.	Precipitation (Rain fall + Snow)	400	100
2.	Surface storage & run-off	160	40
3.	Evaporation: a) Surface Water Evaporation = 5mhm b)Soil moisture Evaporation=10mhm c)Direct Evaporation=70mhm	a)85 b)50 c)105	21.25 12.50 26.25
	Total
	Ground Water Recharge Soil Moisture

From the above table it can be seen that about 40% of the precipitation goes to the sea as run-off and about 15mhm (3.75%) is lost as evaporation.  Rainfall data for the last 100 years is available from hundreds of meteorological stations in India.  Classification and analysis of this data shows that over the last century, there has not been any appreciable shortfall in the precipitation at a given location.  The rainfall data does not corroborate the view that the problem of water scarcity is caused by shortage of rainfall.  The endowment is good and fairly steady, but the scarcity is increasing.  The loss of the incident precipitation is increasing, the demand is increasing, and as a result, the existing resources are depleting faster than ever.  These are perhaps the major causes of water scarcity.

Water Harvesting

It is in this context that water harvesting has immediate scope. Water, in nature, exists in three phases: solid (Ice), Liquid (Water) and Gas (Vapour).  It is a dynamic compound, ever on the move.  Water harvesting is the practice of holding water in place or in the desired phase for some period for human utilisation at a later stage (Athavale, 2003).The Rajiv Gandhi National Drinking Water Mission of the Government of India gave a more comprehensive definition of water harvesting.  A slightly modified and up-dated version is given by Dr. R.N. Athavale in his book

"Water Harvesting and sustainable supply in India." According to Dr. Athavale, the term water harvesting refers to "the collection and storage of natural precipitation and also other activities aimed at harvesting surface and ground water, prevention of losses through evaporation and seepage, and all other hydrological studies and Engineering intervention, aimed at conservation and efficient utilisation of the limited water endowment of a physiographic unit, such as a water shed".

According to the above comprehensive definition of water harvesting, arrive activities like construction of in-situ rain water collection structures, construction of farm ponds, check dams to arrest under ground flow, artificial recharge through wells, control of evaporation, soil conservation practices etc. Rainwater harvesting is a restrictive use of the term water harvesting as it is covered under the comprehensive definition of water harvesting. 

Rain Water Harvesting

The term rainwater harvesting refers to the direct collection of precipitation without allowing it to escape as surface run-off.  There are two types of rainwater harvesting.  They are 1) Roof water harvesting and 2) In-situ water harvesting.

Roof Water Harvesting

A traditional method of roof water harvesting comprises of collection of precipitation incident on roofs and storing it in a waterproof sump at ground level for use during periods of scarcity.  This was done out of sheer necessity in regions of India which had low rainfall like Gujarat and Rajasthan.  Roof water harvesting was also resorted in coastal areas where the water is saline.But modern construction methods do not take into account the need for a roof water harvesting system.  The growth of population, increase in urbanisation, rapid industrialisation, and indiscriminate increase of built-up areas has increased the demand on the common water supply systems.  These schemes are bursting at their seams due to an increase in demand and dwindling sources.But, rather late than never, the importance of roof water harvesting as a supplementary captive source of water supply is being realised.  The rainwater falling over the roof is being collected in sumps or is

recharged into a dug well or open well. In the hilly regions of Western Ghats and North East India, which receive very high rainfall, roof water harvesting has become necessary.  This is because the springs, which used to be the source for water supply in these regions started dwindling due to deforestation.  The amount of water supplied by the springs has become inadequate due to increase in demand also.

In some parts of Andhra Pradesh, Madhya Pradesh, Gujarat, and Rajasthan, the ground water contains fluorides beyond the permissible limit of 1.5mg/litre.  In parts of West Bengal, ground water contains arsenic beyond the permissible limit of 50 micrograms per liter.  In the above cases also, roof water harvesting is useful, because it contains no harmful substances like fluorides and arsenic.  In areas where the ground water quality is good also, it is desirable and cheaper to recharge the ground water reservoir through a percolation pit in the ground or through an existing open well or tube well.

The sump in which the rainwater is stored is covered by a lid to avoid entry of sunlight.  This prevents growth of algae and pollution and also prevents mosquito breeding.  The water has to be disinfected before use by disinfection using bleaching powder or hypo solution.  The sump is to be cleaned before monsoon arrives.  The first run off from the roof is allowed to flow out as it may contain dirt, bird excreta etc from the roof.  This is accomplished by putting a bye-pass line and valve in the pipeline taking the roof water to the sump.  A small filter box filled with sand and gravel is placed at the entry point to the sump to clean the water entering the sump.

Besides acting as a buffer captive storage of water, roof water harvesting has the indirect advantage of reducing flooding in urban areas due to reduction in run-off.  If R.W.H is used by a large number of houses as in a colony for spot recharging of ground water, it helps restoring water table and improving quality of ground water.  The most significant benefit of R.W.H is that it reduces the demand on the already inadequate water supply system.

The amount of roof water which can be collected out of the total precipitation depends on 1) The amount of rainfall 2) The area of the roof and 3) The type of roof.  There is no scientific data for collection efficiency of various types of roofs.

The percapita daily requirement of water for drinking and cooking is generally taken as 20 litres.  For household of 5 members, this will work out to 100 litres per day.  We can assume the summer season to last 100 days in a year.  The total drinking and cooking water demand for a house hold for 100 days would therefore be 10,000 litres.  Adopting a seasonal rain fall of 2 metres, which is fairly reasonable in Kerala, a roof area of 40 m2 has to theoretically yield 80m3 of water (80,000 litres) in a season.  But we have to consider the collection efficiency of the type of roof.  Dr. R.N Athavale in his “water harvesting and sustainable supply in India” has given the following collection efficiencies for various types of roofs.

1.  Cement Concrete         -  85%
2.   Tin Sheet                  -  75%
3.   Baked Tiles              -  60%
4.   Thatch                     -  40%

Hence for a cement concrete roof of 40m2 area and 2 metres of rain fall in a season the roof water available for collection would be 68m3 (68000 litres) and for thatched roof it would be 32m3 (32,000 litres).  But creating such storage would be expensive, we can at least create a storage of 10,000 litres equivalent of summer drinking and cooking water demand of a house hold.  The excess roof water could be used for recharge of ground water.

The roof water harvesting has come into practice in many cities and towns.  In Chennai, it has become mandatory to incorporate roof water harvesting system in building projects in order to get sanction.  In Kerala State also it is gaining momentum slowly.  The practice adopted is to store some roof water in an under ground sump and to use the surplus for ground water recharging to augment the yield of wells and tube wells in the plot.  An integrated approach would be to divert the surface run off from open ground/lawn/garden along with surplus roof water to an underground recharge facility.  A sediment trap or filter bed is used to remove debris and suspended particles before entering the recharge facility.  The recharge facility could be a dug well, bore well, pit or soak way etc.  A soak way is a bore well having a diameter of about 30cms and depth 3 to 10 meters with a slotted pipe.  A filter bed sump 60cmx60cmx60cm is placed on top of the bore below ground level.  The roof water is charged into this sump.

In – Situ Water Harvesting

The rain received in the area was quickly removed as run-off.  The topography is laterite surface with pre-Cambrian granite and quartzite beneath.  Four trenches of size 8mx2mx4m were excavated in this 1-hectare plot of mango plantation.  These trenches were lined to hold water.  Each trench could hold 64m3 of water.  Each trench could irrigate 40 mango saplings in a year.

Another example of in-situ water harvesting noted in the book is for drinking water supply in Cherapunji in Meghalaya.  Cherapunji receives the highest rainfall in the world, yet it suffers water scarcity in the pre-monsoon months. The traditional water supply in Cherapunji is by springs. This has become inadequate due to dwindling of springs and increase in water demand. The Meghalaya P.H.E.D. introduced roof water harvesting.  It comprises of collection of rainwater from house rooftops through gutters, its filtration, and storage above ground in a reservoir.  These systems are sufficient for 90 days at 10 litres per capita per day.

Another application of in-situ water harvesting is augmentation of springs in hilly regions.  In these regions, the top soil is a laterite cap about 10cms in thickness.  When this cap is penetrated, there is soft weathered rock below.  Such an intervention will allow percolation of rainwater.  This would result in increase of spring discharge.

The coastal strip of Konkan extending from Valsad in Gujarat to Kasargod in Kerala has the dual problem of water scarcity and saline water intrusion.  These problems could be addressed by in-situ water harvesting.

Conclusion

Rainwater harvesting is a method of choice in India and particularly in Kerala to supplement public water supplies and to recharge ground water aquifers.  Like Chennai Corporation, other major corporations are making roof water-harvesting mandatory in future constructions. We have to depend on dug wells as well as the preferred and sustainable source of drinking water. Dug wells have less iron/fluoride/arsenic/salinity than bore wells that tap deeper than aquifers. Dug wells can be recharged  by rainwater using pits, soak ways etc.Aquifer storage and recovery (ASR) technique, involving recharge of a well during rainy season and use of the storage during dry season to tide over water scarcity can be put to use advantageously in Indian cities and villages. There should be restriction on bore wells and use of ground water.  Sewage after tertiary treatment in the case of urban agglomerates could be used for limited uses of flushing and gardening to reduce the load on public water supply systems. A concerted and coordinated effort on the part of the Government and Public in scientific rainwater harvesting would definitely solve the impending water crisis. 

Acknowledgement

I wish to thank Er. P.Ramachandran and all members of the Cochin Chapter of the IPA for giving me an opportunity to make this presentation on a topic of relevance in this era. 

I also record my sincere thanks to my able and smart Office Assistant, Mrs. Deleshya Manoj for her help and support in documentation and organizing this presentation matter. 

References

1. Athavale R.N. 2003, Water Harvesting and Sustainable Supply in India, Centre for Environment Education, Ahmedabad. 
2. Chaturvedi, M.C. 1985, Water Resources of India-an over view, Sadhana Vo1-8, PT-1 
3. Postel’s. 1997, Lost Oasis, W.W Norton & Co. N.Y. 
4. Rao, Y.P. 1981, climate of the Indian sub continent, Chapter II in climates of Southern & Western Asia, Ed. Taka hashik and Arakawa.H, Vo1-9 of World Survey Climatology, Elsevier.
5. Sharma, S.K. 2000-Rainwater Harvesting – An alternative technology for fresh water augmentation, in proceedings of National Seminar on Water Harvesting, published by Central Ground Water Board, New Delhi