Rain Water Harvesting: A Solution to Water Problems

20062020

Rajesh Deoliya

[rajeshdeoliya@gmail.com]

The inconsistent rainfall patterns, increasing temperature conditions coupled with other factors like increasing population, change in Land Use and Land Cover are some of the core issues in the water scarcity in most of the part of country. The Hydrological cycle is a complex cycle where any change/modification regionally or locally affect the availability of water at regional and local level. In India south west monsoon is the main contributor of water. About 80% of the rainfall in India occurs during the four monsoon months (June–September) with large spatial and temporal variations (i.e variant as function of time) over the country. Such a heavy concentration of rainfall results in a scarcity of water in many parts of the country during the non-monsoon period. A data compiled by Ministry of Statistics and Programme Implementation (MoSPI), Government of India indicate that per capita water availability in year 1951 was 5178 m3 per year (population 361 million) while in year 2011 it reduced to 1544 m3 per year (population 1211 million), it is projected that in year 2051 the per capita water availability will further decrease to 1174 m3 per year (projected population 1628 million). We are in water stressed situation which is expected to continue in coming decades. 

EFFECT OF WATER CRISIS:

Any change in land use and land cover and change in the topography of water sheds affect  the flow of rain water to the water bodies which is diverted from its defined course leading to inappropriate supply of water to reservoirs and sheet flow of veered off water in the form of flood, further, eroding valuable soil and vegetation.  Since peninsular water bodies do not receive water from snow melting, the role of rain water in rejuvenation of water bodies is foremost and require multifold efforts from government agencies and individuals. The reservoirs feed water for irrigation, drinking water, industries and energy sectors. Any paucity of water in these water bodies affect user. Industries are forced to close, the Mangalore Refinery and Petrochemicals (MRPL) partially shut down its unit for shortage of water supply from the Nethravathi river in year 2019. Earlier also it faced such situations. In Chennai, in year 2018, many IT units faced partial shutdown for paucity of water. The frequent droughts affect agriculture and drinking water supply also. Hence, augmentation and conservation of water resources is challenge for success of developmental process. The rain water harvesting therefore, if sincerely implemented can bring respite in short term and long term also.

THE RAIN WATER HARVESTING:

The national green building norms give priority for implementation of rain water harvesting. Some states like Madhya Pradesh has adopted a policy that every house should have a rain harvesting structure, without this provision the local bodies do not give building permission. However, in the absence of zeal to strictly implement these provisions by statutory agencies and ignorance with the beneficiaries,  the rain water harvesting practices are not receiving proper attention leading to failure of a good cause and practice.

India receives about 1086 mm annual rainfall. The country is divided into 36 meteorological sub divisions covering almost all type of geographic heterogeneity. The surface area distribution of rain fall is given below which suggests that the high rainfall receiving area in the country  is limited and confined to north-eastern states. The majority of area i.e about 72 percent falls in category which is vulnerable to rapid period of droughts or in other words "water stressed".

Category	Rainfall (mm)	Area %
Dry	0-750	30
Medium	750-1150	42
	1150-2000	20
High	Ø  2000	8

source:  Statistical yearbook-2016  MoSPI

Drought and Flood Periods:

If we think that south west monsoon rainfall itself is enough to meet out our water requirements then we are extremely wrong. A glance over the following table will reveal that in the past 140 years i.e from year 1870 to 2010, for which rainfall data is available, the flood events are much lesser than the drought events, meaning that the floods will not be able to recoup the water bodies and water shortage will remain in place and with time it will deteriorate further.

Period	Years of Flood ( Rainfall anomaly exceeding + 10 percent)	Years of Drought ( Rainfall anomaly below - 10 percent)
1870-1880	1874,1878	1873,1877
1881-1890	-	-
1891-1900	1892,1893	1899
1901-1910	-	1901,1904,1905
1911-1920	1916,1917	1911,1918,1920
1921-1930	-	1928
1931-1940	1933
1941-1950	1942,1947	1941
1951-1960	1956,1959	1951
1961-1970	1961	1965,1966,1968
1971-1980	1975	1972,1974,1979
1981-1990	1983	1982,1985,1986,1987
1991-2000	1994	-
2001-2010	-	2002,2004,2009
Total periods of flood and drought	15	25

Source:  recompiled from statistical year book,2016, MoPSI

It is clear that period 1961 to 1990 and 2001 to 2010 have witnessed frequent cycles of drought. However in later years also ( for which rainfall data is unavailable) the rainfall was generally less than normal but it was not qualified to fall in the flood or drought category. The sporadic cases of localized drought and flood have been there between year 2011 to 2019 like in Bihar and Rajasthan. The above table also gives us idea that if planned in proper and scientific way the rainwater harvesting can be a long term solution.

The table above infers that in last 140 years we have witnessed 15 flood situations and 25 drought situations, so if surplus water from flood is stored then the impact of drought periods can be minimizes or it can be avoided. Since the flood is generated due to veered off rain water, a proper and effective mechanism of rain water harvesting will be helpful in achieving the desired results. Broadly we can identify the areas where rainwater harvesting can be practiced:

Potential Rainwater Harvesting  Areas	Possible Rainwater Harvesting Structures	Result
Agriculture	Contour bunds, check dams, gully plugs, nala bunds, recharge pits and trenches along the slopes, Tank, Dug well, bore well recharge structure, Rooftop rainwater harvesting structure (where roof is available in building or shed). The farm field slopes can be shallow trenched to form a network of drain to collect flooding ( sheet flow) water to nearby natural drainage  to ultimately meet water body. It can also be stopped in between for recharge.	It will meet drinking water requirement for few days and recharge water table. The storage tanks can provide water for winter crops.
Industrial	Roof rain water harvesting, water tanks, gully plugs in surrounding areas, percolation tanks, and recharge wells. In mining areas, mine pits work as good rainwater harvesting structure, garland drains to control rainwater flow to identified structures to avoid flooding.	To meet out industrial water requirement – part or full. The mine pits due to their large size serve as good source of ground water recharge and meet water requirement to industry and sometimes of  surrounding villages.
Drinking water and Irrigation	Domestic Use for drinking purpose:  Roof rainwater harvesting in bore wells, storage in underground water tanks. Irrigation:  Watershed management in command area like gully plug, nala bund, contour bund, check dams, percolation ponds etc.	Recharge of ground water and meet water requirement for some days as per storage capacity for domestic users. In irrigation structures to augment storage capacity of the reservoirs.
Forest	Contour bunds, garland drains to direct flow of water to recharge structures, trenches, recharge pits, percolation tanks, small ponds/ furrows, gully plugs, nala bunds, check dams etc	Recharge ground water, increase availability of water in the streams and nallas, availability of water to animals within the forest.

COMPUTATION OF RAIN WATER HARVESTING FROM ROOF TOP RAINWATER:

Runoff coefficient for various roofs:

A runoff coefficient is a number that relates the rainfall rate and runoff rate. Using the runoff coefficient, one can calculate how much water passes over a given area per second. These co-efficient for various surfaces are available with central ground water board and central public works department's publications.

Type of Roof	Runoff coefficient
GI Sheet	0.90
Asbestos	0.80
Tiled	0.75
Concrete	0.70

Source: manual of artificial recharge of ground water, cgwb

 Selection of storage tank:

While adopting rain water harvesting method for domestic use, the size of storage tank is matter of concern. This can be calculated with the help of following example:

sl	Parameters ( for drinking water)	requirements
1	No. of persons in the selected household	4
2	Period of water scarcity for the domestic needs	120 days
3	Per capita water requirement	12 liter per day
4	Annual Average Rainfall	1000 mm
5	Area of roof made of country tiles	20 sq m
6	Runoff coefficient for tiled roof	0.75
7	Size of storage tank (in litres) No. of persons in the house hold  x Period of water scarcity (in days) x per capita water requirement ( in lt/day)	4 x 120 x 25 = 12000 lts
8	Water availability from roof top = Annual rainfall (in mm) X Area of roof (in sq.m) X Coefficient  of runoff for the roof	= 1000 X 20 X 0.75 = 15000 liters

RAINWATER COLLECTION NETWORK:

The above referred rain water collection structures and methods are conventional and deal with the small and isolated area approach. A better approach would be to form a dense network of pipes along the drinking water pipes, in the urban areas, opening in the nearby natural drain which may be a stream. nala, pond or even a river. These pipe network should be connected to roof top of each house to receive water.  This network will be one time investment. It will help in avoiding the flood on one hand by diverting rain water and rejuvenate the water bodies on other hand for longer part  of year  (post monsoon) benefiting in securing the supply of water for urban use.

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Disclaimer: 

The views expressed here are of writer only and do not belong to any organization, associated with.