Forget the Drought - Let's Harvest Rainwater

Kshetrimayum Sanatomba *

Water scarcity in Singda Dam area : Rivers in Imphal drying up on 23 April 2014 :: Pix - Deepak Oinam

Over and over again, year after year, the incidence of water scarcity in Manipur, particularly in the cities, has been a regular show and we neither hold back to blame the government nor tried to change our fast lane lifestyle! With ever increasing population coupled with exploitation of the river catchment areas (deforestation), mining in the riverbeds, pollution & over-drawing of water from the rivers, the demand-supply gap quotient has also been escalating.

A primary reason is that we are very much inclined to modernization and this can be attributed to increase in imperviousness area, broken water cycle and too much technology solutions for fulfilling our needs. This necessitates the requirement to explore more possibilities to harness rainwater, which had been practicing since time immemorial by our great great grand parents.

The state falls in one of the rain fed regions in the country and receives appreciable amount of unevenly distributed precipitation throughout the year. The average annual rainfall in the state is around 1400 mm. However, in recently years we can see changes in the rainfall pattern with no record of rainfall in January, 2014 & sharp falls in February, March & April (source: ICRA, Imphal).

Primary objective of this article is to harvest rooftop rainwater during rainy season & store it for meeting the daily requirement during dry months.

Quantity of Harvestable Rainwater from Rooftop.

The method of calculating effectively harvestable rainwater is given as;
Effectively Harvestable Rainwater (Q) = Roof top area in m2 (R) X Annual average Rainfall in m (H) X Co-efficient of Runoff of the material used for Roof top X Co-efficient of evaporation, spillage & first flush (E).

First spell of rain (15 mins.) is flushed out as it carries with it a relatively larger amount of pollutants from air & catchment surface and is called as First Flush.

Example: A small family of 4 members having rooftop area of 50 m2.
Q = 50 x 1.4 x 0.85 x 0.8 = 47.6 Cum = 47,600 liters.
[Annual Average Rainfall in Manipur = 1400mm = 1.4 m]

As per Central Public Health & Environmental Engineering Organization (CPHEEO), India Water Portal, 2010, the average domestic water consumption in Indian Cities is given in Table-1.

Table-1: Average domestic water consumption in Indian Cities for different uses (CPHEEO)

Rainwater is collected on a catchment area (rooftop). Conduit pipe, drains and gutters are required for carrying rainwater from catchment areas to storage tank. Depending upon the type of uses, filtrations are of 4 types;
o Separation or Screening: The first level of filtration that separates out gross pollutants. Simple mesh or Jalis can be used as screens.
o First Flush: At the second level -the first spell of rain containing dissolved impurities are allowed to flow away. It can be manual or automatic.
o Filtration: Filters remove dissolved organic and inorganic particles in the rainwater.
o Sedimentation: Settling tanks are smaller than storage tanks & removes silt and coarse materials.

Caution: For drinking & cooking purposes, proper filtration & treatment of water shall be required. For other uses, only Screening & First Flush will do.