A brief feasibility study for rainwater harvesting for Kanyakumari district with a specific focus on the reconstruction programme undertaken at Arunahenjanvillai and Rajakamagalamthurai

Peeyush Sekhsaria 07/11/06

Rainfall data for Kanyakumari district

Month	Rainfall (mm)
January	29.70
February	20.60
		Summer Rains : March April May
March	47.80	Total rainfall : 332 mm
April	111.70	No. of rainy days:11. 1 rainy day / 8.5 days
May	158.20	Average :30 mm / rainy day
		South West Monsoon: June July August September
June	211.40	Total rainfall: 537 mm
July	150.00	Number of rainy days: 27. 1 rainy day/5days
August	87.60	Average 20 mm / rainy day
September	102.70
		North East Monsoon: October November December
October	246.80	Total Rainfall:549 mm
November	206.20	Number of rainy days: 24 days. 1 rainy day/ 4 days
December	70.30	Average 23 mm / rainy day
	1443 .00

Total roof area available for Rain Water Harvesting	31 sq mts.
Average daily water needs	200 lits / day

From the rainfall chart we can see that

Kanyakumari is a high rainfall area with 1443 mm annual rainfall.

Rainfall is distributed over 10 months of the year: March to December

The total number of rainy days is 62, making the rainfall fairly well distributed over the period in question

The following factors count when deciding on Rain Water Harvesting project

Total Annual Rainfall

Total rainy months

Total rainy days

Roof area available

Daily House water needs

Present and future condition of water supply/ availability (either governmental / village level / personal)

Future water needs (for example: addition of toilets increases the need for water)

Costs involved

Some calculations

Roof area (sq mts) x average rainfall / rainy day(mts) x 1000

= Water Harvesting Capacity of that rainy day

Summer (March, April, May)

31 (sq mts) x 0.03 (m) x 1000 = 930 lits

1 rainy day / 8.5 days

930 / 8.5 = 110 lits / day

Hence average rainwater available on daily basis for use during summer = 110 lits

Southwest monsoon (June, July, August, September)

31 x 0.02 x 1000 = 620 lits

1 rainy day / 5 days

620 / 5 = 124 lits / day

Hence average rainwater available on daily basis for use during south west monsoon = 124 lits

North east Monsoon (October, November, December)

31 x 0.023 x 1000 = 713 lits

1 rainy day / 4 days

713 / 4 = 178 lits / day

Hence average rainwater available on daily basis for use during south west monsoon = 178 lits

What does this mean for Rain water harvesting

That within the limitations of average for the period March to December (10 months of the year)

Every roof top can provide between a minimum of 110 lits to 178 lits/day

A tank of 1000 lits would be suitable to store the water generated during a rainy day

All bathing, toilet and some washing needs can be fulfilled through harvested rain water for roughly 9-10 months of the year.

The annual rainwater harvesting capacity of a single 31 sq mt roof is 44733 lits!

It can safely be assumed that the annual potential is of 40000 lits/year ( 10 % calculated losses)

One could say that rainwater harvesting is feasible for Kanyakumari district

What exactly will the rain water harvesting scheme look like ?

A 1000 lit PVC water tank placed on a raised platform of 40 cm directly below the rain water spout, with a pipe bringing the rain water to the mouth of the tank.

The mouth of the tank is covered by a filter made of fine cotton cloth folded 8 times and stretched over the mouth should suffice.

This filter can be washed as and when required

One time cost involved is for the following

1. PVC tank

2. Raised platform

3. PVC pipe roughly 2 mts in length

4. One white cotton saree