Rainwater collection – how to calculate the volume

Q: How to calculate the volume of a storage tank based on a certain annual rainfall. For our project in Tibet I need technical information on rain water collection, including project design.
(Water and sanitation programme officer, NGO, Tibet)

Answer: In principle, rainwater harvesting is NOT cheap. So, it is either a supplementary source or means with a relatively small storage volume, or people have to be very economic and use more during rainy season and the bare minimum during dry season. If they would accept that then it is a solution. Unless there is a lot of money available. I understand that in your situation, you have a semi-arid condition, so people may be used to be very economic with water.

One option is to collect rooftop rainwater for drinking and cooking, and ground surface runoff/rainwater for other uses. For both you can use groundwater tanks (from simple to advanced, depending on soil stability etc. and availability of funds) and for rooftop rainwater storage an above-ground tank is most common; also to ease the drawing of water.

Important is also to determine the volumes people use for drinking and cooking (say between 2-5 litres per capita day (lcd)) and that for other proposes (say between 10-20 lcd).

Using those figures and the number of people you can calculate the amount of water you need per day or month. If you have to collect those volumes all from rainwater (as the sole source) then you can calculate the required surface area and the required storage capacity. Each surface area has a specific runoff coefficient, i.e. the efficiency of collection or the rainfall minus the wasted water. For hard roofing materials the coefficient is between 05-0.9, which means that of each litre of water fallen on the roof you can collect between 0.5 -0.9 litres! For ground surface areas this coefficient is much lower, depending on soil type and condition: usually between 0.10 and 0.25!

Cumulating the volume of water in a diagram
The simplest way to calculate is cumulating the required monthly volume of water (drinking/cooking) in a diagram, and plot the volume of water required to satisfy this demand.

Demand drinking and cooking Q = number of consumers * C1 * 30 = Q [m3/month]
C1 = daily water consumption for drinking/cooking per capita (say 2-5 lcd)
Demand other purposes Q = number of consumers * C2 * 30 = Q [m3/month]
C2 = = daily water consumption for non- drinking/cooking per capita (say 10-20 lcd)

You can then calculate (from the graph, see for an example of this graph the WELL fact sheet on domestic rainwater harvesting). There is not a good indication for the cost as it depends on volume, materials used, location etc. Cost for underground tanks are usually cheaper as the soil pressure holds the tank and the water pressure. Also simple lining materials may be used if the soil is sufficiently stable.

Operation and maintenance
One more issue is the management, and operation and maintenance of the water system; this is well described in IRC’s Technical Paper 30 – Water Harvesting.
If the rainwater catchment is a privately owned system then the house-owner can control use and manage this. If it is a community owned facility the problems are many and conflicts, misuse, mismanagement, are very common and people get dissatisfied and may turn away for contributing to operation and maintenance costs.

There are many manuals/handbooks and guidelines that can help you in designing a rainwater collection system. Both for surface runoff/rainwater and for rooftop harvesting. Below, some of these key references are mentioned. If you want a quick overview of points you could consult the WELL fact sheet on domestic rainwater harvesting.

The specifics of the tank design depend on many factors, including:

• The rainfall pattern; in your case it is a very short season (6-8 weeks) (sounds as a monsoon) and with only a few heavy storms
• Total annual rainfall; in your case also limited 425-500mm
• Available rooftop surface area; roofing material and slope of roof
• Available rock catchment area or ground surface area that could be used for rainwater catchment; and what is the soil type or any hard impenetrable material
• Availability of other water sources, their reliability (volume), availability (seasonality), accessibility (distance and altitude difference), quality of water at source
• Availability of streams, piped water, tankered water
• Number of people to be served
• External and own funding; level/amount of funding
• Availability and Cost of building materials
• Availability uilding skills
• Consumption of water
• People’s attitude towards water use: very economic as water is very scarce to wasting as water is in abundance
• Acceptance of different qualities for different water uses/ purposes
• Level of management and control over source
• Perceptions and beliefs on water quality influences acceptance (e.g. rainwater has flat taste; groundwater is ‘living’ water

Further reading

IRC FAQ sheet – Rainwaterharvesting.org
Web site of CSE India containing (among other topics) urban case studies

Domestic Roofwater Harvesting Programme of the University of Warwick – Development Technology Unit.
Contains much information based on two research projects, see for example Domestic Roofwater Harvesting in the Tropics: the State of the Art, by Terry Thomas, Warwick University, UK (2003).

Various specialised rainwater networks are united in the Rainwater Partnership, such as:

International Rainwater Catchment Systems Association

International Rainwater Harvesting Alliance

SearNet – Southern and Eastern Africa Rainwater Harvesting Network –

RAIN – Rainwater Harvesting Implementation Network – initiated by Bunker Roy (Barefoot College, India). He has left and launched the Global Rainwater Harvesting Collective.

IRC, January 2007
This response has been provided by the WELL Resource Centre Network with funding from the UK Department for International Development (DFID). Since March 2007 the WELL enquiry service has stopped.

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