SamSamWater Rainwater Harvesting ToolStep 4 of 4 (results)

Summary of results

The optimum size for a storage reservoir for this rainwater harvesting system is 6100 litres (6.1 m³).

Details on the results and calculations can be found below.


Latitude:49.19851 degrees
Longitude:-123.99658 degrees
Roof size:30 square metres
Roof type:metal
Runoff coefficient: 0.9
Water demand:100 litres per day


The average rainfall at this location varies between 48.7 mm in the driest month (July) and 303.1 mm in the wettest month (December). The total annual rainfall in an average year is 1948 mm.

Water availability

A metal roof has a runoff coefficient of 0.9, which means that 90% of the rain can be harvested. Based on this runoff coefficient and a roof area of 30 square metres a volume of 1315 litres (48.7 mm x 30 m² x 0.9) of water can be collected in the driest month (July) and 8184 litres (303.1 mm x 30 m² x 0.9) in the wettest month (December).
The total yearly amount of water that can be collected from the roof is 52600 litres (53m³) in an average year.

Water demand

The water demand is 100 litres per day, which equals to about 3000 litres per month. The total water demand is 36500 litres (36.5 m³) per year.
During 7 months of the year (January, February, March, April, October, November and December) the amount of water that can be collected from the roof is larger than the water demand. This excess water can be stored to be used in the months where the water availability is smaller than the demand.

Required storage

A storage reservoir (tank) can be constructed to collect and store the water during the wet months so this water can be used during the dry months. For this location, roof size and water demand the optimum size of a storage reservoir (tank) is 6100 litres (6.1 m³).

The storage reservoir will be full in January, February, March, April, November and December and then slowly drain until it is (almost) empty at the end of September.

Dry and wet years

This calculation is based on the average monthly rainfall. The actual rainfall differs from month to month and year to year. The amount of available water and filling of the tank might therefore be different and change from year to year.

When constructing a rainwater harvesting system it is important to take this into account. Below is a description of the situation in a dry year (20% chance) and a wet year (20% chance).

Situation in a dry year: to have enough water in a dry year it is necessary to construct a larger tank for the rainwater harvesting system. A rainwater harvesting system with a reservoir of 9600 litres (9.6 m³) should provide enough water all year round in this situation. It should be decided whether or not the rainwater harvesting system should be sized to this situation or not.

Situation in a wet year: there will be more than enough water. There is no need to take additional measures.

Data source

The rainfall data used for this calculation is based on the CRU CL 2.0 dataset which is described in New, M., Lister, D., Hulme, M. and Makin, I., 2002: A high-resolution data set of surface climate over global land areas. Climate Research 21:1-25.

More information

Show information on the construction and maintenance of rainwater harvesting systems.

Literature on rainwater harvesting systems

Water from roofs

Water from roofs

Erik Nissen-Petersen, DANIDA (2006)

A handbook for technicians and builders on survey, design, construction and maintenance of roof catchments (source)
Roofwater Harvesting; A Handbook for Practitioners

Roofwater Harvesting; A Handbook for Practitioners

T.H. Thomas, D.B. Martinson, IRC (2007)

A handbook with guidelines for implementing (low cost) domestic roofwater harvesting systems or programmes. (source)
Construction of wire-reinforced cement mortar tanks

Construction of wire-reinforced cement mortar tanks

Neil Herath, Comminity Water Supply & Sanitation Project (CWSSP), Ministry of Housing, Construction & Public Utilities, Sri Lanka (1995)

this document has been scanned from earlier manuals and standardizations prepared by Neil Herath for CWSSP – Sri Lanka ( April 1995), and as Manual 3 of the Technical Manual Series on Rural Water Supply and Sanitation, Gravity Water Supply Systems: design and construction (January 2005). It is redesigned to look like the 2005 CWSSP manual guiding field work on wirereinforced cement mortar tanks, commonly known as ferrocement tanks (Han Heijnen, October 2010) (source)
RAIN Water Quality Guidelines; Guidelines and practical tools on rainwater quality

RAIN Water Quality Guidelines; Guidelines and practical tools on rainwater quality

RAIN Foundation, RAIN Foundation (2008)

This document addresses RAINs guidelines towards water quality and gives criteria and practical guidelines to improve and maintain an acceptable water quality of harvested rainwater for drinking purpose. (source)
Rainwater harvesting for domestic use

Rainwater harvesting for domestic use

Janette Worm, Tim van Hattum, Agromisa (2006)

Book Agrodok 43: 'Rainwater harvesting for domestic use' with useful and practical applications of rainwater harvesting in developing countries. (source)

Organizations with experience in the construction of rainwater harvesting systems

SamSamWater Foundation

SamSamWater Foundation aims to increase the number of people in developing countries with sustainable and reliable access to water and sanitation.
'SamSam' means 'together'. We work together to reach our goal: safe and reliable water to all! We believe in practical solutions, realised in close collaboration with our partners and beneficiary communities.

RAIN Foundation

RAIN is an international network with the aim to increase access to water for vulnerable sections of society in developing countries - women and children in particular - by collecting and storing rainwater.
Started in December 2003, RAIN focuses on field implementation of small-scale rainwater harvesting projects, capacity building of local organisations and knowledge exchange on rainwater harvesting on a global scale.

Acacia Water

Acacia Water “for solutions in groundwater” was established in 2003 at the Vrije Universiteit Amsterdam (Faculty of Earth and Life Sciences). Acacia Water focuses on groundwater in relation to surface water, environment and infrastructure. This varies from implementation of field measurements and model calculations, to giving training and strategic advice. Acacia distinguishes itself by the cooperation with the University, as a result of which Acacia Water has access to the latest scientific developments in the field of hydrology and environment. We translate this knowledge to innovative and practical solutions.

Meters Feet

Litres Gallons