It is now possible to calculate the amount of rainwater that can be harvested from the roof of houses thanks to a new smartphone app developed by UN Environment and the United Nations Educational, Scientific and Cultural Organization.

The app, a first of its kind, is based on actual meteorological data collected from weather stations across Africa. The data is specific to locations closest to the weather stations, which the app presents as the nearest city.

Promoting rainwater harvesting is becoming increasingly important to ensure greater water security. It offers an adaptation strategy to climate change, providing an opportunity to store rainwater under increasing conditions of high rainfall variability.

The app, a first of its kind, is based on actual meteorological data collected from weather stations across Africa

Rainwater harvesting can also improve livelihoods of women and children by reducing time spent on fetching water: women still spend 16 million hours a day collecting water in 25 sub-Saharan countries.

It can improve household sanitation and health with an improved drinking water source. Rainwater harvesting also contributes to food security, providing water during dry seasons for small-scale agriculture. Although not every single drop of rain can and should be harvested, rainwater is still an underutilized water source in Africa.

The lack of water is a real challenge across Africa. On average, a person needs eight glasses of water (2 liters) per day. According to the Vital Water Graphics report by UN Environment and GRID Arendal, “more than 2.8 billion people in 48 countries will face water stress or water scarcity conditions by 2025. An area is experiencing water stress when annual water supplies drop below 1700 m3 per person”.

Rainwater harvesting can also improve livelihoods of women and children by reducing time spent on fetching water

In many such countries, rainwater can be harnessed easily for domestic and agricultural use. Harvested rainwater can also benefit the environment and ecosystem when used to enhance groundwater recharge and restore vegetative cover. The low cost of rainwater harvesting technologies can be a more attractive investment option in rural areas compared to investing in a main water supply system.

“This app is practical for individuals as well as communities, local governments, and other actors who are planning to install rainwater harvesting systems in Africa,” said Dr. Juliette Biao, Director of Africa Office in UN Environment. “The app is a concrete example of how science and innovation can be packaged to solve day-to-day needs of households in Africa,” she added.

The new app demonstrates opportunities for rainwater harvesting. For example, a person in Turkana county in Kenya can enter her location (Kenya, Lodwar), the length and width of the roof of the house (in metres), the number of family members, and the quantity of water used per day. The app returns the estimated amount of rainwater that can be harvested, and the quantity of water for a family. It also proposes the size of the rainwater harvesting system as well as its estimated cost. Simple sketches showing rainwater systems and how to recharge groundwater are also included in the app. Rainwater harvesting systems can be easily constructed using appropriate technology and locally available materials.

Rainwater can be collected in relatively simple ways. Rainwater that falls in ditches, on rooftops or on other surface areas is collected in storage facilities, such as water tanks or ponds. This water is stored and used for domestic and agricultural purposes. There are also other ways of rainwater harvesting, such as storing it in the ground where it can be used as groundwater for human consumption or for nature.

Ann Kiria, chair of a young women's group in Kajiado in Kenya said, "water harvesting has benefited our community where women have played a key role in constructing water tanks. With the knowledge we have acquired, building a water tank to harvest rainwater is no more out of reach".

Currently the smart phone application is available for Android (Play Store) system for free. You can download it by searching for RWH Africa Interactive Tool.

A web version is also available at http://www.rainwaterharvesting.africa. An IOS (App Store) version will follow soon.