Subscribe
Historically, as social intelligence grew, people realised that human society cannot grow without the bounties of rainwater extending from the wet months to the dry months. Thus grew the resourceful tradition of water harvesting. Wells were an important source of irrigation in groundwater-rich regions, but people learnt to harvest groundwater in other ingenious ways too, especially where water in general and groundwater in particular, were scarce.
The principle of water harvesting is to conserve rainwater — according to local needs and geophysical conditions — where it falls. In the process, groundwater is also recharged. Traditional water-harvesting systems meet domestic and irrigation needs of people. There is ample evidence to show that community management of traditional systems ensured that basic minimum requirements of all individuals were met.
Africa is a “green-water-dependent” continent. Green water is the rain that falls on fields and is captured by soils temporarily. When it seeps into ground, flows to rivers and accumulates in waterbodies that can be harvested / pumped out and further used is called the blue water.
Close to 95 per cent of Africa’s food production uses only rainwater. As there is a lack of water availability and also unsuitable geographical features, only 5.5 per cent of arable land is suitable for irrigation. Ironically, notwithstanding the fact that Africa is the second driest continent of the world, it also loses most of its well-endowed rainwater.
In comparison to its precipitation, the runoff is extremely low. Rainfall constitutes just 20 per cent of the continent’s renewable water resources indicating very low harvesting and recharge. In a study published in the journal Nature in March 2015, Johan Rockström and Malin Falkenmark of the Stockholm Resilience Centre noted, “Ninety-five per cent of Sub-Saharan agriculture depends on 'green water': moisture from rain held in the soil. In large parts of the continent, most rain evaporates before it generates 'blue water', or run-off, so little of it recharges rivers, lakes and groundwater.”
“Most farming communities are a long way from rivers and cannot use irrigation. Arid deserts and semi-arid savannahs comprise 40 per cent of the region's land area. These receive too little surface run-off (less than 100 millimetres a year) to grow maize (corn), rice, millet and sorghum (which requires at least 400 mm per year) using irrigation alone,” said the researchers in the Nature study.
What is the potential of rainwater harvesting in Africa? First, let’s look at the standard rainwater potential a small piece of land offers. Imagine you have a hectare of land and you receive 100 mm of water in the year (common for most of the continent). One can harvest as much as a million litres of water — enough to meet drinking and cooking water needs of 182 people at a liberal 15 litres per day. Even if you are not able to capture all that water — this would depend on the nature of rainfall events and type of runoff surface, among other factors — you could still, even with rudimentary technology, capture at least half-a-million litres a year.
According to a calculation by the Delhi-based advocacy and research think-tank Centre for Science and Environment (CSE), Sub-Saharan Africa has rainwater harvesting potential of 13,365,000 million cubic metres. To make sense of this estimated potential, it is nearly 44,550 times the average volume of water that flows through the Nile River per day.
In November 2006, a seminal study on the rainwater potential of the continent startled the world. The study done by the UN Environment Programme (UNEP) and the World Agroforestry Centre found, “African countries suffering or facing water shortages as a result of climate change have a massive potential in rainwater harvesting, with nations like Ethiopia and Kenya capable of meeting the needs of six to seven times their current populations, the UNEP study estimated.”
“The figures are astonishing and will surprise many,” said Achim Steiner, the then UNEP Executive Director. As per this study, “the overall the quantity of rain falling across the continent is equivalent to the needs of 9 billion people, one-and-half times the current global population.”
About a third of Africa was deemed suitable for rainwater harvesting if a threshold of 200 millimetres of arrival rainfall, considered being at the lower end of the scale, was used. “Africa is not water scarce,” it concluded. “The rainfall contribution is more than adequate to meet the needs of the current population several times over. For example Kenya would not be categorized as a ‘water stressed country’ if rainwater harvesting is considered. The water crisis in Africa is more of an economic problem from lack of investment, and not a matter of physical scarcity.”
“Over the coming years we are going to need a range of measures and technologies to capture water and bolster supplies,” Steiner said.
“Conserving and rehabilitating lakes, wetlands and other freshwater ecosystems will be vital and big dams, if sensibly and sustainably designed and constructed, may be part of the equation too. “However, large-scale infrastructure can often by-pass the needs of poor and dispersed populations. Widely deployed, rainwater harvesting can act as a buffer against drought events for these people while also significantly supplementing supplies in cities and areas connected to the water grid,” he added.
This article is excerpted from the State of Africa’s Environment 2024 report.