Below sandy, dry riverbeds: a medicine against drought

28 September 2021

Water just beneath the surface in parts of Southern Africa? It seems unlikely, but dried up riverbeds sometimes harbor an unexpected resource. However, accessing it is a challenge. The Dutch popular science magazine Quest interviewed IHE Delft’s Annelieke Duker and Pieter van der Zaag about a hidden abundance of water in Southern Africa’s driest months.

This article has originally been written in Dutch by Pepijn van der Gulden, editor at Quest Magazine

Extreme drought occurs when even the rivers are completely dried up. Things look pretty hopeless when the riverbed has transformed into a sandpit, dozens or even hundreds of meters wide. This situation is a regular occurrence in dry areas of African countries like Zimbabwe and Botswana. These meandering sandy plains seem like the last place you would expect to find even a drop of water. However, those dry riverbeds actually provide a great opportunity for finding water, according to Dutch researchers.

Periods of drought have always been a regular occurrence in Southern Africa, but over the past years they have lasted increasingly longer. A continuously reliable source of water sounds almost too good to be true. Can dried-up riverbeds really help against drought? And if so, where is the water?

Reservoir for rain

More rain isn’t really the answer: it just needs to be distributed better. Since the weather gods are hard to bargain with, the best course of action is to retain the water for longer. This would require quite a number of barrels – unless, of course, nature has already provided a comparable solution. That’s why Duker is investigating the dried-up riverbeds. The sand that the rivers carry with them happens to be very effective at retaining water. This means that those wide riverbeds can actually be quite effective water reservoirs. 

Seen from above, the riverbeds are even dryer than their surroundings. This is because the top layer of water has evaporated. However, deeper layers of sand retain a lot of water, Duker explains. ‘You still lose some water to evaporation in the first 50 centimeters, but that’s not a problem once you start going deeper.’ Up to 30 percent of the sandy flats consists of water, because it can’t flow downwards. ‘There’s always an impermeable layer at the bottom made of rocks, for example.’ Even if sand rivers look like a huge sandpit, in reality they can contain large amounts of leftover rainwater across their full width.

Short but powerful

The sandy riverbeds of Southern Africa aren’t always dry. The drought in that region is relative, water expert Annelieke Duker explains. She is researching the potential of these riverbeds at IHE Delft. ‘We’re talking about areas where the entire annual precipitation happens over one or two periods of a few months.’ The amount of rain isn’t the problem here. The only pity is that all the rainfall happens at once. The fertile wet season enables farmers to grow corn, among other crops. The rivers carry enormous amounts of water downstream, says Duker. ‘During the wet season, those sandy beds are soaked with water from the entire river basin.’ Rivers can be dozens or hundreds of meters wide. But this flood is short-lived. In the periods between the wet seasons, there are months of drought. The high temperatures cause the land to dry up quickly, until even the rivers evaporate, leaving behind only a sand river. This happens most of the year, which means agriculture without irrigation is fruitless. Many dry areas therefore also face considerable poverty. 

Yearly refills

What happens when many farmers use these sand rivers? Won’t they be depleted? Water expert Pieter van der Zaag isn’t worried about this scenario. The sand rivers have a big advantage: their water storage is replenished every year during the wet season. ‘Groundwater is a great source of water. But if you use it too often over a longer period of time, the groundwater levels will drop. When that happens, it can take a long time for the levels to recover.’ This risk of long-term damage through excessive use does not apply to sand rivers. Even if you would deplete them, this wouldn’t lead to long-term problems. ‘The river will fill itself completely when the next heavy rainfalls happen. Every year is a new opportunity. That’s one great aspect of sand rivers.’

Googling water

It sounds attractive to extract that water, although it seems like it might be a lot of work. In reality, that’s not too bad either. In sand rivers, you can find water pretty close to the surface, Duker explains. ‘If you stand on top of a bedding and make a hole with your hands, or a shovel, you can access the water. You can often find water at 30 to 50 centimeters’ depth. You can pump the water up with relatively small pumps.’ That actually sounds pretty favorable. So then maybe the sandy riverbeds are well-hidden? On the contrary. In fact, it’s easy to recognize the beds, explains Duker’s colleague Pieter van der Zaag, IHE Delft Professor of Water Resources. ‘They’re easy to spot, due to the color differences. The sand is very brightly colored when it’s dry. Along the banks of the sand rivers, you can often see rich vegetation, even in the dry season. That raises the question: how can those trees be so green, if the surroundings are so dry?’ You can use Google Earth to find suitable riverbeds: satellite images show the flow of sand rivers. The exact amounts of water still have to be determined, Van der Zaag says. ‘We think the potential is significant. We’re trying to map out the exact potential with preliminary studies. Our first estimation for Sub-Saharan Africa is that up to 20 percent of the surface is nearby one of these sand rivers. Around 150 million people live in those areas.’

Breaking with traditions

Making the switch to irrigation from sand rivers also demands mental flexibility, water expert Pieter van der Zaag explains. ‘Those sand rivers occur in parts of Africa that have a dry climate. In many years, rain-dependent agriculture can lead to crop failure.’ Because the rain supply is irregular, many inhabitants traditionally depend on livestock breeding. ‘It rains a lot during the wet season, but you never know where. Cows have legs, so they can walk to wherever the grass grows. As a farmer, you’re stuck to your patch of land: “when is that rain going to fall?”’ Sand rivers can provide reliable access to water, but that requires a different approach. Making the switch to fields is not easy for families that have reared livestock for generations. ‘You can see that clearly with the Maasai in Kenya. Many Maasai look down on growing crops.’ Rearing livestock is real work, waiting for plants to grow is mediocre. It takes time to make the transition to a new system.

Wet and dry

You’re probably thinking: let’s start the battle against drought! Get those pumps fired up and we can irrigate the nearby fields. But if matters were really that easy, wouldn’t local farmers seize that opportunity? Aren’t they much more familiar with their surroundings than Dutch academics? Some farmers have indeed started pumping up water, but it’s a far cry from mass irrigation.
Annelieke Duker
Annelieke is researching which other factors make this water inaccessible.


On the one hand, her research shows that the sand rivers provide many opportunities. Even while a major drought saw faucets in Cape town dry up, farmers still drew water from their hidden source. This also creates the possibility of growing different crops, Duker says. ‘Farmers usually grow corn during the wet season. But sand rivers enable farmers to grow many more vegetables or fruit trees, often in combination with food for livestock. They can grow crops that are hard to come by in these remote areas, and that are more valuable.’

Digital fieldwork in pandemic times

What does a researcher do who was supposed to conduct field research in the Southern African countryside when a pandemic makes travel difficult? The alternative is digital field research, researcher Annelieke Duker explains. ‘What I did: two colleagues in Zimbabwe went into the field to get in touch with farmers. The biggest challenge was that a number of farmers lived in an area without cell phone service.’ Duker’s contacts would pick the farmers up and take them to an area where they were able to make a call. ‘They would send me a message and I would call them. It actually worked quite well.’ Although there were some other problems. ‘Elephants also live there, and they have disturbed the area a few times. Irrigation systems make the area green, which attracts hungry elephants.’ This sometimes meant that visiting the farmers was unsafe, occasionally putting the research on hold.  

Keep it running

But just like the possibilities, the obstacles are numerous, Duker has noticed. Even getting started is quite a challenge. ‘It’s quite a risk to make the investment and pay 400 or 500 euros for a pump.’ To make this investment viable, the fields need to stay green all year round. But keeping the pumps running is tough. ‘One of the biggest problems for the farmers is getting enough resources, especially diesel. They often live in remote areas, so their lives are a constant balancing act: how much money do you have, how much can you spend on fuel to irrigate a part of your land and successfully harvest a crop?’

The harvest also often leads to problems. ‘The crops are often sold not for cash, but traded for a chicken or a goat. But farmers need cash, they have to buy fuel.’ In practice this regularly means that the pumps can’t run, which leads to unsown fields and crop failure.

Location isn’t helping

It doesn’t help that, although the sand rivers are numerous, they are often far away from inhabited areas. This means that farmers need to move their families in order to use the water supply. But even then, the remote location remains an obstacle, Duker says. ‘Farmers in Zimbabwe simply lack proper access to the market. There are no decent roads, which means it is very difficult to sell their harvest and invest in further growth.’

There’s no point in making the water more accessible until these surrounding conditions are fixed. Only when those challenges are met, can the value of the sand river research assert itself, Van der Zaag says. ‘In Kenya we saw the impact of an asphalt road alongside a sand river. Once that was built, everything started happening. It was like night and day.’

Water pumps and solar energy can tackle a few of the obstacles. So how much potential does the new water source hold? Van der Zaag sees considerable possibilities. ‘We are talking about a niche with considerable challenges. You can expect the largest growth to take place in agricultural regions that are already developed. But I expect that sand rivers will see substantial development over the coming decade. And we’re trying to contribute to that.’

Quest Magazine

This article has originally been written in Dutch by Pepijn van der Gulden, editor at Quest Magazine


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