Measurements of isotopic ratios in water samples suggest that to conserve surface and subsurface waters, shallow groundwater should not be pumped intensively.

"We have to develop [a] new agricultural system to overcome climate change as quickly as possible," said Morio Iijima of Kindai University, Japan. "It should be not only an environmentally-friendly system to preserve water balance in this semi-arid region, but also a beneficial one for small-scale farmers."

The seasonal wetlands of north-central Namibia are of great importance to locals, most of whom are small-scale subsistence farmers who use the wetlands to fish, to raise livestock, to cultivate rice and to wash clothes. But the wetlands are also very fragile, and large year-on-year variations in rainfall have led to highly unstable food production. The amount of surface water has varied widely in the last decade, causing either serious deluges or droughts.

Knowing exactly where surface water originates is important, as it could help Namibian farms use their land more sustainably. Since 2011, Iijima and colleagues have been developing a flood- and drought-adaptive cropping system that can preserve water in fragile semi-arid environments. Now they have studied the Cuvelai system seasonal wetlands in north-central Namibia to uncover the origin of surface water, and see whether their adaptive cropping system could help.

To do so, Iijima’s group, which includes members from various Japanese institutions, as well as from the University of Namibia, analysed the isotopic ratios in semi-heavy water (HDO) and enriched water (H218O) taken from rainwater samples over the Namibian wet season from October 2013 to April 2014. Because the isotopes change dynamically with evaporation and condensation processes, which depend on air temperature and air pressure (and hence elevation), their ratios can point to an origin for the water. The researchers also turned to satellite-recorded longwave radiation, which helped to link the movement of water to the existence of convective clouds.

Iijima and colleagues found that about three-quarters of rainwater in the Namibian Cuvelai system seasonal wetlands region comes from water recycled at local scales. That rainfall appears to feed surface waters, which temporarily pool in the wetlands and recharge the shallow groundwater.

Iijima believes that his group’s adaptive cropping system could benefit this fragile water cycle, and warns against the pumping of the shallow groundwater, which has been attempted in other regions of northern Namibia. He would now like to introduce the cropping system, while studying the hydrology of the wetlands in more detail. "We are still on the way to finding the water budget of the ecosystem," he said.

The study is published in Environmental Research Letters (ERL).

Related links

Related stories