A dramatic rise in groundwater in the Sahel

I was looking at satellite remote sensing data when I was struck by something curious. This remote sensing data used gravitational pull to measure shifts in large masses of water on earth. It is really quite extraordinary to think that one can measure water with gravity. It has become quite a powerful tool for scientists to understand water on earth. They can see how much water exists in areas at a resolution of about 320km by 320km. It measures what is called Terrestrial Water Storage, or TWS, which includes surface water, soil moisture and groundwater.

This above data is from Chad, at 13.5N 17E. I looked around at other places along the same latitude in the Sahel, and they were all increasing faster. So I worked on some Python code, pulled all the satellite data, and looked across the 13.5N transect from Senegal to Mali to Burkina Faso to Niger to Chad to Sudan. Lo and behold, they were all increasing faster. It was increasing fastest towards the middle of the continent, in Chad.

[The x-axis here is the longtitude, so imagine you are going across the Sahel. The y-axis is the terrestrial water storage increase acceleration]

The increase in terrestrial water storage means the groundwater is increasing too, which is a big deal since it has a big effect on society’s water supply. All around the world we are experiencing a lot of over extraction of groundwater. So it’s particularly interesting when there are areas with huge increases in groundwater.

So I started looking around to see why this terrestrial water storage was shifting.

One might think that terrestrial water storage is increasing because the rain is increasing. But Susanna Werth, a geodesist who tracks the planet's water mass budget using remote sensing data, studied the Niger basin from 2003 to 2013 and found something puzzling: terrestrial water storage was rising significantly across the Niger River basin and most of its subbasins, yet there was no sufficient increase in rainfall quantity over the same period to explain it [Werth 2017]. Her explanation pointed instead to the behavior of water after it falls: a great deal of rain flows into low-lying areas and from there percolates slowly down to recharge aquifers, a mechanism a number of hydrologists have highlighted for the region.

My own first idea for why there was this massive jump in terrestrial water storage was that it was because of all the restoration work done under the Great Green Wall across the Sahel, and how areas like Burkina Faso, Niger and Chad have undertaken massive rain infiltration projects that has led to the terrestrial water storage increase. And I still think this is the primary reason, coupled with other factors.

In Niger and Chad villagers are digging half-moon crescents and growing vegetation at large scale, which results in more infiltration, less runoff, more water finding its way underground.

[Yacouba Sawadogo]

A farmer named Yacouba Sawadogo brought back this ancient indigenous technique called zai pits in Burkina Faso. Villagers mocked him at first, but with persistence over decades he began to convince fellow villagers of its power. He was eventually called the Man Who Stopped the Desert, and worked with NGOs and government programs to train and mobilize farmers to dig by the millions.

[Zai pits]

The massive water recycling and eco-restoration projects in the Sahel are really quite astonishing in scale. Andrew Millison’s inspiring videos captures the scale and power of these projects.

Having noted this extraordinary rise of terrestrial water storage in the Sahel I was curious what groundwater state around the world was. I looked around for a map of groundwater depths and found the work of Ying Fan, a hydrologist at Rutgers. On her website she writes ‘I am a hydrologist not interested in water per se, but in what water does to shape our landscape through the ages. Water connects all. Little happens without water." She took information from well data around the world and put it into groundwater models to measure how water moves underground, producing a map of groundwater depths across the globe. In that map the Sahel is colored green, indicating that much of the water table there sits at about 5 to 10 meters depth.

[Simulated groundwater tables from Fan’s work]

So zai pits, half moon crescents, and low lying recharge areas are increasing the terrestrial water storage. What about the role of rain? It has been increasing and so is also a factor. Why has Sahel rain been increasing ?

One cause is that rainfall is rising because of changes in the ocean. Climate scientists figured out some time ago that changes in ocean temperature drive climate modes like ENSO, which then affect rain patterns around the world. It turns out that the temperature gradient between sea surface temperatures in the Tropical North Atlantic (TNA) and the Gulf of Guinea drives the ITCZ, the band of intense convection near the equator that migrates north and south with the seasons, and which in turn brings more or less rain to the Sahel depending on how far north it travels. Whether the ITCZ penetrates to 15 or 20 degrees north, deep into the Sahel, or stalls at 10 degrees, watering only the Guinean coast, is the difference between a good year and a failed harvest.

[NOAA data, you can see TNA rising recently]

It turns out that tropical North Atlantic sea surface temperatures have been rising over recent decades, driven by a combination of greenhouse warming and the cleaning up of industrial sulfur aerosols that had been seeding clouds and cooling that stretch of ocean for much of the twentieth century.

But ocean temperatures are not the only reason for the Sahel's wetter recent decades, there is also precipitation recycling. A group at the University of Wisconsin-Madison, led by climate scientist Michael Notaro and graduate student Yan Yu, working with collaborators at Oak Ridge National Laboratory, turned to three decades of satellite observations of vegetation cover across the Sahel, combined with on-the-ground rainfall records and measurements of temperature, humidity and wind [Yu 2017]. What they found was observational evidence for a positive vegetation-rainfall feedback. As plants draw moisture up from the ground through their roots, they release it back into the atmosphere through their leaves. That moisture drifts downwind and falls again as rain somewhere else in the Sahel, growing more plants, which transpire more moisture, which falls as more rain. Vegetation greenness during the late and post-monsoon periods favours enhanced evapotranspiration, precipitable water, convective activity and rainfall, with the precipitation recycling response of comparable magnitude to the evapotranspiration response itself. Notaro described the finding as demonstrating "an elusive feedback mechanism that's been hypothesized for the Sahel for decades."

The satellite remote sensing photos, leveraging gravitational anomalies, tell the powerful story of this dramatic rise in terrestrial water storage, the rise of this source of water for the millions of people in the area. It is important we figure out why its rising, scientifically. All the different factors are probably contributing to some degree: zai pits and half-moon crescents increasing infiltration, large scale eco-restoration, low-lying areas recharging aquifers, rising sea surface temperatures pulling the monsoon northward, declining industrial aerosol emissions warming the tropical North Atlantic, increased precipitation recycling through vegetation, and global warming. I believe the zai pits and half-moon crescents are a key aspect we should be focusing on scientifically, to document better hydrologically and to put into the climate and hydrological models. If we can track hydrologically what millions of small holes in the ground are doing to the water cycle of an entire region, we help develop our scientific understanding of how zai pits, earthworks, and permaculture processes like swales can restore water cycles, watersheds and degraded drylands all over the world.

What has also become even clearer to me through this investigation, is that to document the success of slow water movements around the world, and to convince the public and policy makers of their importance, we can leverage the power of remote sensing satellites.

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References

Fan, Ying, H. Li, and Gonzalo Miguez-Macho. "Global patterns of groundwater table depth." Science 339, no. 6122 (2013): 940-943.

Werth, Susanna, Dave White, and D. W. Bliss. "GRACE detected rise of groundwater in the Sahelian Niger River basin." Journal of Geophysical Research: Solid Earth 122, no. 12 (2017): 10-459.

Yu, Y., Notaro, M., Wang, F. et al. Observed positive vegetation-rainfall feedbacks in the Sahel dominated by a moisture recycling mechanism. Nat Commun 8, 1873 (2017). https://doi.org/10.1038/s41467-017-02021-1