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The New Diviner

Some of the vanished water from Lake Chad may still be available for use.

By Carl Zimmer
Apr 1, 1992 6:00 AMNov 12, 2019 4:43 AM


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Along the southern border of the Sahara, in the luckless region known as the Sahel, a drought has been going on for 20 years now. In the 1970s hundreds of thousands of people succumbed to the effects of crop failures and cattle die-offs, and today the millions of farmers and nomads who inhabit the Sahel are still living on the edge. One potent symbol of their plight can be seen from space: Lake Chad, once as big as Lake Erie, is now a third the size. But in that desolate image Solomon Isiorho sees a reason for hope. Some of that vanished water, he says, may still be available for use.

Isiorho is a Nigerian-born hydrogeologist who teaches at Indiana University in Fort Wayne. He has been studying Lake Chad--which lies on the borders of Nigeria, Niger, Chad, and Cameroon--for the past decade. He was drawn there by an old mystery: Why isn’t Lake Chad salty like, for instance, the Dead Sea? You have rivers flowing into it and none flowing out, Isiorho explains, and there’s a high rate of evaporation since it’s very close to the Sahara. When water evaporates, salt doesn’t, and so the water that’s left behind should become very salty. Yet Lake Chad is practically fresh, containing only 5 percent as much salt as seawater does.

But what if, Isiorho asked, some of the water leaving Lake Chad were not evaporating but seeping into the ground? In that case the salt would go with it. Such seepage occurs when the water table around the lake is lower than the lake surface (if the water table is higher, groundwater flows into the lake), and other researchers had proposed that this might be the case at Lake Chad. Isiorho realized that if the lake really were replenishing the groundwater, it could save lives.

In 1985 he started trying to measure the seepage--which would be almost imperceptibly slow if it occurred at all--by several methods, all of them relatively low tech. One, for example, made use of a seepage meter: a metal drum, open at the bottom end and closed at the top, with a partially filled bag of water connected to a hole near the top. Isiorho planted the open end of the drum in the lake-bottom mud in such a way that the top was underwater. If water was seeping out of the lake, the level in the bag would fall, but it would rise if groundwater was flowing into the lake.

After dozens of such measurements Isiorho has concluded that water is indeed leaking out the bottom of Lake Chad and flowing away at a rate of five inches a day. That adds up to an underground deluge of around 3 billion gallons a year--some 30 percent of the water that flows into Lake Chad, and enough to meet a lot of human needs. I was quite surprised, says Isiorho. They have a large irrigation system southwest of the lake that uses surface water, and the amount of water that gets into the ground is ten times what they use.

As it happens, the groundwater leaving Lake Chad is flowing southwest into Nigeria. Isiorho suspects that over hundreds of thousands of years--Lake Chad has been around since the Cretaceous Period--the water travels all the way to the Benue River, 240 miles away, which flows into the Niger. His goal now is to find out how the water gets to the Benue and where it might be concentrated in usable quantities. It probably flows along underground faults, and in some cases the surface expression of those faults can be found. They are not easy to see from the ground because the features are often only a few yards wide. But they can sometimes be detected on satellite images.

If Isiorho can pinpoint the hidden oases, the people who live in Lake Chad’s drainage basin could switch off between using surface water and groundwater, and one day, perhaps, leave the danger of drought behind them. But Isiorho thinks the impact of his work isn’t restricted to Africa. The drought-driven devegetation of the Sahel, he says, might also affect global weather patterns: If you have an area the size of the Midwest and you decide to clear all the vegetation, that’s going to have an impact on the global environment. I think we all have a stake in what goes on in this area.

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