Hydrology | White Paper

Hydrology

Written by  John C. Cannon 4/6/2016
Whether it’s present in droves as during the long rains of March to May in the Serengeti, or absent for long stretches across the plains with only a single river and a few springs that persist through the dry season, water plays a critical role in the dynamics of the Great Rift Valley.

Water holes and rivers are a source of life and death for the wildebeest and other herbivores that cycle through the Serengeti ecosystem every year in the Eastern Great Rift. The rains determine the paths of the migratory species among them, and they’re critical for keeping the plains filled with grass and not forest.

Along the Western Great Rift’s edge, a series of lakes and rivers have gone through a series of upheavals over just the past 20,000 years, spilling one into the next and creating an unusual and diverse system of alpine lakes – ones that belch natural gas, lakes so salty that only a few species of fish can live in them, and a delicate but critical source of food and transportation for the regions’ human populations (4,5).

Oddly enough, Africa’s biggest body of water, Lake Victoria (and the second largest freshwater lake in the world by surface area after Lake Superior), was not formed by the tectonic shuffling that created Lakes Edward, Kivu, Tanganyika, and many others. Its origins lie in the erosion of a basin over time, not the sudden formation of one as Earth’s subsurface plates move around (5).

At the heart of the Serengeti Ecosystem is the Mara River. Famous for its role in the drama of the annual wildebeest and zebra migration, as several million animals cross from Tanzania into Kenya and the Maasai Mara Reserve through a gauntlet of crocodiles, it’s the only major river that doesn’t run dry (yet) during the dry season (9). The Mara is also a critical link between the Mau Hills Forest in Kenya and Lake Victoria in the west (1,8).

But it’s changing, say scientists. The pH of the Mara River is becoming more basic (higher pH) and saltier, and the amount of water pouring between its banks is down 68 percent in the past 45 years.

Researchers believe that these shifts are happening because of deforestation upstream in the Mau Hills Forest and the “extraction of water for irrigation in Kenya upstream of the ecosystem.” In fact, farming – for which much of that irrigation water is earmarked – is three times what it was 30 years ago (2).

The loss of the forest may continue to compound the problem, as land cleared for agriculture doesn’t inject moisture back into the atmosphere the same way a forest does – hence, there’s less rain to fall at the Mara’s source and throughout the ecosystem.

Studies of rainfall in the area have shown that less rain is falling nowadays, particularly during the “long rains” between March and May on the Serengeti. And the rain that does fall is more erratic and less predictable (3). Other research has shown similar volatility in the Mau Hills Forest as well (2).

That creates problems beyond just the immediate need for plants to have an adequate water supply. First, it makes the whole system less resilient in the face of droughts that are part of the normal cycle over the long-term. One group of researchers found that if the Serengeti is faced with droughts such as those in 1949-52 or 1972-73, the Mara River could run dry.

That would have a devastating impact on not just migrating wildebeest and zebras but the lions, leopards and other carnivores that rely on these prey species. Indeed, the whole system could collapse if disruptions in water supply thwart the migration because it could upset the careful balance negotiated over eons. As things stand, the wildebeest and other migratory animals use certain areas heavily for a time, and then move on, allowing them to recover. Forced to stay in just one place, these millions of animals could destroy the land they depend on (1,2).

In an interesting twist, the heavy rains that fall during March and May each year are responsible for keeping the grasslands of the plains intact. The Ngorongoro Highlands hold repositories of important minerals, such as potassium, sodium and calcium. When the rains wash the salts of these elements to the plains below, the seep into the soil. Not far below the surface, the salts harden forming what geologists call a “petro-calcic horizon.” There’s plenty of space for the roots of grasses to grow, but in most places, tree roots cannot break through this barrier. As a result, the hectares upon hectares of flat grassland that form the basis of the Serengeti Ecosystem remain intact.  

As the science has shown, the robust Serengeti system could be rendered evermore fragile by changes to its water supply. As such, major international organizations UNESCO are investing in the field of ecohydrology to better understand the impacts that we humans could have, through climate change, deforestation, or the damming of waterways, to name just a few, on such a unique place (10).

Climate change is well-studied in East Africa’s Rift Valley, but many scientists admit that it’s difficult to sort out what will happen to the tropical grasslands in Kenya and Tanzania when the temperature rises. A 2010 study shows that, in an area known throughout its geologic history for tumultuous changes, swings in climate were just another factor to add to that roster. By that measure, the changes we see from Anthropocene-era climate change might not be as drastic (4). But the study’s authors were quick to caution that only a modest suite of changes due to climate change was by no means a forgone conclusion.

Perhaps more important is the question of how the plants, animals and people who call the area home will adapt to these changes.


Citations

1. Gereta et al. Ecohydrology as a tool for the survival of the threatened Serengeti ecosystem. Ecohydrology and Hydrobiology. 2013.
2. Dybas. Saving the Serengeti-Masai Mara - Can ecohydrology rescue a key East African ecosystem? Bioscience, 2011.
3. Wakachala et al. Statistical Patterns of Rainfall Variability in the Great Rift Valley of Kenya. Journal of Environmental and Agricultural Sciences, 2015.
4. Olaka et al. The sensitivity of East African rift lakes to climate fluctuations. Journal of Paleolimnology, 2010.
5. East African Lakes. Encyclopedia Brittanica.
6. East Africa's Great Rift Valley: A Complex Rift System. Geology.com.
7. Serengeti National Park: Hydrology and Soils of the Serengeti National Park. Stacey Lyon-Cronk, 2012.
8. Krhoda. The Impact of Resource Utilization on the Hydrology of the Mau Hills Forest in Kenya. Mountain Research and Development, 1988.
9. Mnaya et al. Ecohydrology-Based Planning as a Solution to Address an Emerging Water Crisis in the Serengeti Ecosystem and Lake Victoria. In River Ecosystems: Dynamics, Management and Conservation, 2011.
10. Hydrology & Water Resources. UNESCO.