SESOIL and the Nile

River

It might seem odd to find a reference to the Bahr el Ghazal swamp, a tributary to the Nile River, in the original SESOIL user’s guide, but there it is. In 1980, Siu-On Chan and Peter Eagleson at the Massachusetts Institute of Technology were given the task of determining where the water entering the swamp went. It had been assumed that most of the water evaporated, but no one was sure. They were asked to determine how much water could be recovered if a canal was constructed to bypass the swamp. Reclaimed water would be sent downstream where it would be used by Sudan and Egypt for agricultural production.

Nile Swamps

About 2,500 km south of Cairo Egypt, the White Nile River enters the Sudd and Bahr el Ghazal swamps. Between the heat, humidity, mosquitoes, malaria, parasites, crocodiles, and hippos, the swamps form an impenetrable no man’s land. An intertwined network of lagoons, channels, and lakes, that stretch for hundreds of miles. Flows are so small that it is not possible to determine where the water comes from, or where it goes. Island mats of floating vegetation shift with the wind, making navigation nearly impossible. For humans the swamps contain nothing but death and disease. For thousands of years, starting with the ancient Egyptians, explorers searching for the source of the Nile were defeated by the swamps. Until the 1860s few had ever successfully traversed the swamps. The Bahr el Ghazal swamp is so inhospitable that when Alexine Tinné rented a boat from the owner, the captain sank it so as not to go. Traveling was so difficult that the new captain had to take the paddle wheels off, so the boat could be dragged through the swamp. By 1899, a navigational channel was cleared through the Sudd, however constant maintenance is required to keep it open. The area is so flat that during the wet season water from the White Nile crosses the divide with the Bahr el Ghazal basin to the west forming one huge swamp. At the confluence of the Bahr el Ghazal and White Nile Rivers, it is difficult to determine which is the main source. The Nile explorers were divided between Livingstone, Tinné, and to some extent Burton who thought the source lay to the right with the Bahr el Ghazal River. Others including Speke, Grant, Baker, and Von Sass looked to the White Nile on the left. The issue was not resolved until 1877 when Stanley followed the Lualaba River northwards and found it turned west becoming the Congo, instead of continuing northeast to become the Bahr el Ghazal River.

Bahr el Ghazal

During the annual floods, the Bahr el Ghazal swamp expands to the size of South Carolina (85,000 km 2 ). During the dry season, it shrinks to an area slightly larger than Connecticut (16,600 km 2 ). Of the 33.7 km 3 of water that enter the Bahr el Ghazal swamp every year, only 0.6 km 3 reach the White Nile River. The rest disappears. The annual loss of 33.1 km 3 is larger than the volume of Lake Mead (32.2 km 3 ), the largest reservoir in the United States. There were only two possibilities, the water evaporates, or it migrates via groundwater flow. Most believed the water evaporated in the hot African sun. Still no one knew how much went where. Chan and Eagleson had to find out. The reason was the Jonglei Canal. It was proposed that by bypassing the swamps with a narrow canal, evaporative loss could be significantly reduced. This would send an additional 5 km 3 of water downstream every year (about half the annual discharge from Lake Mead). Recovered water would be divided between Sudan and Egypt. Chan and Eagleson were tasked with determining how much water could be recovered by draining the swamps. They did this by developing a water balance model for the Bahr el Ghazal. Conceptually they divided the swamp into two parts, the papyrus swamp, and the open grasslands. The papyrus swamp was modeled as a circular area, completely surrounded by a donut shaped grassland. Driven by evaporation, the grasslands grow as the swamp shrinks. The most difficult part was determining how much water evaporated from the grasslands. It is obvious that during the wet season precipitation and floodwaters would recharge grassland groundwater. What is not so obvious is that during the dry season groundwater would wick up and evaporate. But, Chan and Eagleson knew this. They assumed that evaporation from the grasslands was just as high as from the swamp, but for only seven months. Because once the water table fell below the capillary rise zone, evaporation from the soil would stop. Based on their modeling they determined that although the rate of groundwater loss, if any, remained unknown, it was clear that evaporation alone could account for the huge loss. They concluded the Jonglei Canal could easily recover the 5 km 3 of water annually.

Drain the Swamp

Environmentalists see the swamps as a gigantic reservoir regulating floods. They believe evaporation from the swamp creates local precipitation. It may be that by diverting water downstream it could disrupt the delicate ecological balance that the regional tribes depend on. Some believe the reduction in local precipitation could turn the region in to a desert. Destroying the wetlands that are home to hundreds of species. Shrinking the swamps would also reduce grasslands available for grazing livestock and wildlife. The 360 km long canal will form a barrier to the migration of animals, people, and their livestock. Recently elephants have taken refuge in the swamps. Not because it is their native habitat, but because of the lack of human hunters killing them for their ivory. Other see benefits in constructing the canal. Less flooding means less mosquito-borne and waterborne diseases. Canal water would be available for local consumption. Providing a stable water supply in areas that currently deal with annual droughts improving agricultural production. The canal would create a navigational short cut, significantly reducing travel times. Although population densities are low, the area has been plagued by war. The canal may only intensify regional conflicts, or perhaps associated infrastructure improvements would bring economic opportunities, reducing tensions.

Construction Starts, then it Stops

Using the world’s largest bucket-wheel excavator, construction of the canal began in 1978. Looking like a Ferris wheel gone mad, the bucket wheel rig swings back and forth, cutting a canal 44 meters wide and 4 meters deep in a single pass, traveling at about 4 km per week. No one can say for sure what the ecological and economic impact of the Jonglei Canal would be, and we may never know. In 1983, after digging 260 km, construction stopped in its tracks. First, the work site was attacked and abandoned; later the excavator itself was destroyed by a missile strike. All part of a civil war that was fought, at least in part, because of the Jonglei Canal. Although construction has stopped, given the growing demand for water downstream, it could just be a question of time until construction resumes. In 1984, Sharron C. Gaudet and Eagleson, at MIT, developed an enhanced water balance model for the Bahr el Ghazal swamp. They wrote the SIM model that simulates: precipitation, evapotranspiration, and streamflow in the Bahr el Ghazal. With input parameters such as: the number of storms, and soil pore disconnectedness index, it would be strangely familiar to any SESOIL user. Modeling indicates that the Bahr el Ghazal swamp would be significantly smaller following construction of the complete canal system. Eagleson had a much larger impact on the SESOIL model than many people realize. Bonazountas and Wagner in the 1981, SESOIL user’s guide state that “The hydrologic cycle is based on a statistical dynamic formulation of vertical water budget at a land-atmosphere interface (Eagleson 1978), adapted to account for monthly simulations.” Perhaps even more surprising is how much unsaturated zone modeling has to do with the geopolitical stability of the Nile River Basin.