River Run Dry

For the past 11 years, the Great Ruaha River in Tanzania has stopped flowing for extended periods every dry season. Previously, the river was perennial. The recent drying is caused by diversions for irrigation upstream of the Usangu Wetlands, a valuable freshwater ecosystem. Between 1970 and 2004, the irrigated rice area increased from approximately 10,000 hectares to 45,000 hectares.

Inspection of the dried river bed of the Great Ruaha River, at the outlet of the Usangu wetlands. Photo credit: Bruce Lankford.

The drying has brought social conflict between upstream and downstream users. In the dry season, some women and children have to walk up to 20 kilometers every day to find water. There is also harm to Ruaha National Park, which is located downstream of the Usangu Wetlands. The river is a crucial dry season water source for the park, and its drying has had serious ecological impacts. In some years, many hippopotami, fish and freshwater invertebrates die.

Researchers at the International Water Management Institute (IWMI), together with partners from Sokoine University in Tanzania and the University of East Anglia in the United Kingdom, conducted a hydrological study to estimate the environmental flow of the Great Ruaha River required to maintain its basic ecological functions. Results showed a requirement of an average of 635.3 million cubic meters per annum (equivalent to 22% of mean annual runoff) and an absolute minimum dry season flow of 0.5 cubic meters per second through the national park.

The Great Ruaha River is one of the most important waterways in the Rufiji Basin of Tanzania. Water from the river and its tributaries is used for hydropower production and irrigation, making vital contributions to the livelihoods of rural households. The headwaters of the Great Ruaha River drain through the rich alluvial plains of Usangu, which have both permanent swamps and seasonally flooded grasslands. This is a habitat for over 400 bird species and many other animals and plants. The Usangu Wetlands also support agriculture, fisheries, livestock rearing and light industry. In some villages, 95% of households benefit from the wetlands.

Balancing the tradeoffs between economic growth and resource degradation and depletion is a key objective of the Rufiji Basin Water Office. In their study, IWMI and its partners used the desktop reserve model (DRM) to quantify the environmental flow requirements of the river downstream of the wetlands. The model is useful when a rapid appraisal is required but data is limited. Like the building block method developed by South African scientists, it is underpinned by the premise that, under natural conditions, different components of flow play different roles in the ecological functioning of a river. Another model was used to simulate the hydrology of the Usangu Wetlands and estimate the flow into them required to maintain downstream environmental flows.

Allocating water to maintain the river would require tradeoffs between the environment and other users. Maintaining the dry season minimum flow requires that current water abstractions be reduced by 60%. However, irrigated rice is the main source of income for 30,000 poor rural households. It is also important for national food security, as up to a quarter of Tanzania's rice is grown in the catchment. Reducing abstractions would therefore have serious social and economic repercussions.

Alternative options, including tradeoffs between different environmental needs, should be considered. For example, active water management to reduce evaporation in the wetlands would improve flows to the national park. Better water management on farms would reduce wastage. Currently, many villages rely on irrigation canals for their domestic water supply, requiring that diversions be maintained throughout the dry season. Since much of the diverted water is lost through seepage and evaporation, significant water savings may be possible from alternative options for domestic supply, such as groundwater. This would reduce the need for surface water withdrawals.

The maintenance of aquatic ecosystems is a prerequisite for sustainable development. In an environment of worsening water scarcity, decisions regarding the allocation of water must consider environmental implications. However, estimating water requirements for wetlands in water-stressed catchments where peoples' livelihoods dependent on water abstraction is a task far from trivial. It is important to consider not only environmental requirements, but also the economic and social implications. In such situations, understanding flow regimes and hydrological functioning is essential for informed decision making.