Hydrological investigation of the Ihefu wetland, Usangu, Tanzania

As part of the RIPARWIN project a detailed investigation was conducted of the hydrology of the Ihefu wetland, located on the Usangu Plain in south-west Tanzania. This was undertaken because, as a consequence of increasing upstream withdrawals, since the early 1990s water levels in the Ihefu have dropped below a critical level and outflows from it have ceased in the dry season. The consequent drying up of the Great Ruaha River, downstream of the Ihefu (in some years for more than 100 days), has had a disastrous impact on the ecology of the Ruaha National Park, where many animals depend on the river for water. In 2001, the Government of Tanzania promised to return the Great Ruaha River to year round flow by 2010. The key questions that this component of the RIPARWIN project aimed to answer were: i) what flow is required downstream of the wetland? and ii) how much water needs to flow into the wetland to maintain this outflow?

The study

The study used satellite images to investigate changes in land cover and the area of the wetland over time and developed a hydrological model to determine water fluxes and the water budget. The model simulated the wetland as a simple reservoir. Flow data collected from downstream of the wetland were analyzed to quantify changes in the flow regime and the desktop reserve model developed in South Africa was used to estimate minimum environmental flow requirements.

Results

The study found that, depending on rainfall and its distribution, there was considerable seasonal and year-to-year variation in the wetland area. Since 1958, increasing diversions of water have caused average dry season inflows to the Ihefu to decrease from approximately 15.0 m3s-1 to 4.3 m3s-1 (i.e., a 70% decrease). This has led to a reduction in the average minimum dry season area of the wetland from approximately 160 km2 to 93 km2 (i.e., a 40% decrease) and a decline in average dry season outflows of more than 90% in some months (Figure 1). Results from the desktop reserve model indicate that to maintain the basic ecological functioning of the river requires an average annual allocation of 635 Mm3 (i.e. equivalent to 22% of the mean annual runoff) and the absolute minimum dry season flow should not be lower than approximately 0.6 m3s-1 (Figure 2). To ensure the minimum outflow of 0.6 m3s-1 an average dry season inflow to the wetland of 7 m3s-1 must be guaranteed. To achieve this, the dry season flows in the perennial rivers discharging into the wetland would have to be apportioned so that 20% is used for anthropogenic purposes and the remaining 80% discharges into the wetland. Although there is significant potential for improving upstream irrigation efficiency, given the socio-economic importance of current levels of water withdrawal, this division may be difficult to achieve in the near future. Consequently consideration needs to be given to other options, including engineering solutions for water management within the wetland itself.   

 
Figure 1: Illustration of the progressive decline in dry season outflows from the Ihefu wetland over 3 time periods

 
Figure 2:Time series of monthly observed flow and estimated environmental flow (1958-1973) for the Great Ruaha River within the Ruaha National Park (note log scale on the y-axis)

Publications

Kashaigili, J.J., Mbilinyi, B.P., McCartney, M. and Mwanuzi, F.L. (2004). Dynamics of Usangu Plains Wetlands: use of Remote Sensing and GIS as Management Decision Tools.  Presented at the 5th WATERNET/WARFSA Symposium Windhoek, Namibia, 2nd –4th November 2004.

Kashaigili, J.J., McCartney, M., Mahoo, H.F., Mbilinyi, B.P., Yawson, K.D., Lankford, B.A. and Tumbo, S. (2006). Use of a hydrological model for environmental management of the Usangu Wetlands, Tanzania Research Report. Colombo, Sri Lanka; International Water Management Institute (forthcoming)

Contact
For further information contact Matthew McCartney (email: m.mccartney@cgiar.org) or Japhet Kashaigili (email: jkashaigili@yahoo.co.uk).

  RIPARWIN was a multi-disciplinary investigation of water allocation and management in a water stressed catchment in Tanzania. The project was co-managed by the Overseas Development Group (ODG) of the University of East Anglia, UK; the Soil Water Management Research Group (SWMRG) of the Sokoine University of Agriculture, Tanzania; and the IWMI-South Africa Office. Further details can be found at:  http://www.iwmi.cgiar.org/Africa/files/RIPARWIN/05/Riparwin.htm