Water availability and use dynamics and the sustainability of water resources management in the great Ruaha river catchment in Tanzania
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Date
2007
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Abstract
The purpose of this study was to investigate sustainability of water resources
management in the Upper Great Ruaha River Catchment (UGRRC) and the impacts
of water availability and use dynamics to the downstream river flows. Trend
analysis, regime shift analysis, low flow analysis and generation of indicators of
hydrologic alteration (IHA) were among the methods used to investigate variability
of rainfall and river flows. Water abstraction and use patterns were investigated
through intensive hydrometric monitoring and social survey methods. An integrated
river basin decision-making framework was developed and used to assess the
sustainability of water resources management.
The study found out that although river flows entering the UGRRC have not changed
much between pre 1980 and post 1980 time windows, split sample analysis of the
flows showed that the mean annual runoff exiting the UGRRC decreased from
2537.55 Mm 3 to 2053.77 Mm 3. The dry season flows also decreased by 57% between
the two time windows and the decrease, found to be due to human interventions
taking place in the plains, is statistically significance at 5% significance level.
Analysis of IHA parameters revealed a progressive decline in flows lower than Q 30 .
The analysis showed that 1-day minimum flow exiting the UGRRC decreased from
2.572 m 3 /s to 0.1221 m 3 /s; Q 90 decreased from 2.720 m 3 /s to 0.266 m 3 /s; zero flow
days have increased from 0.25 days to 22 days per annum in the post-impact period;
and the minimum flows now, start two weeks earlier as compared to the pre-impact
window. This implies a faster depletion rate of dry season flows in the UGRRC.iii
Comprehensive assessment of water demands and water resources of the Mkoji sub-
catchment revealed that during the dry season water resources are the limiting
production factors as they are not enough to meet the current requirements for
irrigation, let alone other water use sectors. The formal water rights were found to be
problematic as in eight out of the 12 studied river systems water rights were higher
than the actual river flows. However, the granted formal water rights were much
higher compared to the actual water requirements resulting into over-abstractions of
water above what is needed for crop production. As such streams run dry half way
through the sub catchment as water that would have kept them flowing throughout
the year are used up for irrigation. The result is that downstream water users suffer
more from water shortages and some sub-catchments (e.g. Mkoji) are now closed
during the dry season, contributing zero flows to the Great Ruaha River.
Assessment of sustainability showed that current water resources management
practices in the UGRRC are unsustainable and if maintained, they could lead to
severe social, environmental and economic consequences. The study concludes that
there is a need to review the formal water rights to conform to current water
availability and requirements and to improve monitoring and data management
system in order to fulfil the mission, goals and objectives of water resources
management in Tanzania. This study has demonstrated the value of combining
different research methods and analyses and the role of simple decision support tools
to assist in reaching and evaluating decisions concerning sustainable water resources
management.
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Keywords
Water resources management, Downstream river flows, Great ruaha river catchment, Water, Water availability