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«Temporal Flow Variations: A Challenge for Water Management in Tanzania Patrick VALIMBA Department of Water Resources Engineering, Faculty of Civil ...»

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The pattern of decadal April flow changes could be attributed to high rainfalls in the 1960s through the early 1970s (Valimba, 2005) which might have contributed to substantial groundwater recharge that sustained April flows higher through the 1980s decade of reduced rainfall than in the 1950s. The consistent increase of average May flows in consecutive decades, decreasing average April flows and increasing end of April and May rainfall amounts related to long rains (March-May rains) shifting (Camberlin et al., 2003; Valimba, 2006) strongly suggest their contribution towards substantial flow peaking in May. Unlike in other eastern coastal basins of Tanzania such as Wami River basin where flow peak has shifted from April to May (Valimba, 2007b), there is no shifting in northeast Tanzania.

Fig 3: Changing monthly flow seasonality in selected catchments in northeast Tanzania. Allows show the nature of change (increase: up; decrease: down).

Fig 4: Changing weekly flow seasonality in selected catchments in northeast Tanzania.

Fig 5: Multi-year variability of maximum May flows in selected catchments in northeast Tanzania.

Low flows Except for the catchment of 1B4A with lowest flows in September/October where no changes were identified, lowest flows occurring in February and March in other catchments have either show no consistent changes (1DD1, 1DC2A) or consistently changed (1C1) (Fig 3). The percentage changes, however, indicated a consistent increase of 40-70% (February) and 80-102% (March) low flows at 1DD1 while low flow at 1C1 have consistently decreased by 24-53% (February) and 34-74% (March) (Table 2). Unlike the March low flow augmentation at 1DD1, the low flow reduction at 1C1 has been accompanied by shifting occurrence from February to March (Fig 3).

The changes of average monthly flows could be attributed by similar changes of low flow frequency rather than volume as indicated by relatively smooth flow hydrographs during the low flow period (Fig 4). That is, flows are either persistently below or above a reference (1950s) low flow magnitude. However, time series analysis of February/March flow magnitudes indicated augmenting flow minima at 1DD1 and 1B4A and slightly declining flow minima at 1DC2A and 1C1 (Fig 6).

Whilst the positive trends towards low flow augmentation might be advantageous for basin water resources planning, the declining low flows might substantially increase the current stress and water-scarcity conflicts in the region.

Fig 6: Multi-year variability of minimum March flows in selected catchments in northeast Tanzania.

CONCLUSIONS

This study has indicated the spatially variable patterns of seasonal flow variations in northeast Tanzania with flow peaking in April/May in most parts of northeast Tanzania and in December in rivers draining the northern part of the eastern arc mountains. Over the decades of available flow observations (1952-2005), the study has identified significant changes in high and low flows in the region. The changes of high flows were related to augmenting flow peaks in May while changes of low flows corresponded to flow reduction and/or shifting occurrence from February to March.

High flow augmentation corresponded mainly to increasing persistent high flows although in some catchments peak flow increase played additional role. Similarly, low flow reduction or increases were mainly affected by persistently low flows while reduction of flow minima contribute to the situation in some catchments. The major implication of this observation is on the management of the scarce water resources in northeast Tanzania, which is currently under increasing stress for use in various socioeconomic activities. It is proposed among the many relevant measures that increasing water flows during the high flow period should be stored to augment and stabilize low flows during the dry season to ensure adequate water supplies to communities.

REFERENCES

Camberlin, P., Okoola, R., Diop, M. and Valimba, P. (2003) Identification des dates de demerrage et de fin de la saison des pluies: Applications à l’Afrique de l’Est et au Senegal, Publ. Ass. Int. Climatologie, 15, 295 – 303.

MNRT (2005) Hydrological analysis of the Eastern Arc Mountain Forests, Ministry of Natural Resources and Tourism, United Republic of Tanzania, 132p.

Valimba, P. (2005) Rainfall variability in southern Africa, its influences on streamflow variations and its relationships with climatic variations, Unpublished PhD Thesis, Rhodes University.

Valimba, P. (2006) Spatio-temporal variations of rainy seasons in Northeast Tanzania and their relationships with ENSO and SST variations, Unpublished manuscript.

Valimba, P. (2007a) Spatial variation of hydrological floods during the short rains in Northeast Tanzania, Proc. International Conference on Climate and Water, Helsinki (Finland), 3-6 September 2007.

Valimba, P. (2007b) Initial Environmental Flow Assessment: The Wami Hydrology, Wami-Ruvu Basin Water Office, 49p.



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