Michael Owor (PhD, 2006-2010)

Groundwater/surface-water interactions on deeply weathered surfaces of low relief in the Upper Nile Basin of Uganda

Funding

• Commonwealth Academic Staff Scholarship (UGCA-2006-141)
• University of London Central Research Fund (£4644)
• UCL Graduate School (£1424)
• International Foundation for Science (Sweden)

Project Partners

Directorate of Water Resources Management (Uganda)

Thesis Publications

• Owor, M., Taylor, R.G., Mukwaya, C. and Tindimugaya, C., 2011. Groundwater - surface water interactions on deeply weathered surfaces of low relief: evidence from Lakes Victoria and Kyoga. Hydrogeology Journal, Vol. 19, 1403-1420.
• Owor, M., 2010. Groundwater - surface interactions on deeply weathered surfaces of low relief in the Upper Nile Basin of Uganda. Unpublished PhD Thesis, University College London (UK).
• Owor, M., Taylor, R.G., Tindimugaya, C. and Mwesigwa, D., 2009. Rainfall intensity and groundwater recharge: evidence from the Upper Nile Basin. Environmental Research Letters, Vol. 4, doi:10.1088/1748-9326/4/3/035009
• Owor, M., Taylor, R.G., Thompson, J., Mukwaya, C. and Tindimugaya, C., 2009. Monitoring groundwater - surface water interactions in the Upper Nile Basin of Uganda. In: Groundwater and Climate in Africa, edited by R. Taylor, C. Tindimugaya, M. Owor and M. Shamsudduha. IAHS Publication No. 334, pp. 68-75.

Project rationale

Low relief (plateau) regions of eastern and southern Africa feature extensive areas of shallow, open surface water as permanent and seasonal wetlands. They are particularly prominent in Uganda where Lake Victoria and Kyoga wetlands occupy approximately one-sixth of the nation’s land area. Regionally, surface waters provide vital developmental (e.g. hydro-electric power generation, water supply), ecological (e.g. aquatic habitats) and socio-economic (e.g. fishery) services. Wetlands furthermore, control floods and reduce nutrient loading to adjacent surface waters (Kansiime et al., 2003; Mwanuzi et al., 2003). Although historical records are limited, water balance studies in the Victoria Nile Basin highlight the importance of the direct input of rainfall in sustaining surface water levels (Taylor and Tindimugaya, 1996; Taylor and Howard, 1996). There is, therefore, considerable uncertainty about how climatic change and variability will impact precipitation patterns and hence, water levels and the areal extent of shallow lakes and wetlands in the Lake Victoria and Victoria Nile basins. The role of groundwater in maintaining lake and wetland water levels during periods of low or absent rainfall on the African plateau has received little attention and remains very poorly understood. Critically, neither a conceptual nor a numerical representation of the interaction between surface water and groundwater presently exists. Recently, changes in the level of Lake Victoria have occurred as a result of variable precipitation and excessive dam releases at Jinja (Kull, 2006), but how these changes influence the nature of interaction between surface water and long-term stored groundwater remains unstudied. The impact of changing water levels on solute transport between groundwater and lakes and wetlands also remains unresolved. This latter uncertainty is of considerable importance in the Lake Victoria and Victoria Nile basins where changes in land use have occurred in response to population growth and rise in both commercial and subsistence agriculture (Odada et al., 2004). In terms of groundwater, the impact of recent, intensive abstraction for town water supplies on adjacent and wetland water levels in the basins has not been assessed.

Objectives

The study seeks to improve the understanding of the interactions between groundwater and surface waters on the low relief surfaces (e.g. African plateau) in the Lake Victoria and Victoria Nile basins of Uganda. Specific research objectives include:

• to characterise the lithologic interface between groundwater and surface waters and thereby resolve the origin and evolution of wetland environments on low relief surfaces (e.g. African surface)
• to evaluate the role of climatic changes and climatic variability as well as groundwater abstraction on the hydraulic gradient between surface waters and groundwater
• to assess the hydrological and hydrochemical fluxes between groundwater and surface water under conditions of climate change and variability together with groundwater abstraction

Working hypotheses

• A temporally and spatially dynamic interaction exists between groundwater and surface waters which enable significant fluxes of solutes and possibly contaminants to occur.
• Groundwater contributes significantly to the maintenance of surface water levels during dry periods.

Research methodology

Research will initially examine and model regional relationship between climate and water levels both at the surface and subsurface using available meteorological, hydrological records that include borehole hydrographs and remote sensing data. Critically, study sites will be constructed in two areas of contrasting land use and groundwater development (rural and urban/peri-urban) in Uganda to enable not only monitoring of groundwater and surface-water levels (Figure 1) and chemistry (solutes, stable isotopes) but also to characterise the lithological interface between groundwater and surface water. Numerical modelling of flow and solute fluxes (MIKE-SHE, NETPATH) will test conceptual models and integrate evidence from regional and study-site assessments.

Preliminary Findings

A review of water-level fluctuations recorded by the Water Resources Management Department (WRMD) in Uganda since November 1998 (Figure 2) show negligible to slightly negative trends in groundwater storage for most of the stations except for Pallisa water levels are rising (slope = 0.04, r² = 0.5). Slightly more negative trends are observed at Entebbe, Nkozi, Nkonkonjero (slope = -0.02, r² = 0.3 to 0.7). Cyclic seasonal fluctuations are apparent at most of the monitored stations.

Figure 2. Trends in groundwater levels, normalised with respect with mean monthly data from November 1998 to December 2005, for selected monitoring stations within the Victoria Nile basin, Uganda. (data: Water Resources Management Department of Uganda).

A comparison of changes in basin water storage indicated by the borehole hydrograph and lake stage at Entebbe with satellite-derived GRACE (Gravity Recovery and Climate Experiment) datasets (Figure 3). There is a reasonable correlation among changes in water storage indicated by GRACE datasets and groundwater (r² = 0.4) and surface water (r² = 0.4) levels at Entebbe. From February 2003–to May 2006, there are negative trends in groundwater levels (slope = -1.4, r² = 0.6), lake levels (slope = -0.8, r² = 0.9) and water storage indicated by GRACE datasets (slope = -0.3, r² = 0.5). Surface water compares most favourably with observations from GRACE. Larger discrepancies occur between the GRACE and groundwater storage estimates, with the latter showing higher fluctuations in specific periods (July 2003 and March 2005). The trends indicate that there is a general deficit between total water and surface water storage that may be supported by groundwater and soil moisture.

Figure 3. Trends in water storage (equivalent water thickness), normalised with respect to monthly means from 2003-2006, from GRACE datasets (half-width of the equivalent Gaussian smoothened over 400 km, 500 km and 750 km) as well as lake stage and borehole hydrograph records at Entebbe. Groundwater-level fluctuations have been converted using an assumed specific yield of 0.23 (Taylor et al., in review).

GRACE data reflects variations in terrestrial water storage that necessarily includes groundwater, soil moisture and surface water over a defined minimum region size (Rodell et al., 2006; Winsemius et al., 2006; Yeh et al., 2006). There is, of course, an obvious discrepancy between point measurements of groundwater storage and area-averaging inherent to gridded GRACE datasets. The relationship between observations from GRACE and monthly changes in groundwater storage is significantly improved when the mean from 10 regional stations is employed (r² = 0.6). GRACE storage change estimates depend on satellite measurement errors, the spatial averaging algorithm used to remove the correlated errors present in the GRACE gravity field coefficients, the larger area relative to the data, and the sparse temporal sampling of the ground measurements (Yeh et al., 2006). This is still a good first estimate for this data-poor region, and will be explored further by incorporating additional spatial and secular groundwater and soil moisture data from contemporary monitoring stations in the basin, before a more rigorous comparison is made.

Figure 4. Trends in water storage (equivalent water thickness), normalised with respect to monthly means from 2003-2006, from GRACE datasets (half-width of the equivalent Gaussian smoothened over 400 km, 500 km and 750 km) as well as lake stage and mean groundwater-level fluctuations (10 regional stations) have been converted using an assumed specific yield of 0.23.

Acknowledgements

Groundwater and surface water data derive from observational network managed by the Water Resources Management Department of Uganda. GRACE data were processed by D. P. Chambers, supported by the NASA Earth Science REASoN GRACE Project, and are available on the Nasa website.

References

• Chambers, D.P., 2006. Evaluation of New GRACE Time-Variable Gravity Data over the Ocean. Geophysical Research Letters, Vol. 33, L17603, doi:10.1029/2006GL027296.
• Kansiime, F., Naslubega, M., van Bruggen, J.J.A., and Denny, P., 2003. The effect of wastewater discharge on biomas production and nutrient content of Cyperus papyrus and Miscanthidium violaceum in the Nakivubo wetland, Kampala, Uganda. Water Science and Technology, 48(5), 233-240.
• Kull, D., 2006. Connections Between Recent Water Level Drops in Lake Victoria, Dam Operations and Drought. Accessed on 2 February 2007.
• Mwanuzi, F., Aalderink, H. Mdamo, L., 2003. Simulation of pollution buffer capacity of wetlands fringing Lake Victoria. Environment International, Vol. 29, 95-103.
• Odada, E.O., Olago, D.O., Kulindwa, K., Ntiba, M., Wandiga, S., 2004. Mitigation of environmental problems in Lake Victoria, East Africa: causal chains and policy options analyses. Ambio, 33, 19-23.
• Rodell, M., Chen, J., Kato, H., Famiglietti, J.S., Nigro, J., Wilson, C.R. (2006). Estimating groundwater storage changes in the Mississippi River basin (USA) using GRACE. Hydrogeology Journal, DOI 10.1007/s10040-006-0103-7.
• Taylor, R.G., Tindimugaya, C., 1996. The role of groundwater in the Victoria Nile Basin of Uganda: implications of a catchment water balance. In: Comprehensive Water Resources Development of the Nile Basin: Action Plan, Proceedings of the IVth Nile 2002 Conference, Kampala (Uganda), pp. A-87 to A-94.
• Taylor, R.G. and Howard, K.W.F. 1996. Groundwater recharge in the Victoria Nile basin of East Africa: support for the soil-moisture balance method using stable isotope and flow modelling studies. Journal of Hydrology Vol. 180, pp. 31-53.
• Taylor, R.G., Tindimugaya, C., Barker, J.A., Barrett, M.H., Macdonald, D., Howard, G., Kulabako, R., Nalubega, M. and Rwarinda, E. and , in review. Ground water flow and storage in weathered crystalline rock: evidence from Uganda. Ground Water
• Winsemius, H. C., Savenije, H. H. G., van de Giesen, N. C., van den Hurk, B. J. J. M., Zapreeva, E.A. and Klees, R. (2006). Assessment of Gravity Recovery and Climate Experiment (GRACE) temporal signature over the upper Zambezi. Water Resources Research, 42: W12201-W12209.
• Yeh P.J.-F., Swenson, S.C., Famiglietti, J.S., and Rodell, M. (2006). Remote sensing of groundwater storage changes in Illinois using the Gravity Recovery and Climate Experiment (GRACE). Water Resources Research, vol. 42, W12203.