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Country/Region: Southern Africa

Project start: 2013.01.01 , Project end: 2017.10.31

Project status: 2017.10.31 , Completed

Deutsche Version 

Background 

In many parts of southern Africa, groundwater resources are the most important source of drinking water for the local population. In addition to deeper groundwater resources, shallow groundwater resources are particularly important because they can be tapped by the population with simple means and at low cost, as demonstrated by hand-dug wells in northern Namibia and southern Angola. The transboundary Cuvelai-Etosha Basin (CEB) located there is a groundwater-dependent ecosystem. The alternation of wet and dry seasons, periodic droughts and floods, as well as climatic changes such as the shifting of the onset and end of the rainy season, influence the dynamics of groundwater resources and thus the life cycle of plants, animals and humans. Increasing population pressure also has a direct impact on groundwater resources. Yet these vulnerable systems often lack basic knowledge about the quantity and dynamics of their reserves.

An international collaborative research initiative involving scientists from Angola, Botswana, Namibia, South Africa, Zambia and Germany was launched in 2010 with the aim of establishing a Southern African Science Service Centre for Climate Change and Adaptive Land Management (SASSCAL). The initiative, funded by the German Federal Ministry of Research, Technology and Space (BMFTR), was created to respond to the challenges of global change and to generate long-term scientific results and socio-economic benefits for the entire Southern African region. To this end, problem-oriented research on adaptation to climate and climate change and sustainable land management will be carried out through interdisciplinary research in the fields of water, climate, agriculture, biodiversity, forestry and capacity development.

As part of the SASSCAL project, the Federal Institute for Geosciences and Natural Resources (BGR) participated in the tasks "Improving the understanding of groundwater related processes and establishing groundwater budgets for water management purposes" (Task 10) and "Support of the establishment of an MSc Program of Applied and Environmental Geology at the University of Namibia, Windhoek" (Task 82). Together with a partner institute at the University of Namibia (UNAM), extensive field campaigns were carried out to deepen and qualify the knowledge base on the physical processes that significantly control groundwater recharge. Groundwater recharge can be divided into a direct component (infiltrating precipitation) and an indirect component (delayed infiltration into oshanas, depressions or sedimentary riverbeds). Both components are strongly influenced by the physical properties of the unsaturated zone, such as the nature of the sedimentary layers in the soil or the land cover.

As part of the above-mentioned Task 82, a Master's degree programme was designed and developed in close cooperation with the UNAM, which will enable the training of urgently needed young talent in the environment/water sector in the partner countries. As part of an integrated approach, the students were also directly involved in the work of Task 10 to provide practical experience on the ground. Master's and doctoral scholarships from projects in the partner countries complemented this integrated approach.

Project area

In the first phase of SASSCAL, BGR's study area was limited to the CEB. This transboundary basin, which extends almost equally across Angola and Namibia, is home to about two-thirds of Namibia's population (2.3 million inhabitants) and has unique geological and hydrological characteristics. The aquifer system, known as the "Ovambo Basin", deviates from the surface catchment and was formed in the Tertiary period when the Okavango River was still flowing into the Etosha Basin (Miller et al., 2010). Previous BGR projects had already provided a good infrastructure for research (e.g. boreholes for groundwater monitoring). In addition, results from geophysical investigations were available and could be used to support the planned activities. This made the CEB an ideal research area for scientific studies dealing with the complex interactions between aquifer systems and groundwater recharge processes in the unsaturated zone.

Main research areas

BGR's research approach focused on a detailed investigation of the unsaturated zone, where a complex interplay of soil type, vegetation and spatio-temporal precipitation distribution creates very heterogeneous conditions for groundwater recharge. The main questions were:

  • How can data quality improvement help to improve groundwater assessment for water management purposes?
  • Which methods will serve best for local groundwater recharge estimation in arid environments? Is it possible to regionalise these methods?
  • Is it possible to sustain shallow groundwater resources as drinking water supply, taking into account climate and land use changes?
  • Can groundwater supply be supported by artificial recharge?

A combination of field and laboratory methods, calculations and interpretations based on stable water isotopes (deuterium and oxygen-18), remote sensing data and numerical modelling were used to answer these questions. A total of seven field visits focused on soil sampling for isotopic hydrological characterisation and soil hydraulic analysis, tracer experiments, vegetation studies and climate, soil moisture and groundwater monitoring. The field data were used to characterise infiltration processes and water movement in the soil-plant-atmosphere continuum under different conditions (soil, climate, vegetation) and to estimate groundwater recharge. Both established methods and further developments of existing approaches were used. The results were then used to develop and validate numerical models to describe processes relevant to groundwater recharge for shallow and deeper aquifers. The work has enabled, for the first time, an informed assessment of these groundwater resources and provided the basis for sustainable water management.

Results

  • Methods based on stable water isotopes can contribute to a detailed investigation and better understanding of the processes relevant to groundwater recharge at the soil-plant-atmosphere interface.
  • The measurement of stable water isotopes in soil water, developed in the project and applied for the first time in a semi-arid region, allows the recording of water transport processes with high spatial and temporal resolution.
  • Isotope hydrological tracer methods allow the estimation of important variables required for the parameterisation of numerical models (e.g. root depth and root density distribution) as well as the direct quantification of potential groundwater recharge on an annual basis.
  • Groundwater recharge in shallow aquifers is heterogeneous and strongly dependent on the type of terrain.
  • The estimated groundwater recharge rates for the analysed landforms are
    • between 9 and 20 mm/year on a long-term average for vegetated sites in deep sandy areas, but with strong annual variations (e.g. 2013–14: 29 mm/year, 2014–15: 0 mm/year, 2015–16: 4 mm/year),
    • between 20 mm/year and 77 mm/year as a long-term average in unvegetated deep sandy areas and ephemeral riverbeds, and
    • 3 to 4 mm/year long-term average in deflation basins (local depressions in the northeast of the CEB).
  • Groundwater recharge is highly unlikely above Iishana (seasonally flooded areas in the centre of the CEB).
  • The low-lying Ohangwena-2 (KOH-II) aquifer in the Angolan highlands is very likely to be a recent recharge area.
Photo of BGR's field research camp "Eenhana Forest" in the eastern part of the Cuvelai-Etosha Basin
BGR's field research camp "Eenhana Forest" in the eastern part of the Cuvelai-Etosha Basin

Conclusions

  • The greatest potential for groundwater recharge occurs on vegetation-free, deep sandy sites (ephemeral riverbeds, sand dunes, vegetation-free areas).
  • Local perched aquifers offer potential for sustainable use and targeted groundwater recharge. These aquifers can be developed into an important part of the rural water supply due to their good accessibility, the rapid regeneration of water resources (e.g. by only one rainy season) and the availability of good quality water (no salinisation problems). The next step, however, is to carry out extensive geophysical exploration and further scientific research at a regional level.
  • The deep-lying Ohangwena-2 (KOH-II) aquifer can serve as an important strategic water resource to bridge prolonged periods of drought. Extended monitoring and research into the exact age and recharge rate of KOH-II is required.
  • Despite the uncertainties, the estimated recharge rates provide a basis for regionalisation of groundwater recharge across the unsaturated zone based on different landforms.

The projects Groundwater for the north of Namibia and Kalahari, supported expertise for the BGR team in this basin, where the majority of Namibians live (Lindenmaier et al., 2012).

Selected activities

Literature

Papers

  • BEYER, M., GAJ, M., HAMUTOKO, J., KÖNIGER, P., WANKE, H. & HIMMELSBACH, T. (2015): Estimation of groundwater recharge via deuterium labelling in the semi-arid Cuvelai-Etosha Basin, Namibia. – Isotopes in Environmental and Health Studies, 51(4): 533–552. DOI: 10.1080/10256016.2015.1076407
  • BEYER, M., GAJ, M., KÖNIGER, P., HAMUTOKO, J.T., WANKE, H., WALLNER, M. & HIMMELSBACH, T. (2018): Isotopenhydrologische Methoden (2H, 18O) zur Bestimmung der Grundwasserneubildung in Trockengebieten: Potenzial und Grenzen. – Grundwasser, 23(1): 73–90. DOI: 10.1007/s00767-017-0381-0
  • BEYER, M., KÖNIGER, P., GAJ, M., HAMUTOKO, J.T., WANKE, H. & HIMMELSBACH, T. (2015): A Deuterium-based labeling technique for the investigation of rooting depths, water uptake dynamics and unsaturated zone water transport in semiarid environments. – Journal of Hydrology, 533: 627–643. DOI: 10.1016/j.jhydrol.2015.12.037
  • GAJ, M., BEYER, M., KÖNIGER, P., WANKE, H., HAMUTOKO, J. & HIMMELSBACH, T. (2016): In situ unsaturated zone water stable isotope (2H and 18O) measuements in semi-arid environments: a soil water balance. – Hydroly and Earth System Sciences, 20: 715–731. DOI: 10.5194/hess-20-715-2016
  • GAJ, M., KAUFHOLD, S., KÖNIGER, P., BEYER, M., WEILER, M. & HIMMELSBACH, T. (2017): Mineral mediated isotope fractionation of soil water. – Rapid Communications in Mass Spectrometry, 31(3): 269–280. DOI: 10.1002/rcm.7787
  • HAMUTOKO, J.T., WANKE, H., KÖNIGER, P., BEYER, M., GAJ, M. (2017): Hydrogeochemical and isotope study of perched aquifers in the Cuvelai-Etosha Basin, Namibia. – Isotopes in Environmental and Health Studies, 53(4): 382–399. DOI: 10.1080/10256016.2016.1273913
  • HIMMELSBACH, T. (2017): Tiefe, semi-fossile Grundwasserleiter im südlichen Afrika: Hydrogeologische Untersuchungen im Norden von Namibia. – GMIT – Geowissenschaftlichen Mitteilungen, 67: 7–18. ISSN: 1616-3931.
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  • KÖNIGER, P, GAJ, M., BEYER, M. & HIMMELSBACH, T. (2016): Review on soil water isotope-based groundwater recharge estimations. – Hydrological Processes, 30: 2817–2834. DOI: 10.1002/hyp.10775
  • WALLNER, M., HOUBEN, G., LOHE, C., QUINGER, M., HIMMELSBACH, T. (2017): Inverse modeling and uncertainty analysis of potential groundwater recharge to the confined semi-fossil Ohangwena II Aquifer, Namibia. – Hydrogeology Journal, 25: 2303–2321. DOI: 10.1007/s10040-017-1615-z

Technical Report

LINDENMAIER, F., DILL, H.G., DOHRMANN, R., FENNER, J., GERSDORF, U., KAUFHOLD, S., KRINGEL, R., LUDWIG, R,, MILLER, R.McG., NICK, A., NOELL, U. & WALZER, A. (2012): Results of Analysis from Drill Holes on the Cubango Megafan. – Technical report prepared for the Kalahari Research Project "Development and Dating of Duricrusts as Base for a Paleogeographic and Hydrogeological Model of the Cuvelai-Etosha Basin in North Namibia". 50 pp.; Hanover.
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Presentations

  • BEYER M. et al. (2017): Drought-induced anomalies of soil water stable isotopes enable identification of deep root water uptake. – German Association for Stable Isotope Research (GASIR) - Meeting, Hanover, Germany.
  • BEYER M. et al. (2017): Ecohydrological feedbacks in water-limited environments: combining drought-induced changes of soil water stable isotopes and labeling experiments. – International workshop on “Isotope-based studies of water partitioning and plant-soil interactions in forested and agricultural environments”, Florence, Italy.
  • BEYER M. et al. (2015): Deuterium (2H) as applied tracer in the vadose zone: experiences from semiarid Namibia. – German Association for Stable Isotope Research (GASIR) - Meeting, Heidelberg, Germany.
  • BEYER M. et al. (2014): Artificial deuterium labeling for a quantification of groundwater recharge in semi-arid regions. – German Association for Stable Isotope Research (GASIR) - Meeting, Munich, Germany.
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  • GAJ, M. et al. (2015): Limits and potentials determining stable water isotopes using in-situ measurements and soil water extractions. – German Association for Stable Isotope Research (GASIR) - Meeting, Heidelberg, Germany.
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  • GAJ, M. et al. (2014): Isotopic enrichment of stable isotopes (2H and 18O) in the soil-plant-atmosphere continuum: Implications for seasonal water uptake and evaporation patterns in a semi-arid environment. – IAH-International Congress, Marrakech, Morocco.
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  • KÖNIGER, P. et al. (2014): Soil water stable isotope based groundwater recharge estimations: Examples from humid and semiarid areas. – IAH-International Congress, Marrakech, Morocco.
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Posters

  • BEYER, M. et al. (2017): Constraining water uptake depths in semiarid environments using water stable isotopes. – EGU General Assembly, Vienna, Austria.
  • BEYER M. et al. (2016): Inferring root water uptake depths from water stable isotopes at different system states: Combining natural abundances and labeling experiments. – Joint European Stable Isotopes User group Meeting (JESIUM), Ghent, Belgium.
  • BEYER, M. et al. (2014): Deuterium labeling of soil water movement in the Cuvelai-Etosha Basin, Namibia. – EGU General Assembly, Vienna, Austria.
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  • GAJ, M. et al. (2016): How do soil types affect stable isotopes ratios under evaporation: A post corrected evaporation fingerprint of the Cuvelai-Etosha-Basin, Namibia. – Joint European Stable Isotopes User group Meeting (JESIUM), Ghent, Belgium.
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  • GAJ, M. et al. (2016): Stable isotopes of soil water are affected by clay minerals: A post correction approach for dry soils based on physicochemical soil properties. – EGU General Assembly, Vienna, Austria.
  • GAJ, M. et al. (2015): In-situ measurements of pore water stable isotope composition in a semi-arid environment and their implications to spatio-temporal variability. – EGU General Assembly, Vienna, Austria.
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  • GAJ, M. et al. (2015): Modeling unsaturated zone water stable isotope depth profiles in an evaporation dominated environment. – IAH-International Congress; Rome, Italy.
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  • GAJ, M. et al. (2014): How do soil types affect stable isotope ratios of 2H and 18O under evaporation: A Fingerprint of the Niipele subbasin of the Cuvelai - Etosha basin, Namibia. – Proceedings of EGU General Assembly, Vol. 16, Vienna, Austria.
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  • GAJ, M. et al. (2014): Recharge estimation in a semi-arid environment using stable isotope methods: A discussion on in-situ field and laboratory techniques. – German Association for Stable Isotope Research (GASIR) - Meeting, Munich, Germany.
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  • KÖNIGER, P. et al. (2014): Stable isotope and hydro chemical variability along the Calueque-Oshakati Canal in the Cuvelai-Etosha Basin, Namibia. – EGU General Assembly, Vienna, Austria.
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Doctoral Theses

  • BEYER, M. (2016): Quantitative studies along the soil – vegetation – atmosphere interface of water – limited environments: practice-oriented approaches based on stable water isotopes, modeling and multivariate analysis. – Mitteilungen des Instituts für Wasserwirtschaft, Hydrologie und landwirtschaftlichen Wasserbau, Leibniz Universität Hannover, Heft 102, ISSN 0343-8090 Link
  • GAJ, M. (2016): Recharge estimation in a (semi-)arid environment using soil water balance, stable isotopes and modeling approaches. Albert-Ludwigs-Universität Freiburg

Master Theses

  • ADAM, B. (2015): Modelling the response of stable isotope fractionation in soil water profiles with uncertainty. KIT Karlsruhe.
  • BAHLMANN, L. (2016): Rainfall characteristics and their implications for groundwater recharge in deep vadose zones. A case study in the Cuvelai-Etosha Basin (CEB), Namibia. Leibniz University, Hanover.
  • MAHINDAWANSHA, A. (2015): Intercomparison of laboratory techniques for determination of stable isotopes in soil water. University of Darmstadt.
  • SCHILLING, M. (2015): Indirect groundwater recharge in the Cuvelai-Etosha Basin (CEB), Namibia. Investigating the importance of Oshanas, Deflation Pans and Ephemeral River Beds. Leibniz University, Hanover.
  • SHEHU, B. (2014): Correction of GRACE data for the influence of surface water, soil and vegetation moisture. A case study on the quantification of groundwater recharge rates in the Cuvelai-Etosha-Basin (CEB), Namibia. Leibniz University, Hanover.
  • VILLANUEVA, R. (2015): Direct groundwater recharge in Cuvelai-Etosha-Basin (CEB), Namibia. The use of DAISY for modelling unsaturated zone water movement in a semiarid climate. Leibniz University, Hanover.

Further Literature

  • MENDELSOHN, J., JARVIS, A., & ROBERTSON, T. (2013): A profile and atlas of the Cuvelai-Etosha Basin. – 170 pp., RAISON & Gondwana Collection, Windhoek, Namibia. ISBN: 978-99916-780-7-8
  • MILLER, R. M., PICKFORD, M., & SENUT, B. (2010): The geology, paleontology and evolution of the Etosha Pan, Namibia: Implications for terminal Kalahari deposition. – South African Journal of Geology, 113(3): 307–334.

Film

Namibia's Groundwater Treasure – The SASSCAL-Project & Technical Cooperation (30 min, in German) Link

Partner

Dr Heike Wanke and Dr Martin Hipondoka, Faculty of Geography, University of Namibia (UNAM), Windhoek, Namibia

Funding information

SASSCAL / 01LG1201L

Your contact

Prof. Dr. Georg Houben

Phone: +49 511 643-2373

Georg.Houben@bgr.de