The Swakop River Basin is located in the central western part of Namibia and stretches over the Khomas, Erongo and Otjozondjupa Regions. It is the centre of the uranium industry of Namibia.

The Ministry of Agriculture Water and Forestry (MAWF) is leading the implementation of Integrated Water Resources Management (IWRM) in Namibia. As per the definition of the Global Water Partnership (GWP), IWRM is seen as a process, which promotes the coordinated development and management of water, land, and related resources in order to maximize socio-ecological well-being of the people in an equitable manner without compromising the sustainability of vital ecosystems. In Namibia, IWRM is gradually implemented at basin level, in all its major river basins, in line with the new Water Resources Management Act, 11 of 2013. The Basin Management Approach is promoted as an appropriate mechanism for stakeholders’ participation in water resources development and management. Consequently, Basin Management Committees (BMCs) are established for the application of IWRM principles at the river basin level. The primary purpose of Basin Management Committees is to ensure equitable access to and sustainable use of water resources and also to protect, develop, conserve, manage and control water resources in the respective basins.

To comply with the Water Resources Management Act, 11 of 2013, the MAWF is supporting the Swakop Basin Management Committee with the formulation of a water resources management plan for the basin.

The information required for the IWRM study has been outlined in the terms of references.

From sources and archive such as GROWAS, OMBMC, IWRM website, Omaruru Municipality NamWater, Namibia Statistics Agency, DWAF and so forth relevant data and information will be collected and converted to formats suitable for the project. A project database will be created and data manipulation will include the formulation of thematic maps that will depict geographic perspective of the basin, catchment characters such as rivers, population density, towns, boreholes, water quality, surface water and groundwater flow etc., within an Arc-GIS environment. The generated maps will form part of the visual aids in compiling the status report.

The desk study will include an impact assessment investigating the impact of current land use which includes agriculture, bush encroachment and prosopis on basin hydrology. The assessment will offer recommendations for optimisation of water use in the different land use sectors.

• Review of existing data sources (including links to websites) and identification of gaps
• Definition of water resources, integrated runoff-recharge hydrological and geohydrological model of the entire catchment
• Impact assessment of land use (including agriculture, bush encroachment and Prosopis) on the hydrology of the basin and proposals for economic optimisation of water use by this sector
• Status of stakeholder participation, awareness-raising and water education

• Water demand and conservation, management of abstraction licences (permits)
• Water quality management and pollution prevention, including vulnerability assessment of water resources to pollution
• Monitoring and reporting: Collection, interpretation and sharing of data, database and GIS development
• Readiness and response plans in case of flood events or water supply disruptions
• Capacity-building of the Omaruru Basin Management Committee
• Institutional development and capacity-building

A comprehensive implementation framework of the basin plan has to be compiled, outlining the role players (key stakeholders) and timeframe of the actions completion.

• <todo>Runoff daily for the last 25 years (since 1990) for all stations within the basin</todo>
• <todo>Rainfall, Temperature and relative humidity daily for the same period for at least three, better 5 stations</todo> covering the west east transsect
• <todo>Groundwater levels in the alluvial aquifer from boreholes daily</todo>
• <todo>Production data for alluvial boreholes</todo>
• <todo>Dam Data, volume-stage relationship, stage time series, area and abstraction</todo>
• <todo>Geological map</todo>
• <todo>Borehole map</todo>
• <todo>Station map for Runoff and met stations</todo>

Meteorological data are available for stations at Walvis Bay and Windhoek. Daily rainfall is available at Walvis Bay and at Windhoek. Windhoek is the elevated station with higher rainfall and semi-arid conditions. Walvis Bay is located at the coast in an arid climate with low rainfall amounts.

Maximum and minimum temperature are also given at Walvis Bay (Tmax, Tmin) and at Windhoek (Tmax, Tmin) on a daily basis and are used to evaluate potential and actual evaporation from open water surfaces and vegetated land. Relative Humidity is given at both stations at 8:00, 14:00 and 20:00 (see Walvis Bay 8 a.m., 2 p.m. and 8 p.m. and Windhoek 8 a.m., 2 p.m. and 8 p.m.).

Meteorological data can be used to calculate evaporation. Evaporation rates for Namibia are given in the Namibian Evaporation Map. Although these values represent potential evaporation, they seem to be rather high and exagerated. Therefore, an independent re-evaluation of evaporation is carried out based on station data.

Runoff in the Swakop Basin is monitored at 5 stations.

• Graph: runoff as a function of time.
• Annual sums from 1.10. until 30.09.
• calculate mean specific runoff: annual runoff in liters (or mm) divided by basin area in m²
• calculate runoff coefficient: mean annual specific runoff in mm/m² as percent of annual rainfall in mm/m² - the expected value is 4 %.

The runoff coefficient of surfaces in the Swakop basin varies strongly. As shown by the unit runoff map of Namibia the Swakop basin hosts some of areas with the highest runoff coefficients in Namibia. Granite is producing up to 30 % of runoff per unit area. However, in the lower part of the Swakop basin, areas with low runoff coefficients of only 1-5 % prevail. Therefore, most of the runoff is produced in the upper part of the basin, not only because of higher rainfall but also because of higher runoff coefficients. In the lower part of the basin runoff production is small. In addition runoff is reduced by transmission losses in alluvial channels.

An integrated hydrological and geohydrological model of the entire Omaruru catchment will be developed. The main purpose of this model is to aggregate available hydrological information and provide runoff and groundwater recharge as key indicators for integrated water resources management. The model will have a straightforward graphical user interface (GUI). It is based on a graphical modelling framework in which the water storage and hydrological processes are visualized as graphic elements instead of programming code. A similar model has been developed with success for the Swakop catchment within the Strategic Environmental Assessment for the Erongo Region.

The Omaruru Geohydrological Model will represent the basin and sub-basin structure, rivers and aquifers as compartments and will include water schemes and abstraction points. The model has a compartment structure. Flows between compartments are described based on physical principles. This concept has been applied very successfully in the Swakop basin: Alluvial aquifers along the Omaruru are sub-divided into management compartments based on geological evidence and hydrogeological information. Water levels, water storage, evaporation, runoff and recharge and also water quality can be specified for each of these compartments. Each compartment is associated with a sub-basin with specific surface properties, land-use characteristics and storage properties.

The model is driven by hydro-meteorological data (rainfall and evaporation parameters temperature, relative humidity and solar energy balance. These data can be loaded to the model (if readily available). In case, data are not available or available with delays in data processing average monthly seasonal regimes of rainfall and evaporation are available that can be used as proxy input data. In addition, hydrological station data, boreholes with their respective abstraction rates water storage and distribution infrastructure are included.

The model runs on a monthly or daily basis (as chosen by the user) and converts rainfall to runoff and soil water storage and soil water storage to evaporation and groundwater recharge based on approaches that have been developed in Namibia or proven right in this regional context (Namibia antecedent runoff model as in Hughes & Metzeler (1998) evaporation model for alluvial aquifers (Hellwig (1973), and recharge estimation methods for direct and indirect alluvial recharge (Klock, Kuells, & Udluft, 2001, Dahan et al., 2008, Kuells, 2000). Runoff is routed through the Omaruru and recharge to local aquifers and to the alluvial aquifer compartments is calculated. The user can retrieve runoff hydrographs and tables of available resources at each of the compartments and groundwater level and current storage for each of the local aquifers.

The geohydrological model facilitates integrated water resources management as it integrated all impacts and converts these to water levels and storage volumes compared to potential storage and juxtaposed to critical management lines (red lines). Critical water levels can be defined and projections run how water levels will drop in the future given the known and current uses. Options for demand management are included: The response of the aquifer systems to management options can be simulated and evaluated.

The geohydrological model can also be used as decision support system. Boreholes are included and scenarios for different abstraction rates can be run (see Bittner, Marx & Külls, 2011: SEA for the Central Namib Uranium Rush: - Geohydrological Model of the Swakop River, Phase II.). The model will show the impact on all other connected groundwater compartments. The model can therefore be used for impact assessment and to simulate requested abstraction rates, before water rights are issued.

Finally, the geohydrological model will include features to provide summary tables for sub-basins and compartments for management purposes. Theses summary tables are compatible with recommendations for IWRM standards and with the UN water accounting approach. The hydrology and management of Omaruru dam can be included as an option.

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Bittner A. (2010): Numerical Groundwater Model and Water Balance of the Swakop/Khan River System. Groundwater Specialist input to the Strategic Environmental Assessment of the Central Namib ‘Uranium Rush’, Windhoek.

Bittner, Marx & Külls, 2011: SEA for the Central Namib Uranium Rush: - Geohydrological Model of the Swakop River, Phase II.

CSIR (1997): An Assessment of the Potential Environmental Impacts of the Proposed Aquifer Recharge Scheme on the Khan River, Namibia. Final Report by CSIR Division of Water, Environment & Forest Technology to Rössing Uranium Limited, Swakopmund, Namibia.

DWA (1992): Unit Runoff Map for Namibia. Department of Water Affairs, Ministry of Agriculture, Water and Rural Development.

GKW BICON & PARKMAN (1996): Water supply to the Central Namib Area of Namibia. Final Report: Volume 4: Existing Fresh Water Sources. Produced for the Department of Water Affairs and Kreditanstalt für Wiederaufbau, Windhoek.

HELLWIG, D. H. R. (1973): Evaporation of water from sand, 3: The loss of water into the atmosphere from a sandy river bed under arid climatic conditions. Journal of Hydrology, 18(3-4), 305-316.

Hughes & Metzeler (1998) Assessment of three monthly rainfall-runoff models for estimating the water resource yield of semiarid catchments in Namibia, Hydrological Sciences—Journal—des Sciences Hydrologiques, 43(2)