The Omaruru River Basin is located in the central western part of Namibia and stretches over the Khomas, Erongo and Otjozondjupa Regions. This basin is one of the most urbanized and popular tourist destinations in the country. The Basin Management Committee was established in 2008 to undertake coordinated development and management of water land and related resources in the Omaruru River Basin. The Basin is defined by two major ephemeral rivers, namely; the Omaruru and Swakop Rivers which are west-flowing rivers that flow during major rainfall events. Figure 1 show the Omaruru River catchment area.

The Omaruru River Basin is located in the central western part of Namibia and stretches over the Khomas, Erongo and Otjozondjupa Regions. This basin is one of the most urbanized and popular tourist destinations in the country. The Basin Management Committee was established in 2008 to undertake coordinated development and management of water land and related resources in the Omaruru River Basin. The Basin is defined by two major ephemeral rivers, namely; the Omaruru and Swakop Rivers which are west-flowing rivers that flow during major rainfall events. Figure 1 show the Omaruru River catchment area.

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.

<olmap id=“olMap” width=“740px” height=“400px” lat=“-23.0” lon=“16.1” zoom=“12” statusbar=“1” toolbar=“1” controls=“1” poihoverstyle=“1” baselyr=“terrain” gpxfile=“:river.gpx” geojsonfile=“:mines.gsojson” kmlfile=“:en:projects:tailings.kml” summary=“Environmental monitoring”> -23.1,16.1,60,.8,marker-green.png, Borehole
Uranium: 8 µg/L
Chloride: 122 mg/L </olmap>

To comply with the Water Resources Management Act, 11 of 2013, the MAWF is supporting the Omaruru 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.

The following basic data need to be collected and summarized:

• Location
• Demographic overview and water demand
• Water utilisation and management
• Water quality
• Environmental issues
• Social issues
• Institutional and planning issues
• Review of objectives in Vision 2030, Millennium Development Goals, NDPs, NBSAP 2 and local development agendas that should be considered in the basin management strategies to be developed.

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

The review will inform the current status of the basin’s demography, water demand, utilization and management as well as quality will be carried out. The review will also investigate the environmental and social issues in the basin as well as issues that are related to institutional matter and planning. Further a review of Vision 2030, Millennium Development Goals, NDPs, NBSAP 2 and local development strategies will be considered as in-put into basin management strategies. An analysis of stakeholder participation, awareness raising and water education will also be done.

• 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 #dbassukas:2021-04-20>Borehole map</todo>
• <todo>Station map for Runoff and met stations</todo>

A very good and detailed introduction to IWRM is given by IWRM in Omaruru Basin. According to Strohbach (2008) the Omaruru Basin has a basin area of 11,579 km². Struckmeier & Rambow (2002) have described the hydrogeology of Vogelsberg within the Omaruru Basin. The use of scarce water resources is described by Schneeweiss & Müller (2009). Geyh & Ploethner (1995) have investigated the isotope composition of groundwater in the Omaruru Basin. The water management at Roessing Uranium was has already been described by Smit & Brent (1991). In a student project a water balance of the Omaruru basin has been established in 2003 /2004.A good insight to undergroundwater storage and aquifier response to artifical recharge in the Omararu delta is given by Aquifer response to artifical discharge, Omaruru Delta Namibia and (see e.g. Seely et al. )

Fig. Map of the Omaruru Basin provided by IWRM in Omaruru Basin

A peculiarity of the Omaruru Basin is the OMDEL dam, a recharge dam in an ephemeral river (see e.g. Seely et al. ). A summary of hydrological conditions and data is given by Namwater 2014.The geography and associated characteristics of Omaruru catchment area is given by Jacobson et al.(1995), page 138.

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 Omaruru Basin is monitored at Etemba station (Station number: 2971M02). The station is located at Lat:-21.43333333 and Long:15.66666667 at an elevation of 1072 m. Daily data of runoff since 2006 indicate a strong inter-annual variability of runoff. The file is also available as comma separated value file.

• 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 Omaruru basin varies strongly. As shown by the unit runoff map of Namibia the Omaruru 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 Omaruru 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.

This report provides a detailed characterization and evaluation of the Omaruru catchment runoff. The report focuses on the statistical analysis of data obtained from six runoff gauging stations in Omaruru between the periods 1965-2015. The data obtained for analysis include daily series runoff volume for the period 1965-2015 for all the six gauging stations.

A detailed description of the methods used to analyze the obtained data; the results of the analyzed data and conclusions pertinent to understanding the runoff characterization of Omaruru catchment are well documented in this report.

The method of data analysis include transformation of daily runoff volume into annual volume; specific annual volume determination as well as ratios such as event duration; event volumes and event correlation ratios. Other calculations include event synchronization and runoff coefficient determination. All calculations can be found in the appendices.

The results of analyzed data show that runoff volume decreases from the upper catchment area to the lower catchment towards the Atlantic Ocean. The maximum annual runoff volume is recorded in Etemba while the least is recorded in Henties Momentum a station few kilometers downstream of the Omdel dam. In particular, the average runoff coefficient as analyzed using the obtained data falls at 1.65% a value smaller than the runoff coefficient values reported in several literature of runoff and runoff coefficient in arid and semi-arid regions. (Hughes & Metzler., 1998; Mosert et al., 1993; DWA 1992). The average runoff event duration for the six gauging stations falls at 10.5 days per event.

Fig.1. Basin Map of Namibia showing Runoff Gauging Station Location developed by C.Külls

This report finds that there is a high transmission loss in runoff volume from upper catchment to the lower catchment. This is evident in the variation in yearly runoff event volume and in runoff event occurrence across each gauging station. For the entire period 1965-2015, runoff event occurrence consistency throughout the six gauging stations was recorded only in the year 1988 (see appendix 1) There were slightly high degree of runoff event occurrence consistency in the years 1987 and 1990 though for these years runoff event occurred only in four of the six gauging stations while for all other years there were high degree of inconsistencies in the runoff event occurrence across the six gauging stations.

Furthermore, the Omdel dam situated west at a point where the least runoff volume is recorded would imply that the dam receives only very little as most substantial runoff is generated at the upper catchment and does not reach the dam due to transmission losses.

The principal statistical methods used to address the objective of this report consist of integration as summation, percentile analysis, summary statistics and frequency distribution graphs.

Integral Summation method was used to evaluate and transform daily runoff volume series into annual runoff volumes. The resulting annual runoff volumes were used to calculate the specific runoff volume. The yearly runoff coefficient were obtained by dividing the specific runoff volume by theoretical standard rainfall and the result multiplied by 100. Integral summation was also used in calculating the runoff event occurrence duration alongside peak flow and volume for each off the event occurrence. Summary statistics was used to synchronize the annual runoff event occurrences while percentile analysis was employed to evaluate runoff event occurrence distribution.

The methods used to address specific objectives are summarized in table 1.0 Details of statistical methods are provided in the following text.

Table 1.0: Objective and Statistical Method

Integral summation

Integral summation was used to address the objective of transforming the daily runoff volumes into yearly runoff volumes. The annual runoff volumes for the period 1965-2015 were calculated for each of the six gauging stations.

Integral summation was also used to obtain runoff event duration; associated runoff event duration peak flow values and event occurrence volumes.

Summary Statistics

Summary statistics was employed in describing the specific runoff values for each runoff gauging station area. The specific runoff values was calculated as (Annual Runoff Volume/Area). Similarly, the runoff coefficient was calculated as (Specific Runoff value/Standard Rainfall *100). Summary statistics was also used to address the objective of event occurrence synchronization.

Quartile Analysis

Quartile analysis was used to describe the frequency of events for each of the stations and the relationship between events for each of the six stations. Also explained with quartile analysis are the peak flow of event occurrences and runoff volume for each event occurrence.

Pictorial description of the quartile analysis of the objectives of this report can be found in the result and appendices section.

This section provides a detailed insight into the runoff statistics of Omaruru catchment largely focusing on the data analysis of the obtained data from the six gauging stations in the catchment. Fig. 2 shows the annual runoff record of the Omaruru catchment for the period 1964 through 2015 while the summary statistics illustration in table 2.0 below shows the characterization of runoff event occurrence for each gauging station; the maximum, minimum, average and other variable quartile statistics of the event occurrence duration, peak flow value per day and volume.

Fig.. Omaruru Annual Runoff Record

Further analysis of correlation between events and the runoff pattern are shown in the single volume charts below.

<barchart> 60|Omburu:12.6, Omaruru:54.8, Etemba:30.0, Nei-Nei:1.6, Sabrina:7.9, Henties-Momentum:0.6 </barchart>

Fig.: Omaruru Runoff Volume-1988

<barchart> 60|Omburu:14.0, Omaruru:42.9, Etemba:34.4, Nei-Nei:12.5, Sabrina:4.6, Henties-Momentum:0.0 </barchart> Fig.: Omaruru Runoff Volume-1990

<barchart> 60|Omburu:33.7, Omaruru:51.2, Etemba:39.5, Nei-Nei:1.1, Sabrina:0.0, Henties-Momentum:0.0 </barchart> Fig.: Omaruru Runoff Volume-1976

<barchart> 60|Omburu:12.7, Omaruru:28.6, Etemba:0.0, Nei-Nei:0.0, Sabrina:6.9, Henties-Momentum:0.0 </barchart> Fig.: Omaruru Runoff Volume-1994

The years 1987, 1988 and 1990 were the years which runoff event occurrence showed great consistencies across the gauging stations. In particular runoff events occurred across the six gauging stations in the year 1988 while for 1987 and 1990 runoff events occurred in five of the six gauging stations. The other years showed lower degree of consistency in runoff event occurrence.

The data analysis shows a decreasing correlation in runoff volume and runoff event occurrence from the upper catchment to the lower catchment. The single volume charts above (Figures 3-6) & Fig.7 below are supporting pictorial views of a decreasing correlation in runoff volume and runoff event occurrence. The average specific volume and runoff coefficient are given in table 3.0

Fig.: Runoff Event Correlation (Omaruru & Sabrina)

Figure 7 illustrate the relationship between event volumes recorded at Omaruru a station upstream and Sabrina a station downstream; a close look at the chart shows that high volumes are recorded at Omaruru however only a small fraction gets to Sabrina. The average specific volume and runoff coefficient are given in table 3.0

Table: Specific Runoff & Runoff Coefficient

In addition to the event correlation data given in the preceding charts and tables, event frequency charts chosen for a selected stations are given in figures 8-10.

<barchart> 120|0:0, 10:118, 20:13, 30:6, 40:1, 50:0, 60:0,70:1, 80:0, 90:0, 100:0 </barchart>

Fig.:Omburu Event Frequency Chart

<barchart> 30|0:0, 10:25, 20:10, 30:6, 40:5, 50:2, 60:5,70:0, 80:1, 90:0, 100:0 </barchart>

Fig.:Omaruru Event Frequency Chart

<barchart> 30|0:0, 10:20, 20:0, 30:0, 40:0, 50:0, 60:0,70:0, 80:1, 90:0, 100:0 </barchart>

Fig.:Sabrina Event Frequency Chart

The runoff volume recurrence interval was derived by compiling yearly peak runoff volume for one of the gauging stations (Etemba), these volumes were then ranked by their magnitude. See Appendix B for detailed calculations of recurrence interval. Recurrence Interval RI is given as: (n+1)/ (n+1-m). n - Rank m – Relative Ranking

Fig.: Etemba Recurrence Interval Chart

Appendix B contains a detailed calculation of annual runoff volumes, annual specific runoff, runoff coefficients, event durations, event volumes, correlation between events alongside other associated statistics and calculations on runoff volume recurrence intervals.

The obtained runoff data which this statistical analysis focused on has provided a sufficient means of characterizing the runoff pattern of Omaruru catchment. The primary elements of catchment runoff pattern characterization: runoff coefficient, runoff volumes, flow per unit area, runoff event occurrence and correlation between events have been identified and detailedly analyzed in this report.

The decrease in the runoff event occurrence from the upper catchment through the lower catchment translates into evident runoff transmission losses. The presented results in the preceding figures and tables are pictorial supporting data which show evident transmission losses in the runoff pattern of the Omaruru catchment.

Finally, as evident in this report the Omdel dam built with the primary function of temporarily storing ephemeral flood water for Omdel aquifier recharge appears to be situated at a location where runoff volumes are least. A means of abstracting runoff from locations of higher runoff volumes have to be closely considered.

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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.

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