dc.description.abstract
Located in the western part of the Cuvelai Basin, the Iishana system is a transboundary region covering parts of southern Angola and northern Namibia. Hydrologically, this region is characterized by a network of episodically water-bearing channels in which numerous pans are embedded. These pans, which fill up during the rainy season, form an important water resource for the rural population, especially for agricultural and domestic use.
The Iishana system is one of the most densely populated areas in southwestern Africa, and this high population trend is increasing (NamStat 2013). To date, the majority of the population (80–90%) currently lives in rural areas. However, (small) cities are experiencing steady growth.
The semi-arid climate in this area has distinct rainy and dry seasons and is characterized by high interannual variability, resulting both in intense droughts and in strong flood events. As a result, water is sometimes a scarce resource in this region. The strong population growth and the temperature increase predicted as a result of global climate change will put further pressure on available water resources. However, as this region is also subject to volatile rainfall dynamics, in addition to droughts, the Iishana system also experiences repeated, severe flood events. Most recently, flood events occurred in 2008 to 2011, 2013, and 2017, resulting in the loss of life, the loss of crop yields and consequent loss of livelihood for many people, and the destruction of key infrastructure elements.
To date, there has been no complex 2D-hydrodynamic model for the Iishana system and no transferable modeling approach to identify potential locations for water storage and facilitate the planning and development of flood retention measures.
In this study, various methods have been developed and applied to address these issues. This has allowed for the validation of existing findings as well as the discovery of new insights, which are briefly summarized below.
First, an investigation was performed to test the influence of topography on hydrology, with a special emphasis on infrastructure elements. The focus here was on improving the raw DEM for subsequent calculations. For this purpose, filter corrections were performed on the TanDEM X raw data, and road dams, culverts, and bridges were recorded by means of kinematic surveys.
As a result, the definition of the flow paths was improved. It became clear that northern roads, especially those running from east to west, have a strong influence on the runoff behavior in the study area due to their height and their orientation orthogonal to the water flow of the Iishana.
Based on the corrected DEM and the application of a modified Blue Spot Analysis, further new findings emerged. Approximately 190,000 pans with a total storage volume of about 1.9 km³ and a total area of 4,021 km² were identified. The part of the study area located in Angola accounts for two thirds of the potential storage volume while only one third of the storage volume is in Namibia. Furthermore, about one third of all pans are located in the episodically water-bearing channels.
Based on previous results in other regions, a calculation of the surface-volume relationship (SA/V rate) was performed for the first time for the Iishana system. This enabled the identification of about 2,000 pans that are primarily suitable for an expansion of storage volume.
Using continuous and spatio-temporally varying TRMM precipitation data, a 2D-hydrodynamic modeling and reconstruction of the 2008/2009 flood event was performed using the FloodArea model.
Although the results represent a snapshot, they nevertheless contribute to an improved understanding of the interconnected runoff system and highlight potential flood hazards. Depending on the weighting of evapotranspiration in the calculation of the model, the potential storage volume can be quantified between 0.116 km³ and 0.547 km³. The total inundation area was calculated at 1.860 km². In addition, three main runoff paths were identified, of which the central and the eastern runoff paths pose a particular threat to the regional capital of Oshakati.
Furthermore, with the help of the model, for the first time it was possible to identify areas where, after the end of the rainy season, water availability is naturally shortest (Namibia) or longest (Angola).
Based on these numerous, new results, scenario calculations for neighboring catchments as well as calculations for other precipitation periods can be performed in the future. Thus, the duration of water availability after the end of a rainy season can be determined and possible locations for retention measures can be identified for various locations.
en
