Understanding Hydrological Processes in an Ungauged Catchment in sub-Saharan Africa

Ungauged catchments can be found in many parts of the world, but particularly in sub-Saharan Africa. Information collected in a gauged catchment and its regionalisation to ungauged areas is crucial for water resources assessment. Especially farmers in semi-arid areas are in need of such information. Inter and intra-seasonal rainfall variability is large in these areas, and farmers depend more and more on additional surface and groundwater resources for their crop production. As a result, understanding of the key hydrological processes, and determination of the frequencies and magnitudes of stream flows, is very important for local food production. This is particularly true for the ungauged Makanya catchment in Tanzania, which is the subject of this study. In the absence of long-term hydrological data, hydrological processes have been studied through a multi-method approach. Regular rainfall and runoff measurement devices were installed in a nested catchment approach. High spatial and temporal resolution data have been collected over a period of 2 years to capture all the hydrological processes. Spring samples have been taken to identify groundwater flow systems. Hydrograph separation with hydrochemical data has been performed to identify and quantify the origins and flow pathways of the water during flood flows. Electrical resistivity tomography (ERT) has been used to map the subsurface structure at selected sites. Finally, a conceptual model has been developed to test the hypothesised conceptualisation of the flow paths. Agricultural practices by farmers in the catchment vary as a function of location as they are influenced by the local climate, water resources availability and soil type. Three zones with distinct features have been identified within the study area. In the highlands, it is cooler, rainfall is more abundant and there are perennial springs. Here irrigation is practiced as supplementary irrigation in the wet season and as full irrigation during the dry season. In the midlands only supplementary irrigation is practiced using the remainder of the perennial streams coming from the highlands. Here the high probability of occurrence of dry spells requires supplementary irrigation. In the lowlands, base flow has dried up and spate-irrigation is practiced during the rainy season, whereby flash floods are diverted from the main river onto the farm land. In the highlands, the occurrence of perennial springs is defined by the geology. These springs discharge a substantial amount of water. In the midlands, few springs exist, yielding a substantially lower amount of discharge, with poorer quality. Abundance of spring water from the highlands is both used in the highlands and the midlands for irrigation. However, due to the many diversions, the rivers do no longer reach the outlet of the catchment, as they do not exceed the infiltration capacity of the alluvium. Only large flash floods reach the spateirrigation system in the lowlands. Two types of floods have been analysed in the catchment, induced by different types of rainfall. High intensity rainfall can generate flash floods which reach the spate-irrigation system. Smaller floods infiltrate into the alluvium before reaching the spate-irrigation system. It has been observed that during small floods, the vast majority of the flood originates from groundwater (> 90 percent). During an extreme event, the groundwater contribution is still about 50 percent of the total flow, increasing the outflow from the groundwater reservoir a 100-fold during the peak flow and causing a substantial base flow increase after the event. The time of concentration is extremely short, within one to two hours the flood peak reaches the outlet of the catchment flowing into the spate-irrigation system. The rainfall during the March 2006 event was such an extreme event, with intensities as high as 50 mm hr-1, while the entire event lasted only 4-5 hours. During this event, rainfall intensities exceeded the infiltration capacity of the soils and overland flow was common. The flows exceeded also the absorption capacity of the alluvium of the main valley of the catchment, replenishing the local aquifer and feeding the downstream spate-irrigation system. The temporal resolution of the observed data is 15 min. This level of detail was necessary to capture the rapid catchment response during peak flows. The observation data, unfortunately is not complete as extreme events damaged the structures and affected the reliability of the observations. Moreover, 7 out of 10 instruments were lost, stolen or damaged which hampered the data collection. A conceptual hydrological model has been developed to test the rainfall-runoff hypotheses. The model is able to model in a process-based fashion the flows at the foot of the mountain, incorporating the observed hydrological processes. The model yields good results for a simulation of a 2-year period at hourly time step (Nash-Sutcliffe efficiency: 0.79 and Log Nash- Sutcliffe: 0.90). This model contains a large number of parameters, of which several parameters could be identified from the data. However, automatic optimisation of the remaining parameters is hampered by equifinality. Hence, the strategy chosen is step-wise calibration using multiple performance criteria judging hydrograph performance visually. If the hydrological model would be used for upscaling farming practices, then one should realise that the hydrological processes in the valley of the catchment are different from the highland processes and need to be studied in further detail before the model can be upscaled to the catchment level. The current hydrological and water resources situation, as in many parts of Africa, is also a result of anthropogenic influences. Increased water usage in the upper parts of the catchment generated the need for agreements between highland and midland water users. No agreements are in existence between these two groups and the lowland farmers. With the base flow no longer reaching Makanya, the farmers were forced to change their irrigation practices. Changes in land use in the upstream parts have also impacted on the hydrological processes. However, with a lack of historical data, we were unable to quantify this. The downstream farmers are not solely negatively affected, as flash floods generated in the highlands reach Makanya, replenish the unsaturated zone and local groundwater bodies and deposit fertile sediments. Currently, there is a balance between the upstream and downstream farmers, whereby yields are produced by each system. However, it is unsure how future increases in water requirements will affect this balance.