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.WatermanagementCivil Engineering and Geoscience

Spatial rainfall variability and runoff response during an extreme event in a semi-arid catchment in the South Pare Mountains, Tanzania

OA fund TU Delft This paper describes an extreme flood event that occurred in the South Pare Mountains in northern Tanzania. A high spatial and temporal resolution data set has been gathered in a previously ungauged catchment. This data was analysed using a multi-method approach, to gather information about the processes that generated the flood event. On 1 March 2006, extreme rainfall occurred in the Makanya catchment, (300 km2), where up to 100 mm were recorded in Bangalala village in only 3 h. The flood was devastating, inundating large parts of the flood plain. The spatial variability of the rainfall during the event was very large, even in areas with the same altitude. The Vudee sub-catchment (25.8 km2) was in the centre of the rainfall event, receiving about 75 mm in 3 h divided over the two upstream tributaries: the Upper-Vudee and Ndolwa. The peak flow at the weir site has been determined using the slope-area method and gradually varied flow calculations, indicating a peak discharge of 32 m3 s?1. Rise and fall of the flood was very sharp, with the peak flow occurring just one hour after the peak of the rainfall. The flow receded to 1% of the maximum flow within 24 h. Hydrograph separation using hydrochemical parameters indicates that at the floodpeak 50% of the flow was generated by direct surface runoff (also indicated by the large amount of sediments in the samples), whereas the recession originated from displaced groundwater (>90%). The subsequent base flow in the river remained at 75 l s?1 for the rest of the season, which is substantially higher than the normal base flow observed during the previous rainy seasons (15 l s?1) indicating significant groundwater recharge during this extreme event.Civil Engineering and Geoscience

Determining spatial variability of dry spells: A Markov-based method, applied to the Makanya catchment, Tanzania

With a growing world population and a trend towards more resource-intensive diets, pressure on land and water resources for food production will continue to increase in the coming decades. Large parts of the world rely on rainfed agriculture for their food security. In Africa, 90% of the food production is from rainfed agriculture, generally with low yields and a high risk of crop failure. One of the main reasons for crop failure is the occurrence of dry spells during the growing season. Key indicators are the critical dry spell duration and the probability of dry spell occurrence. In this paper a new Markov-based framework is presented to spatially map the length of dry spells for fixed probabilities of non-exceedance. The framework makes use of spatially varying Markov coefficients that are correlated to readily available spatial information such as elevation and distance to the sea. The dry spell map thus obtained is compared to the spatially variable critical dry spell duration, based on soil properties and crop water requirements, to assess the probability of crop failure in different locations. The results show that in the Makanya catchment the length of dry spell occurrence is highly variable in space, even over relatively short distances. In certain areas the probability of crop failure reaches levels that make rainfed agricultural unsustainable, even close to areas where currently rainfed agriculture is successfully being practised. This method can be used to identify regions that are vulnerable to dry spells and, subsequently, to develop strategies for supplementary irrigation or rainwater harvesting.Water ManagementCivil Engineering and Geoscience

Modelling stream flow and quantifying blue water using modified STREAM model in the Upper Pangani River Basin, Eastern Africa

Effective management of all water uses in a river basin requires spatially distributed information of evaporative water use and the link towards the river flows. Physically based spatially distributed models are often used to generate this kind of information. These models require enormous amounts of data, if not sufficient would result in equifinality. In addition, hydrological models often focus on natural processes and fail to account for water usage. This study presents a spatially distributed hydrological model that has been developed for a heterogeneous, highly utilized and data scarce river basin in Eastern Africa. Using an innovative approach, remote sensing derived evapotranspiration and soil moisture variables for three years were incorporated as input data in the model conceptualization of the STREAM model (Spatial Tools for River basin Environmental Analysis and Management). To cater for the extensive irrigation water application, an additional blue water component was incorporated in the STREAM model to quantify irrigation water use (ETb(I)). To enhance model parameter identification and calibration, three hydrological landscapes (wetlands, hill-slope and snowmelt) were identified using field data. The model was calibrated against discharge data from five gauging stations and showed considerably good performance especially in the simulation of low flows where the Nash–Sutcliffe Efficiency of the natural logarithm (Eln) of discharge were greater than 0.6 in both calibration and validation periods. At the outlet, the Eln coefficient was even higher (0.90). During low flows, ETb(I) consumed nearly 50% of the river flow in the river basin. ETb(I) model result was comparable to the field based net irrigation estimates with less than 20% difference. These results show the great potential of developing spatially distributed models that can account for supplementary water use. Such information is important for water resources planning and management in heavily utilized catchment areas. Model flexibility offers the opportunity for continuous model improvement when more data become available.Water ManagementCivil Engineering and Geoscience

Modelling stream flow and quantifying blue water using a modified STREAM model for a heterogeneous, highly utilized and data-scarce river basin in Africa

Integrated water resources management is a combination of managing blue and green water resources. Often the main focus is on the blue water resources, as information on spatially distributed evaporative water use is not as readily available as the link to river flows. Physically based, spatially distributed models are often used to generate this kind of information. These models require enormous amounts of data, which can result in equifinality, making them less suitable for scenario analyses. Furthermore, hydrological models often focus on natural processes and fail to account for anthropogenic influences. This study presents a spatially distributed hydrological model that has been developed for a heterogeneous, highly utilized and data-scarce river basin in eastern Africa. Using an innovative approach, remote-sensing-derived evapotranspiration and soil moisture variables for 3 years were incorporated as input data into the Spatial Tools for River basin Environmental Analysis and Management (STREAM) model. To cater for the extensive irrigation water application, an additional blue water component (Qb) was incorporated in the STREAM model to quantify irrigation water use. To enhance model parameter identification and calibration, three hydrological landscapes (wetlands, hillslope and snowmelt) were identified using field data. The model was calibrated against discharge data from five gauging stations and showed good performance, especially in the simulation of low flows, where the Nash–Sutcliffe Efficiency of the natural logarithm (Ens_ln) of discharge were greater than 0.6 in both calibration and validation periods. At the outlet, the Ens_ln coefficient was even higher (0.90). During low flows, Qb consumed nearly 50% of the river flow in the basin. The Qb model result for irrigation was comparable to the field-based net irrigation estimates, with less than 20% difference. These results show the great potential of developing spatially distributed models that can account for supplementary water use. Such information is important for water resources planning and management in heavily utilized catchment areas. Model flexibility offers the opportunity for continuous model improvement when more data become available.Water ManagementCivil Engineering and Geoscience

Hydrograph separation and scale dependency of natural tracers in a semi-arid catchment

WatermanagementCivil Engineering and Geoscience

On the value of combined event runoff and tracer analysis to improve understanding of catchment functioning in a data-scarce semi-arid area

Civil Engineering and Geoscience

Enhancing capacities of riparian professionals to address and resolve transboundary issues in international river basins: Experiences from the Lower Mekong River Basin

This paper analyses the design and impact of capacity building programmes aimed at enhancing capacities of riparian professionals to address and resolve transboundary issues in international river basins. The case study is a programme developed by the Mekong River Commission (MRC). A post-training evaluation was applied to assess its impact in terms of individual capacity enhancement and change (use and application of knowledge, factors hampering application, and change in function and opportunities within the organisation). The design of the Capacity Building Programme of the MRC Flood Management and Mitigation Programme required a well balanced range of subjects (such as IWRM (integrated water resources management), model and decision support systems, and international water law). The post-training evaluation, 6 months after the last training workshop, showed an increase in familiarity with the topics for all 37 respondents, with the highest increase for the respondents with few years of working experience and from training and education institutions. The relevance of the subjects taught was highlighted by 95% of the respondents, and 78% of the participants had already used some of the acquired knowledge in their job. The respondents indicated that they did not have sufficient opportunities to apply all knowledge. The phased implementation and training of lecturers during the training workshops had a good impact, directly through increasing involvement in facilitation and delivery of the capacity building programme and through the use of the knowledge gained in short courses and development of curricula at their institute. For these types of capacity building programmes, a few recommendations can be made. The selection of participants is crucial for the application of the learned knowledge in their work. The integrative nature of transboundary water issues calls for a capacity building programme addressing a wide range of subjects, which can be understood by a wide range of professionals from different sectors. Training methods should also address this integrative nature through, e.g. roleplays and case studies. A successful capacity building programme needs to address the three levels of capacity building (enabling environment, organisations, and individual staff) and involve national and regional training and education institutes.Water managementCivil Engineering and Geoscience