DEVELOPMENT OF REGIONAL FLOW DURATION CURVES FOR SELECTED RIVERS IN UPPER NIGER RIVER BASIN, NIGERIA

ABSTACT

It is common knowledge that the end user of stream flow data may necessarily not have any prior knowledge of the quality control measures applied in their generation. Thus, this study provides an intuitive attempt to provide an independent quality indicator to boost the confidence in the use of stream flow data by developing regional flow duration curves for selected ungauged station of the upper Niger River Basin, Nigeria. Toward this end, stream flow data for seven gauging stations covering some sub-river basin in the upper Niger River Basin was effectively mobilised; in this case, monthly stream flow data covering a range of eleven to fifty three years period. The flow duration curves from the gauging stations were fitted with three distribution models; i.e., the logarithmic, power and exponential regression equations. For regionalisation, the parameterisation was carried out in terms of the drainage area alone; just for simplicity of models. Results obtained showed that the exponential regression equation, in terms of R2 had the best fit. Though the regionalised model was simple, measurable agreement was obtained during the calibration and validation phases. However, considering the length of data used and probable variability in the stream flow regime, it is not possible to objectively generalise on the quality of the results. Against this backdrop, it suffices to take into cognisance the need to use an ensemble of catchment characteristics in the development of the flow duration curves and the overall regional models; this is important considering the implications of anthropogenic activities and hydro-climatic variations.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background of the Study

Hydrologic techniques such as estimation of water yield at specific locations, determination of response of a river basin for a given precipitation input and estimation of design storm/floods for water resources structures, if capably applied are essential for the sound development and management of water resources. The estimation of water potential for a particular water body finds relevance in the face of changing population dynamics. Generally water use out paces the growth in population and thus create demand for water that frequently impingent the available supply. As noted by Langbein and Iseri (1995), the water demand- supply conflict and the associated problem of it spatio- temporal fluctuations or variability require that robust approach for water analysis be available. It in important considering the fact that planning for water resources development must depend upon availability of hydrological information which forms the basis of sustainability. One of the largely used tools to evaluate flow rates and the river regime is the flow duration curve (FDC). This is a cumulative frequency curve that shows the percentage of time that a given discharge is equalled or exceeded during a fixed period (Ubertini, 2010). This plot provides information on the probability of occurrence or otherwise. The choice of this tool is important because it is one of the most informative methods of displaying the flow characteristics of a stream throughout the range of discharge, without regard to the sequence of occurrence. Basically, the water nature of the slope for a particular flow duration curve (FDC) gives an ideal of the storage, characteristic of a river. The flow duration curve may well indicate the watershed rainfall- runoff  characteristic,  and  have  been  applied  in  many  aspects  of  water  resources development and water environment protection. These curves are derived in gauged river data, but are essential also in rivers where the systems for collecting and managing water information are inadequate.

Different approaches exist to solve this problem; they can be categorised into two groups, namely:

1.   The statistical models: The flow estimation in ungauged sites is obtained by simple regression models, which help to connect flow statistics or parameters to known characteristics of the basins.

2.   The second group is represented by rainfall – runoff models which allows for simulation and evaluation of the time series of river flows within a catchment. This method is useful in order to evaluate the interaction between the physical components of the water basin.

In line with the definition according to Bandaragoda (2001), a river basin in basically a hydrologic response unit; i.e, in defines an area which constitutes in the overall the geography unit. For any river system, there are quite a number of water related human interventions, including water storage, diversion, regulation, distribution, application, pollution, purification, and other associated acts; according to Sunaryo (2001), these action help to change the natural system. From time in memorial, human development have centered around the banks of rivers for water for agriculture, domestic and industrial purpose, hydro- power generation, e.t., this fact was reiterated by Adeloye (2002), especially the use of river for assimilation of human wastes. The availability of this water for various uses is highly variable, both temporally and spatially. As a consequence, it is often not possible to rely on river water in its natural occurrence to meet these needs without one form of human intervention or another (Adeloye, 2002). According to George (2005), water as a resource must be culturally defined because water by itself is not productive. Its use requires some minimum level of social and economic infrastructure for it to be productive.  Nigeria have altogether over 162 large, medium and small dams that have been constructed and are being operated. They have a total storage capacity of more than 30109 m3, i.e. less than 10% of the country’s total potential surface water resources. Eighty five percent of the larger dams are located in the Sudano-Sahelian zone of the country. A sample of 52 dams indicates that 79% have domestic industrial water supply components, while 33% have irrigation as a major use to which the stored water is used for; 4% are also for hydro-electric   power generation (HEP); 29% for fisheries and 16% for recreation. Because of these notable facts, Oyibande (1995) posted that the available dams irrespective of size contribute in quantifiable measure to flood mitigation and in advertency affect wetlands in their respective downstream areas.

The three largest hydropower dams are under operation and control the flow of the Niger and Kaduna rivers. These are Kainji, Jebba and Shiroro with total active capacity of 18.6 x 109m3 and total power capacity of 1920 MW. In terms of storage usage however, irrigation accounts for 36%, water supply 3% and hydropower 61% (Oyebande, 1995). Accurate demand survey and assessment is a pre-requisite for efficient reservoir operation of these dams to meet the objectives of their construction. In Nigeria, the range of purposes served by storage reservoirs includes water Supply for irrigation, domestic and industrial uses, hydroelectric power (HEP), increasing water depth for navigation, flood control, reclamation of low-lying lands and recreation. Some of these uses conflict (e.g. flood control and HEP and other uses), thus, priorities   and proper balancing need to be carefully considered. In order to carry out proper reservoir operation, certain basic data and drawings such as flow duration curves (FDC) are required at the operation and maintenance office. Unfortunately, most of the above drawings and data are not available in many operation and maintenance (O&M) offices at dam sites. As a result, most of the monitoring works needed for efficient and proper reservoir operation are not carried out. In reservoir management, the reservoir operational guideline should take into account reservoir inflow, evaporation losses, and irrigation demand both in wet and dry seasons, among other demands. Observed hydrological data at dam sites are indispensable for such considerations. Unfortunately, the scarcity of stream flow data is a common problem, as shown by the large number of studies addressing the regionalization of flow duration curves (FDCs) for different geographic regions around the world e.g, Le Boutivier and Waglen in Castallarin, A., Galeati, G., Brandimarte, C., Montanari, A., and Brath,  A., (2004), Castallarin, A., camerani, G, and Brath, A., (2007), Viola, F, Noto, L.V Cannarozzo, M. and loggia La.G (2010), Patils, (2011). The situation in developing countries in this light in more dire; for instance, as reported by Mantra  and Ahaneku (2009), the problem of stream flow data mobilisation in due to a lost of reasons like pecuniary related (i.e., fund,), and for a large extent, lack of proper data archiving and retrieval system.

Considering the dearth of stream flow data in terms of both quantity and integrity, the regionalization of FDCs appears therefore to be an essential operative tool when dealing with ungauged river basins or short stream flow records. That is, river basins of which the data is not identified or have limited amount of stream flow observations, the flow duration curve can be obtained by implementing the regional analysis. The ideal here is them search for a potential candidate curve based on available information of surrounding areas or basins to generate the possible flow duration curve. This concept in predicted on the assumption of hydro meteorological similarity of the neighbouring basins.

1.2       Statement of the Problem

The statement of the problem this study in informed by the following specific problem of increase hydrological importance Viz:-

1.   Dearth of long and continuous stream flow data resulting from paucity of stream gauging station, in-a-in the corresponding relevant information base.

2. Improper inventory of available surface water bodies limits any probable quantitative water resource planning strategy.

3.   Hydrological data archiving and retrieval/assessment pose a daunting challenge. That is, the choice of an appropriate data length and quality is extremely impaired, especially the absence of stage- discharge rating curve in most river basin.

1.3       Aim and objectives of the study

The aim of this study is to develop regional flow duration curves (FDC’s) for the ungauged sub-basins of the Upper Niger River Basin. In view of the general aim of the research, the specific objectives of the study are to:

1.   develop  regional  flow  duration  models  for  purposes  of  understanding  the hydrologic signatures of the sub-basins.

2    quantify the reliability and effectiveness of the regional approach and derive flood and low flow indexes for the ungauged catchment.

3    determine  the  possibility  of  effective  generalisation  of  the  result  of  the regionalisation approach in the face of data austerity; i.e., likelihood of optimising the hydrologic parameters of the regional models.

1.4       Justification of the Study

Regionalization is shown to be a powerful tool in transferring hydrological information from gauged sites to other remote ungauged sites within the homogeneous region. The developed regional models are used in predicting accurately hydrological information, such as sediment rating curves, maximum flood events, unit graphs, and mean annual floods for ungauged basins and is needed for the hydrological design of water resources development systems. The result of this study will therefore be of tremendous benefit to the Government and non-governmental organizations involved in water resources planning and operations in Kaduna, Gbako and Gurara river sub-basins to sustain the rapid socio – economic development in the Federal Capital Territory (FCT) and its neighbouring states, especially quantification of water potential of rivers for hydroelectric power (HEP) generation, increasing water depth for navigation, flood control, reclamation of low-lying lands and recreation.

1.5 Scope and limitation of the study

The study is basically limited to the establishment of regionalised flow duration curves for the selected ungauged rivers in the Upper Niger River Basin and the determination of the model performance characteristics. Beside this, the study was mired by the absence of long and continuous stream flow data.

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