CHAPTER ONE
        INTRODUCTION
1.1     Background to the study
The water is the most important natural resource especially in the arid or semi-arid zones that face high population growth, scarcity of freshwater, irregularity of rain- fall, excessive land use change and increasing vulnerability to risks such of drought, desertification and pollution. Thus, the availability and the sustainable use of this resource become the core of the local and national strategies and politics in these regions.
Hydrology is the main governing backbone of all kinds of water movement and hence of water-related pollutants. Understanding the hydrology of a watershed and modeling different hydrological processes within a watershed are therefore very important for assessing the environmental and economical well-being of the watershed. These models can offer a sound scientific framework for watershed analyses of water movement and provide reliable information on the behavior of the system.
    The degradation of hydrological resources has made it essential to encourage management practices based on knowledge of spatial and temporal changes in the quantity and quality of water, in order to ensure the suitability of water supplied for different uses. This can be assisted by using hydrological and water quality models to simulate a wide range of processes in hydrographic basins, such as the production of water and sediments and the dynamics of point and nonpoint sources of pollution.
Hydrological models are powerful tools to represent water-resource availability and behavior in drainage basins under many applications, such as climate change, flood, drought, runoff and nutrient movement (Abbaspour et al., 2015). They can assist in the planning and decision-making processes for environment protection and the guarantee of water availability for future uses (Da Silva et al., 2015; Fatichi et al., 2016).
The ability of hydrological model to produce satisfactory predictions is necessarily correlated to adequate sensitivity analysis and model calibration (Song et al., 2015). Hydrological models, such as SWAT, incorporate several parameters (climatic, hydrological and others) obtained theoretically and through field data collection. Some of these contribute greatly to model outputs (sensitive parameters), while others have minor relevance (non-sensitive parameters) (Van Griensven et al., 2006).
Osun is a river that flow southward through southern Nigeria into Lagos lagoon and Atlantic Gulf of Guinea. It is one of the several rivers ascribed in local mythology to have been women who turned into flowing waters after some traumatic event frightened or angered them. Osogbo GPS coordinates are 7˚46’15.74’’N and 4˚33’25.13’’E. Due to the potential of Osun river basin as a major source of water supply, this study will develop a plan that will allow for the sustainable management and treatment of the water in the catchment area.
1.2    Problem Statement
Osun River Catchment main source is one of the major sources of water supply, drinking and commercial purposes. It therefore important to know sensitive hydrologic parameters that contribute and hinder the stream flow out and sediment concentration in the catchment area.
1.3   Aims and Objectives
The main aim of this project is to investigate the use of modeling tool for sensitivity analysis of stream flow hydrologic and sediment parameters. Specific objectives achieved are:
i.    Simulation of the hydrological process of the watershed using temporal and spatial data.
ii.    Prediction of the stream flow and sediments yield of the watershed.
iii.    Determine the influence of the hydrologic and sediment parameters on stream flow and sediment yield prediction.