CHAPTER ONE
1.1     INTRODUCTION
Water is an integral part of life, as human beings derived from the environment several services that are necessary for the survival, Water is one of the basic needs that human beings cannot live without; indeed water is life! Therefore, water-related (hydrological) ecosystem services provided by the environment (e.g provision, regulation and purification of freshwater) are quite valuable and important for human well-being. This underscores the importance of sound watershed management for continued provision of hydrological ecosystem services. From a hydrological point of view, a watershed includes all land contributing water (surface and ground water) to a reference point.     
  It is therefore obvious that land comprising of any watershed would generally be under other uses such as forests, agriculture and urban centers, which might commonly be considered ‘primary’ land uses. This means that watersheds provide other important ecosystem services, beside provision of hydrological ecosystem services. In some cases, enhanced provision of some ecosystem services may also lead to reduced capacity of watersheds to provide other services e.g. intensive cultivation to maximize food production may also lead to increase in soil erosion and consequently degradation of water quality.
Sediment yield is the amount at a point of interest in a particular period of time which occur due to heavy rainfall, are normally as tones per year or kilogram per year.
A large part of failure to achieve reasonable estimates of average annual sediment lies in particles of extrapolating relationship derived from field data with no consideration of appropriateness for future conditions.
Sediment yield is affected by many factors such as climate, soil, relief, vegetation and human influence. Runoff refers to as the part of water cycle that flow over land as surface water.
     Runoff has been used as a variable representing climatic conditions and includes not only the water that travel over the land surface and through channels to reach a stream but also interflow, the water that infiltrates the soil surface and travels by means of gravity toward a stream channel.
      In this study offa water shed is simulated to predict the surface runoff and sediment yield. The spatial and temporal variation obtained can be used as a decision support tool in the management of the water shed.
1.2   PROBLEM STATEMENT
Recent flooding issues in Nigeria as a result of excessive rainfall has resulted into runoff and sediment yield in the area. Therefore in order to manage the water shed sustainability, it is necessary to predict the variation of sediment yield and runoff in the area which can serve as a data base in managing the watershed.   
1.3    AIM AND OBJECTIVES
The aim of the research was to investigate the spatial and temporal variation of sediment yield and runoff in Offa watershed.Specific objectives achieved are:
i.    Predict the surface runoff and sediment into watershed
ii.    Develop temporal variation map of sediment yield and runoff
iii.    Develop spatial variation map of sediment yield and runoff