INVESTIGATION OF THE SPATIAL AND TEMPORAL VARIATION OF SEDIMENT YIELD AND SURFACE RUNOFF IN OFFA WATERSHED, KWARA STATE
ABSTRACT
In this research, an hydrological modelling tool, soil and water assessment tool (SWAT) used to investigate the spatial and temporal variation of sediment yield in a watershed. The model was run for 31years using spatial data such as Digital Elevation Model, soil map, land use and precipitation, wind and solar radiation. The results showed that the maximum value ofsurface Runoff was estimated as 19100.034mm in the year 2005 while the minimum surface Runoff was 1000.671mm in the year 20017. The maximum value of sediment yield was estimated as 2340.532mm in the 2005 and the minimum value was estimated as 34.769mm in the year 2003
TABLE OF CONTENTS
Title page
Certification ii
Dedication iii
Acknowledgement iv
Abstract v
Table of content vi
List of Figures ix
CHAPTER ONE: INTRODUCTION
1.1 Introduction 1
1.2 Problem Statement 2
1.3 Aims & Objectives 2
1.4 Justification 3
1.5 Scope of Study 3
1.6 Description of Study Area 3
CHAPTER TWO: LITERATURE REVIEW
2.0 Literature Review 5
2.1 Sediment Source Analytical Techniques 5
2.2 Sediment Yield Measurements 6
2.3 Field Measurements of Sediment Yield 7
2.4 Sediment Yield Modeling 7
2.5 Brief Description of Selected Hydrology Models 8
2.5.1 RIBASIM 8
2.5.2 WEAP 9
2.5.3 Realm Resource Allocation Model 9
2.5.4 HSPF Model 10
2.5.5 AGNPS Model 10
2.5.6 SWAT Models 11
2.6 SWAT Model Description 11
2.7 Water Shed Hydrological Modeling. 11
CHAPTER THREE: METHODOLOGY
3.0 Methodology 13
3.1 Model Selection and Description 13
3.2 Model Input Data 14
3.3 Digital Elevation Model (DEM) 14
3.4 Soil Map 15
3.5Weather Data16
3.6 SWAT Model Set-up and Run 16
3.7 Water Shed Delineation 16
3.8 Land use map of the watershed 17
CHAPTER FOUR: RESULTS AND DISCUSSION
4.0 RESULTS 19
4.1 Temporal Variation of total means for surface runoff 19
4.2 Temporal variation of total means for sediment yield 20
4.3 Temporal variation of annual means for sediment yield 21
4.4 Temporal variation of annual means for surface runoff 21
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.0 Conclusion 23
5.1 Recommendation 23
References LIST OF FIGURES
Figure Description Page No.
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1.2
1.3
3.1
3.2
3.3
3.4
3.5
4.1
4.2
4.3
4.4
Map of Nigeria
Offa watershed
Kwara State Map
Technical workflow of the study
Digital elevation model of the study area
The Soil map of the study area
Watershed delineation
Land use map
Temporal variation of surface Runoff
Temporal Variation of annual means for sediment yield
Temporal Variation of annual means for sediment yield
Temporal variation of annual Means 4
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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
1.4 JUSTIFICATION
The Spatial and temporal variation of sediment yield and runoff in the catchment area can serve as a decision support tools to water managers in the area to plan a sustainable management of the water shed.
1.5 SCOPE OF STUDY
The scope of the work was limited to the study of temporal variation of sediment using temporal data of 31years from 1986 to 2016. the analysis was based on preliminary modeling results obtained from digital elevation model (DEM) land use and soil map to predict the optimum variation of sediment yield and runoff generation in Offa water shed.
1.6 DESCRIPTION OF STUDY AREA
Offa water shed is located in offa local government area of kwara state Nigeria. It is one of the main water supply in its locality, it is about 43km from Ilorin the capital of kwara state it fall within the latitude 8.1393o N and 4.71740 E. The map is shown in figure 1.1-1.3
Figure 1.1: Map of Nigeria
ImageFigure 1.2:Kwara State Map
Figure 1.3: Offa Water Shed
ImageImage
CHAPTER TWO
2.0 LITERATURE REVIEW
The chapter review existing literature on approaches for measuring/ determining sediment yield, sediment source and also existing models for predicting sediment transport and sediment yield.
2.1 SEDIMENT SOURCE ANALYTICAL TECHNIQUES
Determining the source of sediment &associated nutrient and contaminant is an important issue for the management of water quality (caitcheon, 1998). Spatial sediment tracing can be achieved by measuring the relative contribution for a whole drainage network can be established (caitcheon, 1998). In addition to spatial source tracing doted sediment cores from channel.
Symander and strunk (1992 in nagleetal. 2007), described some of the difficulties with the use of sediment to identify source areas.mappingerosional feature in water shed for sediment source tracking could involve using photos, map, fieldsurveys, erosion pin and troughs. Assembling information on suspended sediment source has proved difficult using the traditional direct monitoring techniques GIS and remote sensing techniques and other modeling programmes such as USLE, WEPPhave recently been employed for sediment source tracking in catchment models are appealing because they are cheaper to use. They also most effective for source analysis where the models have been applied and calibrated models are used by reviewing existing data and consulting with those who are familiar with basin condition (Gellis 2010) however with very large basin some model are associated with large errors.
Secondly some are very complexand require very complex input data sets such as hydrology, rain full interception by vegetation. Water balance, plant growth and residue decomposition of catchment which make them unsuitable in the developing countries where such explicit data in difficult to generate (e.g WEPP & EUROSEM).The modeling approaches to overcome the limitation of the emprical USLE concentrate on physically based erosion models such as SWAT.
2.2 SEDIMENT YIELD MEASUREMENTS
Sediment yield is the amount of sediment load passing the outlet of a catchment, that is the sediment load normalize for the drainage area and in net result of erosion & deposition processes with a basin these materials are of three different kinds, dissolved load (consisting of soluble materials carried as chemical ions), suspended load (containing clay &silt held up by the turbulent flow) and bed load which include larger particles.
2.3 FIELD MEASUREMENTS OF SEDIMENT YIELD
Measuring&estimating sediment yield has long been subject to confusion and uncertainty (Thomas, 1985) because various method have been developed to measure sediment yield and they include measurement of suspended sediment load &water discharge (akrasi 2005; khanchouletal.2010), measuring total eroded soil and depositedsediments in small catchments and measuring sediment volumes in lakes or reservoirs (Verstrarteen andPoesen,2001).
The idea to estimate the sediment yield of rivers would be to measure suspended sediment concentration and water discharge continuously and use the product function as an estimate of sediment discharge. The use of sediment discharge rating curve to estimate yield is however problematic because suspended sediment concentration are known to be variable for a given discharge because storm flow hydrograph usually, but not always are characterized by his her suspended sediment concentration than subsequent flows of similar magnitude.
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