Hydrological Update of the Watershed Resources Management Model and Gis-Based Application to the Humid Tropical Environment of South-East Nigeria

Abstract

Hydrological models are useful tools for prediction and understanding of hydrological phenomena which underpin watershed resources utilization. A watershed model simulates hydrological processes in a more holistic manner across the entire watershed area. Most watershed models are fairly complex; interactions among watershed hydrological components are essentially nonlinear and take place on a wide range of temporal and spatial scales. The Watershed Resources Management (WRM) model was developed in Canada in 1991 and published by Mbajiorgu (1995a & b). The objectives of this study were to: (i) update the hydrological component models of evapotranspiration and canopy interception, (ii) develop a MAPWINDOW GIS Input Application for WRM model input files preparation and data processing, (iii) employ the Shuffled Complex Evolution – University of Arizona (SCE-UA) optimization scheme for automatic calibration of WRM model, and (iv) validate the WRM model for application to the humid tropical environment of South-East Nigeria. The spatial structure and numerical solution scheme of the original WRM model were retained for distribution of hydrological responses and parameter specification. Recent advances in physically-based modelling of hydrological processes with particular regard to canopy interception and evapotranspiration were implemented. Modular components of the model are as follows: Initialization routine; Timing routine; Rainfall-event routine; Ponded-infiltration routine; Runoff routine; Saturation-runoff routine; Kinematic-f1ow routine; Conservationstructures (terraces) routine; Grassed-waterways routine; Hydraulic-structures (reservoirs) routine; Culvert routine; Evapotranspiration-event routine; Baseflow routine; Soil-moisture accounting and Subsurface-lateral flow routines. WRM input application was developed as a plug-in to MAPWINDOW GIS using Visual Basic programming language. The input application facilitates analyses of geospatial data, extraction of the watershed drainage network, and creation of WRM model input files. Model parameters often do not represent directly measurable entities but must be estimated using other means as well as model calibration. Automatic calibration was implemented by the SCE-UA method. The model was calibrated and validated using observed runoff and sediment yield data from the Upper Ebonyi River Watershed, South-East Nigeria. Three statistical evaluation techniques, namely, Coefficient of determination (R2 ), Nash-Sutcliffe Efficiency (NSE), and Percent bias (PBIAS) were employed to evaluate the WRM model vii application, in addition to graphical comparisons of simulated/predicted and observed time series. An input application software plug-in to Mapwindow GIS was developed for WRM model. Optimum values for six parameters of WRM model were generated from the calibration technique. Simulated runoff rates and sediment yield data were generated for the Upper Ebonyi River Watershed and compared/matched with observed time series from June to October, 2013. From the results obtained during calibration and validation phases of the model application, the coefficient of determination values were 0.83 to 0.99 for runoff prediction and 0.99 for sediment yield. NSE values of 0.62 and above were obtained for runoff prediction and of 0.57 and above were obtained for sediment yield prediction. PBIAS values of calibration and validation phases of runoff prediction were less than ± 12% which is considered a very satisfactory model performance. However, for sediment yield simulations, the model did not perform as well since PBIAS values ranged from 42% to 77% with the acceptable limits being ± 55%. The WRM model as hydrologically updated has thus been satisfactorily applied to a humid tropical environment quite distinct and different from the Atlantic Canadian environment of the model’s original development. 

Table Contents

Title page

Dedication i

Acknowledgement ii

Abstract iii

Contents v

List of Figures viii

List of Tables xi

Chapter One: Introduction 1

1.1 Background 1

1.2 Watershed Management 2

1.3 A Model for Watershed Resources Management 3

1.4 Objectives and Scope of the study 4

1.5 Justification 4

Chapter Two: Literature Review 6

2.1 Models 6

2.1.1 Hydrologic Modelling 9

2.2 Existing models of Watershed Hydrology 11

2.3 Watershed Hydrologic Processes Description and Modelling 11

2.3.1 Interception 11

2.3.2 Estimation of Snowmelt 14

2.3.3 Evapotranspiration 15

2.3.4 Infiltration and Unsaturated subsurface flow 18

2.3.5 Saturated subsurface flow 27

2.3.6 Overland and Channel flow 29

2.3.7 Reservoir Routing Model 36

2.3.8 Erosion and Sedimentation 38

2.3.8.1 Physically-based erosion models 39

2.3.8.2 Upland Erosion 40

2.3.8.3 Channel Erosion 42

2. 4 Modelling Equations for Conservation structures 43

2.4.1 Culvert 43

2.4.2 Terraces 45

2.4.3 Vegetated or Grassed waterways 45

2.5 Coupling the surface and subsurface process descriptions 46

2.6 Geographic Information System (GIS) 47

2.6.1 Integration Strategy 49

2.6.2 Digital Elevation Model Data 50

2.7 Parameter Estimation 51

2.7.1 Model Calibration 52

2.7.2 Automatic Calibration 55

2.8 Model Validation 58

2.9 Model Evaluation 62

2.10 WRM Model Concepts, Structure and Components Integration 65

2.10.1 Watershed Concepts 65

2.10.2 Model Structure 71

2.10.3 Component Integration: The Systems Approach 73

Chapter Three: Methodology 75

3.1 Modification of Hydrological Component Models 75

3.1.1 The Interception and Evapotranspiration component 75

3.1.2 The Infiltration and Unsaturated-subsurface flow component 79

3.1.3 Saturated subsurface flow component 80

3.1.4 The Overland and Channel flow component 80

3.1.5 The Reservoirs and Structural component 80

3.1.6 Erosion and Sedimentation component 81

3.2 Continuous Simulation Implementation 82

3.3 Program Structure 84

3.3.1 Logic of the Main Subprogram 86

3.4 Model Synthesis 87

3.5 Input Data Processing for WRM model Application Using GIS 101

3.6 Automatic Calibration with SCE-UA Optimizer 105

3.7 Establishing a watershed data file 107

Chapter Four: Results 108

4.1 WRM Input Application Results 108

4.2 Calibration Results 110

4.2.1 Evaluation of Calibration Parameters 110

4.3 WRM Model Output Results 111

4.4 Observed Results 126

Chapter Five: Model Output Analysis and Discussion 128

5.1 Model Calibration Output Analysis 128

5.2 Model Validation Output Analysis 132

5.3 Model Evaluation 138

5.3.1 Model Evaluation on Calibration Results 138

5.3.2 Model Evaluation on Validation Results 141

Chapter Six: Summary and Conclusion 146

REFERENCES 148

APPENDIX I: INPUT APPLICATION SOURCE CODE 157

APPENDIX II: CALIBRATION CODES 176

APPENDIX III: WRM FORTRAN CODE AS UPDATED 201

APPENDIX IV: WRM INPUT DATA FILES 231

APPENDIX V: INPUT PARAMETER DEFINITION 313