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Hydrological modeling of the Congo River basin: Asoil-water balance approach


par Bahati Chishugi Josue
University of Botswana - Masters of Sciences (M.Sc.) 2008
  

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UNIVERSITY OF BOTSWANA

Faculty of Sciences
Geology Department
MASTERS PROGRAMME OF HYDROGEOLOGY

HYDROLOGICAL MODELLING OF THE CONGO RIVER BASIN: A SOIL-WATER BALANCE APPROACH

Josué BAHATI CHISHUGI

A Dissertation submitted to the School of Graduate Studies in partial fulfilment of the requirements for the degree of Master of Science (MSc) in Hydrogeology

SUPERVISOR: Dr B.F. Alemaw
2008

DECLARATIONS

I solemnly declare that this work is the result of my own toiling and has never been submitted anywhere for any award.

Signature of Author

Date

Josue BAHATI CHISHUGI

This dissertation has been submitted for examination with my authority as a University supervisor.

Signature of the Supervisor

Date

Dr B.F. ALEMAW

STATEMENT OF COPYRIGHT

No part of this dissertation may be reproduced, stored in any retrieval system, or transmitted in any form or by any means: electronic, mechanical, photocopying, recording, or otherwise, without prior written permission of the Author or the University of Botswana.

ACKNOWLEDGEMENT

To my Parents, brothers and sisters in D.R.Congo for their unlimited love and support through the many years of school;

To my fiancée, Rachel Moiza Maline, for her Love and endurance,

To the German Academic Exchange (DAAD) for the fellowship which enabled us to complete this M.Sc. degree in Hydrogeology;

To my supervisor Dr B.F. Alemaw, for his invaluable help and encouragement, fruitful advice and patience during my introduction to Water Balance Modelling;

To Dr. T.R. Chaoka, the Head of Geology Department, University of Botswana, for his advices and financial support that regenerated my efforts;

To all the Staff member of Geology Department for their advises and supports;

To my Congolese family in Botswana, in particular Madam P. Kampunzu, Dr. Lukusa and His wife, Papa and Maman Mihigo, Prof. Kitenge's family, V. L. Basira, A. Ibrahim, S. Loly, S.M. Oscar, L. Wani, Dr Chantal and Asina;

To Neovitus Shayio, K. Justin and Kaniki, Tina, Adjoa, and many good people I met in Botswana, Geramny and Malawi;

To my graduate colleagues in Batswana, Z. Chiyapo, M. Brighton «The useless Boy», Oteng, Obone, Pricila, Lynette, Haward, Defaru, Lintwe;

I am truthfully grateful and express my thanks.

EPIGRAPHS

Oh God!
Make me strong to overcome my weaknesses, and
Blessed be Your Name forever and ever.
Amen

Dear Parents, My gods!
Despite your insufficiencies,
You showed me the way to School.
I have understood your divinity!
Thanks for your Love and Sacrifices.

Josué B. Chishugi

ABSTRACT

During the last decade African continent has been characterised by a shortage of water, electricity and food due probably to climatic change and mismanagement of the natural resources. The political stability in Central Africa region, the progress in industrial development in southern and the northern Africa gradually increase the need in water for domestic, agricultural, industrial and environmental uses. The second longest river in Africa, contributing with 30% to the African discharge to the Atlantic Ocean, the Congo contains the second largest forest in the world, after the Amazon, sustains the global climate change regulation and ecosystem stability; therefore, the understanding of its hydrology is of grand importance.

In order to understand and evaluate the spatial and temporal distribution of the Congo River Basin (CRB) water balance, a distributed GIS-based hydrological model, namely Hybrid Atmospheric and Terrestrial Water Balance (HATWAB) initially developed by Alemaw (2006) was parameterised and applied to the Congo basin using Rainfall, Potential Evapotranspiration, soils and vegetation information. The model simulates the Soil-water balance model component, namely the Integrated Vertical Moisture Convergence (C), soil moisture (SM), Actual Evapotranspiration (AET) and Runoff (ROF). The spatial distribution of the simulated components correlate strongly with Rainfall patterns, especially in the high rain fed region (Effective Rainfall >1100 mm/year), corresponding to the central part of the equatorial forest, and extending between 5 and -5 degrees of latitude; whereas some disturbances are observed in the lowest rain fed (Effective Rainfall <1000 mm/year) regions of the basin located the south-eastern and the up-north part of the CRB. The Evapotranspiration Ratio (ETR) shows two main climatic regions, ETR close to 1 for region 1 and ETR<7 for region 2, with an intermediate zone between (07<ETR<0.8). The Annual average SM varies between 0 and 400 mm while the actual Evapotranspiration (AET) varies between 400 and 1700 mm/year with highest values on the water bodies. The based-wide gridded runoff (ROF) varies between 0 and 1400 mm/annum with a annual average of 324 mm/a. Flooded wetland areas and swamps are characterised by 0 to 40 mm/a values while highest runoff (> 900mm/a) are computed on the rivers and lakes. The inland grid ROF varies between 50 and 900 mm per year with an average of 324 mm/year. The accumulated ROF computed at the CRB outlet reaches 44700m3/sec, which is within 5% of marginal error compared to the observed discharge of the Congo River.

A supporting script tool (DEMHydro) was developed to extracts the topographic, topologic and hydrologic characteristics of the basin, using Digital Elevation Model (DEM) information.

TABLE OF CONTENTS

DECLARATIONS II

STATEMENT OF COPYRIGHT III

ACKNOWLEDGEMENT IV

EPIGRAPHS V

ABSTRACT VI

TABLE OF CONTENTS VII

LIST OF FIGURES XI

LIST OF TABLES XIII

LIST OF TABLES XIII

LIST OF ABBREVIATIONS, ACRONYMS AND SYMBOLS XIV

LIST OF ABBREVIATIONS, ACRONYMS AND SYMBOLS XIV

CHAPTER ONE 1

1.0

INTRODUCTION OF THE STUDY

1

1.1

Introduction

1

1.2

Statement of problem

1

1.3

Research objectives

2

1.4

Importance of study

2

1.5

Organisation of the thesis

3

CHAPTER TWO 4

2.0 OVERVIEW OF THE STUDY AREA 4

2.1 The Study Area 4

2.2 Physiography 5

2.3 Hydrology 6

2.4 Climate 8

2.5 Soils 10

2.6 Land cover/use and Population density 11

2.7 Geology 13

2.7.1 Basement formation 13

2.7.2 Surface formations 13

CHAPTER THREE 14

3.0 LITERATURE REVIEW 14

3.1 Hydrological models 14

3.2 Water Balance Model approaches 15

3.2.1 Atmospheric Water Balance Studies 16

3.2.2 Soil Water Balance Studies 16

3.2.3 Surface Water Balance Studies 17

3.2.3.1 Water Balances 17

3.2.3.2 Runoff Mapping 18

3.3 Potential evapotranspiration (ETp) and Effective rainfall determination 18

3.3.1 Estimation of Potential Evapotranspiration 18

3.3.1.1 Net radiation 19

3.3.1.2 Mean Relative Humidity 20

3.3.1.3 Wind speed 20

3.3.1.4 Solar radiation 20

3.3.2 Estimation of Effective Rainfall 21

CHAPTER FOUR 23

4.0 METHODOLOGY 23

4.1 Watershed and streams characteristics 23

4.2 Watershed and drainage network Processing Method 23

4.3 DEM-Hydro processing output maps 25

4.3.1 DEM Visualization and areal distribution over elevation 25

4.3.2 Flow direction map 27

4.3.3 Flow accumulation 28

4.3.4 Drainage network extraction and ordering 29

4.3.5 Catchment and Sub-Catchments extraction 30

4.3.6 Overland Flow map 32

4.4 Watershed characteristics 33

4.4.1 Watershed Geomorphology 33

4.4.1.1 Area and length 33

4.4.1.2 Watershed Shape 35

4.4.2 Morphometric Analysis 35

4.4.2.1 Morphometric network topology 35

4.4.2.2 Horton morphometric parameters 36

4.5 GIS-Based Hydrological Model Development 41

4.5.1 Introduction 41

4.5.2 Water Balance Model development procedure 43

4.5.3 Water Balance Model Development 43

4.5.3.1 Atmospheric water balance 43

4.5.3.2 Terrestrial water balance 44

4.5.3.3 Imbalance estimation 45

4.5.3.4 Rainfall-Actual Evapotranspiration-Soil moisture-Runoff modelling 45

4.5.4 Data sets and software 48

4.5.4.1 GIS and geo-referencing procedure 48

4.5.4.2 Meteorological data sets 48

4.5.4.3 Discharge data 50

4.5.4.4 Digital Elevation Model (DEM) and Mask files 50

4.5.4.5 NDVI and vegetation database 51

4.5.4.6 Soil properties 51

4.5.4.7 Software resources 55

CHAPTER FIVE 56

5.0 MODEL APPLICATION, DATA PRESENATTION AND INTERPRE-TATION RESULTS 56

5.1 Generalities on the Model application 56

5.2 Initial soil moisture 56

5.3 Data presentation and Interpretation results 57

5.3.1 Soil moisture (SM) 57

5.3.2 Actual Evapotranspiration (AET) 59

5.3.3 Runoff 60

5.3.4 Simulated sub-watershed and basin-wide runoff 62

5.3.5 Vertical Integrated Moisture Convergence 65

CHAPTER SIX 67

6.0 CONCLUSIONS AND RECOMMENDATIONS 67

6.1 Conclusions 67

6.2 Recommendations 68

REFERENCES 69

APPENDICES 74

APPENDIX 1: METEOROLOGICAL STATIONS COVERING THE STUDY AREA (FROM FAO/UNESCO CLIMWAT DATBASE) 75

APPENDIX 2: SPREADSHEET MODEL FOR THE PENMAN-MONTEITH CALCULATION METHOD OF ETO (AFTER ALLEN ET AL, 1998) 79

APPENDIX 3: CONGO RIVER DISCHARGE DATA AT KINSHASA 82

APPENDIX 4: ATTRIBUTE TABLE FOR DRAINAGE NETWORK ORDERING 84

APPENDIX 5: HORTON STATISTICS FUNCTIONALITY: DEFINITION OF PARAMETERS 85

APPENDIX 6: HORTON MORPHOLOGICAL PARAMETERS AND STATISTICS FOR SUBSEQUENT STRAHLER ORDER 86

APPENDIX 7 PEARSON PRODUCT MOMENT CORRELATION BETWEEN HORTON AND GIUH 87

APPENDIX 8.A. LOCAL WATER BALANCE FOR SELECTED GRID CELLS IN THE
CONGO RIVER BASIN (TABLE) 88

APPENDIX 8.B. LOCAL WATER BALANCE FOR SELECTED GRID CELLS IN THE
CONGO RIVER BASIN (GRAPHS) 90

APPENDIX 9: SEASONAL DISTRIBUTION OF THE CONVERGENCE MOISTURE SAMPLES FOR SELECTED GRID CELLES OVER THE CONGO BASIN (C) 91

LIST OF FIGURES

Figure 1 General Location Map: Position of the study area in Africa 4

Figure 2 The Congo River Basin Elevation System. The high est station elevation is located in the Tanzanian region while the lowest, at the Atlantic Ocean (Note: this elevation grid is derived

from the elevation of the selected 145 meteorological stations falling inside the study area) 6

Figure 3 Mean Discharge Regime of the Congo River Basin at the Kinshasa gauge 7

Figure 4 Monthly discharge of the Congo River (Kinshasa gauge). Mean 1960-1990 7

Figure 5 Meteorological profile of D.R.Congo 9

Figure 6 Long-term mon thly average of Effective Rainfall (1961-1990) at grid cell. 9

Figure 7 Effective rainfall distribution for three selected grids in the Area 10

Figure 8 Congo Basin Agronomic Soils Map. The polygon limit the Congo watershed 11

Figure 9 Vegetation and Land cover and uses over the Congo River Watershed (after World river resources, 2003) 12
Figure 10 Population density distribution over the Congo River Watershed: Basin area 3,730,881 sq.Km, Average Population Density (people per sq.km): 15, Number of large cities (100,000

people). 12

Figure 11 Hydrological Model Classification 14

Figure 12 DEM Processing flow chart: Extraction of Drainage network, Catchment and Horton

Parameters. 24

Figure 13 Areal distribution at different altitude (The area in a logarithmic scale) 25

Figure 14 DEM visualization map for Cental Africa. The defined colored polygone delineated the Congo River basin. 26
Figure 15 D-8 algorithm: Based on the output Flow direction map, the Flow accumulation operation counts the total number of pixels that will drain into outlets (after ILWIS 3.4 Manual)

27

Figure 16 Flow direction map 27

Figure 17 Histogram of Flow Direction for Central Africa 28

Figure 18 Flow Accumulation map; on top: Entire basin, on bottom: A selected area 29

Figure 19 Stream network map masked by the boundary of the Congo River Basin 30

Figure 20 Extracted sub-catchment map in the Congo Basin 31

Figure 21 Merged sub-watershed with stream network and majors outlet of the CRB 31

Figure 22 Longest flow path map overlayed on the sub-watersheds of the CRB 32

Figure 23 Overland flow distribution in the study area 32

Figure 24 Overland flow distribution in the Ouesso sub-watershed 33

Figure 25 Horton morphometric parameters for 4 selected sub-watersheds in the Congo River

37

Figure 26 Strahler order vs. Stream length map 39

Figure 27 General terrestrial Water Balance model structure 42

Figure 28 Rainfall-Runoff simulation model for a single grid cell 42

Figure 29 Functional relationship between soil moisture and Evapotranspiration (ETa is the actual Evapotranspiration, ETp is the potential Evapotranspiration, SM is the soil moisture, FC is the field capacity and WP, the Wilting point 47
Figure 30: Distribution of Clima tic stations in the study area. The study area covers more than

145 stations 49
Figure 31 Rainfall averaged (1961-190) data from 145 stations. 1. Rainfall, 2. Effective Rainfall 49

Figure 32 Mean Annual Potential Evapotranspiration (1961-1990) map 50
Figure 33 Available soil water vs. soil texture showing estimates of field capacity, permanent wilting point and Available water content. S-Sand, SI-Silt, CL-Clay, F-Fine, VF-Very Fine, L-

Loamy (after Levy et al, online) 53

Figure 34 Hydrological Soil types over the basin 54

Figure 35 Soil Field Capacity in the root zone. 54

Figure 36 Hydrological Soil types over the basin 55

Figure 37 Soil moisture correlation with the latitude 57

Figure 38 Mean annual moisture (in mm) over the Congo basin. 58

Figure 39 Season Soil moisture (in mm per season) over the basin. 58

Figure 40 Mean Annual Actual Evapotranspiration over the Congo River basin 59

Figure 41 Season Actual Evapotranspiration over the Congo basin 60

Figure 42 Mean annual runoff over Congo basin (mm/year) 61

Figure 43 The relationship between precipitation and drigged simulated runoff in the CRB 61

Figure 44 Seasonal Runoff grid runoff maps. Top left: December-February, Top right: MarchMay, Bottom Left: June-August, Bottom right: September-November 62
Figure 46 Seasonal and Spatial distribution maps for Vertical Integrated Moisture Convergence (in mm/month) over the Congo River basin. A: December-February, B: March-May, C: JuneAugust, D: Septembre-November (Negative values correspond to the moisture convergence,

positive values correspond to the moisture divergence). 65

LIST OF TABLES

Table 1 River discharge at KINSHASA gauge (after Vorosmarty et al, 1998) 7

Table 2 Effective Rainfall distribution in the Congo Basin 9
Table 3 Land cover distribution in the Congo River basin (after World river resources, 2003).... 11

Table 4 Summarised Statistics for the DEM 26

Table 5 Summarised statistics for the Flow direction grid map in the area of study. 28

Table 6 Sub-wateshed characteristics of the CRB 34

Table 7 Extra cted sub-watersheds areas of the CRB 34

Table 8 Stream numbers and Bifurcation Ratio for sub-watersheds of the Congo River 36

Table 9 Horton Morphometric Parameters for the sub-catchments in the Congo River 38

Table 10 Stream Length Ration for the different sub-catchments in the Congo River 38

Table 11 Stream Ration for the selected subwatershed 39

Table 12 Drainage density for watersheds of the Congo River 40

Table 13 Rooting depth assigned for various soil textures and SCS soil groupings 51

Table 14 Soil texture distribution in the Congo River basin 51

Table 15 Relationship linking vegetation class, soil texture, rooting depth and moisture

capacities of various soil groups in Central Africa (Source: Alemaw and Chaoka, 2003) 53

Table 16 Subwatershed runoff averages 63

LIST OF ABBREVIATIONS, ACRONYMS AND SYMBOLS

AET actual evapotranspiration

(ea-ed) Vapour pressure deficit (kpa)

AfDB African Development Bank

ASCE American Society of Civil Engineering

AWC Democratic Republic of the Congo

AWF African Water Facility

C Vertical Integrated Moisture Convergence

C.R.B Congo River Basin

C.V.I Vertical Integrated Moisture Convergence

CICOS International Commission of the Congo-Oubangi-Sangha

River Basin

CLIMWAT Cliamtic Database

Slope vapour pressure curve (kPa oc-1)

D.R.C. Democratic Republic of the Congo

DEM Digital Elevation Model

DRO Direct Runoff

dS/dt Change of storage with time

ed actual vapour pressure (Kpa)

EPPT Effective Precipitation

EROS Earth Resources Observation Systems

ET Evapotranspiration

ETo Reference crop evapotranspiration

ETp Potential Evapotranspiration

ETR Evaptranspiration ratio

ETref Reference Evapotranspiration

FAO Food Agricultural Organization

FC Field Capacity

G soil heat flux (MJ m-2 d-1)

GCM General Climate Model

GIS Geographic Information System

GIUH Geomorphological Instantaneous Unit Hydrograph

GW Giga Watt

HATWAB Hybrid Atmospheric and Terrestrial Water Balance

HYDROSHEDS Hydrological data and maps based on SHuttle Elevation

Derivatives at multiple Scales

ILWIS Integrated Land and Water System

Imb Imbalance

IRD International Research Development

ITCZ Inter-Tropical Convergence Zone

L T-1 Length over Time

LAI Leaf Area Index

LDP Longest Drainage Path

LHSOr Lengnth of the Hisghest Strahler Order

LFP Longest flow path

mm millimeter

N Day length

n Day sunshine

n /N relative sunshine fraction

n/a Non applicable

NDVI Normalized Difference Vegetation Index

NEPAD New Partnership for Africa's Development

NOAA-AVHRR National Oceanic and Atmospheric Administration-

Advanced Very High Resolution Radiometer

npix Number of Pixels

npixcum Cumulative Nmber of Pixels

npixpct percentage of number of pixels

P Precipitation

PET Potential evapotranspiration

Q Discharge

Q Water Vapour flux

Qpg Peak discharge

R Runoff

Rn Net radiation at crop surface (MJ m-2 d-1)

Rnl net longwave radiation (MJ m-2 d-1)

Rns net short wave radiation (MJ m-2 d-1)

RO Runoff

ROF Runoff

SADC Souther Africa Development Community

SCS-CN Soil Conservation Service - Curve Numbers

SM Soil moisture

T Average temperature (oc)

Tkn Minimum temperature (K)

Tkx maximum temperature (K)

Tmonth n Mean temperature in month n ( 0C)

Tmonth n Mean temperature in month n-1 ( 0C)

Tpg Time to Peak Discharge

TRO Total Runoff

U2 Windspeed measured at 2m height (m s-1)

U2 Windspeed measured at 2m height (m/s)

UFC Unit field Capacity per unit volume of Soil

UH Unit Hydrograph

UNEP Uneted Nations Environment Programme

USGS Unit field Capacity per unit volume of Soil

UWP Unit Wilting Point per unit volume of Soil

WHC Water Holding Capacity

WP Wilting Point

? Psychometric constant (kPa oC-1)

CHAPTER ONE

1.0 INTRODUCTION OF THE STUDY

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