The relationships of forest and watershed characteristics to soil water retention, storm runoff, erosion, and wave attenuation in Vietnam

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Abstract (summary)

Forests can have a profound impact on the hydrological cycles. Numerous studies in Vietnam, and elsewhere have examined the effects of vegetation cover and geomorphology on hydrological processes at both watershed and regional scales, but the effects of forests in water yield, regulating seasonal water flows, and soil erosion are still in debate. This dissertation focuses on obtaining a deeper understanding about how forests, weather and geomorphology affect hydrological responses and soil erosion in Vietnam.

Dissertation is a collection of four independent studies. The first study characterizes soil water retention of four forest types representing different levels of forest degradation. The results suggest that soil water retention, a function of soil moisture, bulk density, and soil depth; varies among forests, and it depends primarily on litter cover, vegetation cover, and porosity. Forest soil moisture can be predicted by a regression model, with the root square mean error of 3%.

The second study investigates effects of watershed characteristics on runoff in 15 typical watersheds. The watershed factors, which include watershed size, shape, slope and elevation difference, forest cover and distribution, are analyzed in relation to increasing and decreasing peak flow, and daily streamflow variation, in which forest cover and distribution, shape, and elevation difference are found to be significant impacts on storm runoff. Relationships between peak discharge and initial flow and rainfall are statistically significant in this study.

The third study is to define minimum forest areas for protection soils from erosion. A soil loss prediction equation and soil loss tolerance of 10 ton ha-1 y-1 are used to generate an erosion risk map and vegetation index for Vietnam. Required forest areas are calculated by comparison erosion risk with vegetation index.

Finally, wave attenuation is analyzed in relation to initial wave height, cross-shore distances, and mangrove forest structures. From these relationships, minimum mangrove band width for coastal protection from waves is defined and ranges from 40 m to 240 m depending on mangrove structures.

Indexing (details)

Subject
Hydrologic sciences;
Forestry
Classification
0388: Hydrologic sciences
0478: Forestry
Identifier / keyword
Earth sciences, Biological sciences, Soil moisture, Forest hydrology, Watersheds, Wave attenuation, Erosion, Vietnam, Erosion risk map
Title
The relationships of forest and watershed characteristics to soil water retention, storm runoff, erosion, and wave attenuation in Vietnam
Author
Tran, Bao Quang

Acknowledgments
This dissertation would not have been possible without the technical and
emotional support of many people. I would like to first thank my advisor Dr. Melinda
Laituri for her unbelievable thoughtfulness, her invaluable guidance, and her
encouragement during my study and research.
My sincerest gratitude and appreciation is extended to my committee members,
Dr. Dan Binkley, Dr. Paul Doherty, and Dr. Eileen Helmer for their helpful guidance,
comments, suggestions, and friendship throughout the program. I also would like to
thank Dr. Phillip Chapman and Dr. Jerry Vaske for their statistical consultations for
my research, Robert Sturtevant for his helpful and value editing of my writing. It has
been a great honor to work with all of them and other faculty, staff, and students at
Colorado State University as well.
I would like to thank Dr. Vuong Van Quynh, my mentor at Forestry University
of Vietnam, who has contributed so much to this work and my development as a
scientist since I was an undergraduate student. I am deeply indebted to him for
providing me with his experience and knowledge to Giúp to finish this dissertation.
I wish to express my appreciation to the Ministry of Education and Training of
Vietnam (MOET) for their financial support for my study at Colorado State
University. I gratefully acknowledge the funding for my research provided by the
Ministry of Agriculture and Rural Development of Vietnam (MARD). My
dissertation would not have been possible without these crucial funding sources.
I would also like to extend thanks for the field and lab assistance of my
colleagues at the Institute of Forest Ecology and Environment. Many thanks go to
Vietnam International Student Association for the friendship and activities during my
stay at CSU.
Finally, It is impossible to describe how deeply indebted I am to my family for
their love and encouragement, especially my parents for inspiring me to excel
intellectually, my beloved wife Minh Nguyet, and my cheerful son and daughter. I
never would have accomplished this research without their invaluable supports.
Table of Contents
Abstract of Dissertation Hi
Acknowledgments v
List of Figures ix
List of Tables xi
CHAPTER 1 . GENERAL INTRODUCTION 1
1.1. Research Objectives 2
1.2. Dissertation Structure 2
1.3. Potential Contributions of the Vietnam study 4
REFFERENCES 4
CHAPTER 2. THE EFFECTS OF FOREST DEGRADATION ON SOIL
WATER RETENTION IN NOTHERN VIETNAM 6
Abstract 6
2.1 . INTRODUCTION 7
2.2. METHODOLOGY 1 0
2.2.1. Study Sites 10
2.2.2. Data Collection 12
2.2.3. Data Analysis 13
2.3. RESULTS AND ANALYSIS 14
2.3.1. Forests and Structures 14
2.3.1.1. Forest Distribution 14
2.3.1.2. Forest Structures 16
2.3.2. Forest Soil Properties 17
2.3.2.1. Forest Soil Moisture 17
2.3.2.2. Relationship between Topsoil Moisture and Rainfall over time 18
2.3.2.3. Forest Soil Porosity 19
2.3.3. Effects of Environmental Factors on Forest Soil Moisture 20
2.3.4. Soil Moisture Prediction Models 22
2.3.4.1. Prediction Models for rainy days (1) 22
2.3.4.2. Prediction Models for no rainy days (2) 23
2.3.4.3. Model Validation 24
2.3.4.4. Forest Soil Water Retention , 25
2.3.5. Monthly Threshold of Rainfall 27
2.4. DISCUSSION 28
2.5. CONCLUSIONS 30
LITERATURE CITED 32
CHAPTER 3. THE EFFECTS OF WATERSHED CHARACTERISTICS 34
ON STORM RUNOFF RELATIONSHIPS IN VIETNAM 34
Abstract 34
3.1. INTRODUCTION 35
3.2. METHODOLOGY 37
3.2.1. Study Sites 37
3.2.2. Sources of Data 39
3.2.3. Watershed Delineation 39
3.2.4. Data Preparation and Analysis 40
3.3. RESULTS AND ANALYSIS 43
3.3.1. Watershed Characteristics 43
3.3.2. Runoff Characteristics 46
3.3.3. Effects of Watershed's Factors on Runoff Characteristics 48
3.3.3.1. Effects of Forest Cover 48
3.3.3.2. Effects of Forest Distribution 49
3.3.3.3. Effects of Watershed Sizes 50
3.3.3.4. Effects of Watershed Shapes 51
3.3.3.5. Effects of Watershed Slope 52
3.3.3.6. Effects of Watershed Elevation Differences 53
3.3.3.7. Multivariate Analysis Effects of Watershed's Factors on Runoff 54
3.3.4. Effects of Rainfall on Peak flow 56
3.4. DISCUSSION 59
3.5. CONCLUSIONS 62
LITERATURE CITED 63
CHAPTER 4. DEFINING AREAS UNSUITABLE FOR AGRICULTURE IN
VIETNAM WITH A GIS-BASED MODEL OF SOIL EROSION 66
Abstract 66
4.1. INTRODUCTION 66
4.2. METHODOLOGY 69
4.2.1. Study Sites and Data Sources 69
4.2.2. Criteria for Defining Required Forest Area 69
4.2.3. Spatial Analysis to Define Areas Requiring Forest Cover 71
4.2.4. Rainfall Interpolation 73
4.3. RESULTS AND ANALYSIS 74
4.3.1. Rainfall Interpolation and Rainfall Erosivity Factor 74
4.3.2. Erosion Risk and Areas Requiring Forest Cover 76
4.4. DISSCUSSION 77
4.5. CONCLUSIONS 79
LITTERATURE CITED 80
CHAPTER 5. THE RELATIONSHIP OF MANGROVE STRUCTURES 82
TO WAVE ATTENUATION IN COASTAL VIETNAM 82
Abstract 82
5.1. INTRODUCTION 82
5.2. METHODOLOGY 84
5.2.1. Study Sites 84
5.2.2. Data Collection 86
5.3. RESULTS AND DISCUSSION 86
5.3.1. Effects of Mangrove Structures on Wave Height 86
5.3.2. Minimum Mangrove Band Width for Coastal Protection from Waves 90
5.3.2.1. Defining Mangrove Band Width 90
5.3.2.2. Minimum Mangrove Band Width for Coastal Vietnam 93
5.4. CONCLUSIONS 95
LITTERATURE CITED 96
CHAPTER 6. CONCLUSIONS AND RECOMMENDATIONS 97
6.1. Summary and Conclusions 97
6.2. Recommendations for Further Research 99
APPENDICES 102

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