FACULTY OF AGRICULTURAL SCIENCES Institute of Plant Production and Agroecology in the ...

207 Pages·2016·4.4 MB·English

by  Chris Steele

FACULTY OF AGRICULTURAL SCIENCES
Institute of Plant Production and Agroecology in the Tropics and Subtropics
University of Hohenheim
Field: Geomorphology and plant production
Prof. Dr. Georg Cadisch

Sediment, carbon and nitrogen capture in mountainous irrigated rice systems
Dissertation
Submitted in fulfillment of the requirements for the degree
“Doktor der Agrarwissenschaften”
(Dr.sc.agr. / Ph.D. in Agricultural Sciences)
to the
Faculty of Agricultural Sciences
presented by:
Johanna Irma Franciska Slaets

born in:
Herk-de-Stad, Belgium
submitted in: November 2015
This thesis was accepted as doctoral dissertation in fulfilment of the requirements for
the degree “Doktor der Agrarwissenschaften” (Dr.sc.agr. / Ph.D. in Agricultural
Sciences) by the Faculty of Agricultural Sciences at the University of Hohenheim on
January 25, 2016

Date of oral examination: February 15, 2016

Examination committee

Supervisor and Reviewer Prof. Dr. Georg Cadisch
Co-Reviewer Prof. Dr. Hans-Peter Piepho
Additional Examiner Prof. Dr. Roel Merckx
Vice-Dean and Head of Committtee Prof. Dr. Markus Rodehutscord
Acknowledgements
First off, I would like to thank my supervisor, Georg Cadisch, who gave me the
chance to work in this project and in this topic. I was very lucky to have him visit me
often in the field, during which he taught me many things – from operating a four-
wheel drive vehicle, to looking at the big picture and avoiding scientific tunnel
vision. Whenever I hear myself saying the words “Well, it depends – what’s your
hypothesis?” it will be due to his training. To my second supervisor, Hans-Peter
Piepho, from whom I have learned everything I know about statistics, who was
always willing to borrow his editorial hawk eye and who taught me how to bootstrap,
thank you ever so much, and this thesis would have looked very different without
you coming on board.
A big thank you also goes to Thomas Hilger, my direct supervisor, who helped me
solve so many logistical crises and equipment failures in the field that I cannot even
count them, and who always supported me in pursuing new ideas. To Petra
Schmitter, my predecessor, whose many ideas formed the initial seed for this thesis, I
am glad she always stayed involved, no matter where on the globe she found herself
throughout the last years.
As this thesis was part of the Uplands Program, there were a great number of people
instrumental in the logistics and local support. I would like to thank Gerhard
Clemens, Pepijn Schreinemachers and Holger Fröhlich, the project coordinators in
Vietnam, Thailand and Germany, who made life so much easier for all of us PhD
students in the project. Mrs. Hong, Julia Rietze and Birgit Fiedler were very helpful
in navigating the administrative mill that was involved. Peter Elstner assisted me
with the GPS data. Thank you also to all project colleagues, German, Thai and
Vietnamese, without whom the field experience would have been much more lonely
and much less gratifying.
To my wonderful, splendid, spectacular field team, Do Thi Hoan and Nguyen Duy
Nhiem, without whom the dataset would have never existed: thank you for braving
torrential rains, tropical storms, and dirt roads on motorbikes to collect these
samples. You were the best field team I could wish for and it was my privilege to
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work with you. The people of Chieng Khoi commune, especially Mr. Ngoc, Lam, An
and Keo who assisted in the data collection, and lake manager Nguyen Xuan Truong,
thank you very much for making the experience about real people and putting a face
on the “local stakeholders” from literature. Cam on nha, and I hope to visit you all in
Vietnam again one day.
Special thanks also to the lab team at the Central Water and Soil Lab of Vietnam
National University of Agriculture, Dang Thi Thanh Hue and Phan Linh under
supervision of Nguyen Huu Thanh. The field work also wouldn’t have been possible
without the logistic and scientific support of Tran Duc Vien at the Center for
Agricultural Research and Ecological studies of Vietnam National University of
Agriculture.
To Gaby Kircher who was my main German conversation partner and the secret driving
force behind Institute 380a, thank you for everything. And to all my colleagues at 380a,
thank you for years of positive atmosphere, international food, entertaining Christmas
parties and barbecues in summer, and I hope we meet each other again, someday,
somewhere. It’s a small world! To all friends, whether still in Hohenheim, or now
departed, who were there to celebrate the good parts with me and commiserate through
the less good: Scott, Anna and Michelle, Victor, Susanne and Toa, Martina, Angela
Bernal (my fitness-inspiration), the Mensa-gang, old and new (Daniela, Christian, Judith,
Rong, Lucia), thank you all for your support and friendship.
I want to thank my friends and family in Belgium, who managed to keep up with me
throughout so many changes and chaos and sometimes non-responsiveness, your support
has meant so much to me. To my sister, my mother and father, thank you for always
being there for me, whether in person or Skype, through many time zones. Jullie
relativeringsvermogen is onmisbaar en ik denk dat ik er zonder jullie steun niet aan
begonnen was! To Juan Carlos, thank you for everything, but most of all thank you for
simultaneously fuelling my love for science and yet reminding me that when push comes
to shove, life is more important than research, always.
And finally I thank you, unknown reader, for showing interest in my work.

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Table of Contents
Chapter 1 General introduction ............................................................................... 1
1.1 Overview ......................................................................................................... 1
1.2 A history of plant production and land use change in montane Southeast
Asia ................................................................................................................. 2
1.2.1 From traditional swidden agriculture to intensive upland
cropping ............................................................................................. 2
1.2.2 Rice cultivation in changing mountainous landscapes ...................... 5
1.3 The role of sediment in the indigenous nutrient supply of rice ...................... 6
1.4 Effects of sediment re-allocation beyond the catchment ................................ 8
1.5 Methodological knowledge gaps .................................................................... 8
1.5.1 Spatial and temporal measurement choices for sediment and
nutrient loads ..................................................................................... 8
1.5.2 Monitoring constituent concentrations in irrigated watersheds ....... 10
1.5.3 Uncertainty of constituent load estimates ........................................ 10
1.6 Objectives and hypotheses ............................................................................ 11
1.7 Outline of the thesis ...................................................................................... 12
Chapter 2 A turbidity-based method to continuously monitor sediment,
carbon and nitrogen flows in mountainous watersheds .................... 15
2.1 Abstract ......................................................................................................... 15
2.2 Introduction ................................................................................................... 16
2.3 Material and Methods ................................................................................... 20
2.3.1 Exploratory laboratory test: effect of texture and organic matter
on turbidity signal ............................................................................ 20
2.3.2 Study site .......................................................................................... 22
2.3.3 Field monitoring............................................................................... 24
2.3.4 Water sample analysis...................................................................... 26
2.3.5 Model selection ................................................................................ 27
2.4 Results ........................................................................................................... 29
2.4.1 Effect of organic matter and texture on turbidity signal .................. 29
2.4.2 Spatial and temporal variation of hydrological and water quality
characteristics .................................................................................. 30
2.4.3 A linear mixed model to predict SSC, POC and PN........................ 32
2.5 Discussion ..................................................................................................... 39
2.5.1 Linear mixed model more adequate than regression ....................... 39
2.5.2 Turbidity as a direct proxy for catchment nutrient flows ................ 40
2.6 Conclusions ................................................................................................... 43
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Chapter 3 Quantifying uncertainty on sediment loads using bootstrap
confidence intervals ............................................................................... 45
3.1 Abstract ......................................................................................................... 45
3.2 Introduction ................................................................................................... 46
3.3 Material and methods .................................................................................... 50
3.3.1 Discharge and sediment concentration ............................................ 50
3.3.2 Bootstrap resampling procedure ...................................................... 52
3.3.3 Data transformations ........................................................................ 59
3.3.4 Alternative option to simulate errors ............................................... 59
3.3.5 Bootstrap confidence intervals ......................................................... 60
3.3.6 Identifying hydrological drivers of uncertainty ............................... 61
3.4 Results ........................................................................................................... 62
3.4.1 Rating curves and load estimates ..................................................... 62
3.4.2 Width of confidence intervals for sediment loads ........................... 68
3.4.3 Hydrological drivers of uncertainty ................................................. 69
3.5 Discussion ..................................................................................................... 71
3.5.1 Load estimates, data transformations and bias ................................. 71
3.5.2 Confidence interval width and model selection ............................... 73
3.5.3 Bootstrapping discharge and error propagation ............................... 75
3.6 Conclusions ................................................................................................... 75
Chapter 4 Sediment trap efficiency of paddy fields at the watershed scale
in a mountainous catchment in Northwest Vietnam .......................... 79
4.1 Abstract ......................................................................................................... 79
4.2 Introduction ................................................................................................... 80
4.3 Material and Methods ................................................................................... 83
4.3.1 Study site .......................................................................................... 83
4.3.2 Hydrological monitoring .................................................................. 84
4.3.3 Sediment concentration predictions ................................................. 86
4.3.4 Separating sediment sources ............................................................ 86
4.3.5 Sediment load estimates ................................................................... 88
4.3.6 Sediment texture with mid-infrared spectroscopy ........................... 90
4.4 Results ........................................................................................................... 91
4.4.1 Hydrological processes driving sediment flows .............................. 91
4.4.2 Seasonal sediment load trends in the irrigation system ................... 95
4.4.3 Sediment budget for paddy fields .................................................... 97
4.4.4 Watershed sediment yield .............................................................. 100
4.5 Discussion ................................................................................................... 100
4.5.1 Upland sediment contribution to the irrigation system .................. 100
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4.5.2 Sediment trap efficiency of paddy fields ....................................... 101
4.5.3 Buffer capacity of the reservoir ..................................................... 103
4.6 Conclusions ................................................................................................. 104
Chapter 5 Sediment-associated organic carbon and nitrogen inputs from
erosion and irrigation to rice fields in a mountainous watershed
in Northwest Vietnam ......................................................................... 107
5.1 Abstract ....................................................................................................... 107
5.2 Introduction ................................................................................................. 108
5.3 Material and methods .................................................................................. 111
5.3.1 Study site ........................................................................................ 111
5.3.2 Measuring sediment-associated OC and N fluxes at the
watershed scale .............................................................................. 113
5.3.3 Predicting sediment-associated OC and N loads ........................... 115
5.4 Results ......................................................................................................... 119
5.4.1 Sediment-associated paddy OC and N inputs from irrigation ....... 119
5.4.2 Sediment-associated OC and N paddy inputs from erosion .......... 123
5.4.3 Plant-available nitrogen contributions from irrigation and runoff . 126
5.4.4 Organic carbon and nitrogen budgets for irrigated lowland rice ... 127
5.4.5 Relocation of organic carbon and nitrogen to neighboring
catchments ..................................................................................... 128
5.5 Discussion ................................................................................................... 129
5.5.1 Reservoir influence on upland-lowland nutrient re-allocation in
irrigated watersheds ....................................................................... 129
5.5.2 Irrigation and overland flow in the overall rice organic carbon
and nitrogen budget ....................................................................... 131
5.5.3 Nutrient re-allocation beyond the watershed scale ........................ 132
5.5.4 Implications of uncertainty for nutrient balance ............................ 133
5.6 Conclusions ................................................................................................. 134
Chapter 6 General discussion ............................................................................... 137
6.1 Overview ..................................................................................................... 137
6.2 Methodology transfer and scaling issues .................................................... 137
6.3 Drivers of accuracy in constituent transport monitoring for irrigated
watersheds .................................................................................................. 141
6.3.1 The role of measurement error in improving sediment
concentration predictions ............................................................... 141
6.3.2 Hydrological and climatic influences on error for better
sampling strategies ........................................................................ 144
6.3.3 Implications of uncertainty for load studies ................................... 147
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6.4 Extreme rainfall event impact on sediment re-allocation in light of
climate change ............................................................................................ 148
6.5 Sediment buffering capacity of small surface reservoirs ............................ 151
6.6 Escaping the maize trap: the future of maize-rice cropping systems in
montane Southeast Asia .............................................................................. 154
References ............................................................................................................... 157
Summary ............................................................................................................... 177
German Summary .................................................................................................. 181
Appendices .............................................................................................................. 185

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List of Tables
Table 2.1: Textural properties and organic matter content of tested sediments
in the laboratory experiment. Soils with three different dominant
grain sizes were selected from a reference library, and for each of
those, two different organic matter contents were chosen. ................... 21
Table 2.2: Descriptive statistics for storm flow and base flow samples for the
following variables: sampled event rainfall (Pav), discharge (Q),
turbidity (Turb), suspended sediment concentration (SSC),
particulate organic carbon (POC) and particulate N (PN). The
total number of events sampled was 30 in 2010 and 50 in 2011,
resulting in 867 storm flow grab samples. Additionally 376 base
flow samples were collected. (Pos=Position, U=Upper gauge,
L=Lower gauge, NA=Not Applicable). ................................................ 31
Table 2.3: Model fit shown by AIC and Pearson’s correlation coefficient (r)
for Suspended Sediment Concentration (SSC), Particulate
Organic Carbon (POC) and Particulate Nitrogen (PN) for the
tested models, starting from simple linear regression with
turbidity (no transformation applied or autocorrelation fitted) and
stepwise showing the effect of adding a data transformation,
accounting for serial correlation, and adding predictor variables
(n= 756 for SSC, 730 for POC and 478 for PN). .................................. 33
Table 2.4: Standardized regression coefficients ± standard errors for fixed
effects of selected linear mixed model for Suspended Sediment
Concentration (SSC), Particulate Organic Carbon (POC) and
Particulate Nitrogen (PN) for each of the four measurement
locations: the upper and lower gauge of the river and the
irrigation channel. * and ** indicate that the coefficient is
significant at the level α=0.05 and α=0.01, respectively.
(Turb=Turbidity (NTU), Q=Discharge (m s-1), Pcum=
Cumulative rainfall (mm), Pdur=Time elapsed since start of the
event (min), NA=Not Applicable) ........................................................ 37
Table 3.1: Annual sediment load estimates (in Mg per year) for the two
years of the study directly estimated without bootstrapping, and
load estimates with 95% confidence interval limits and interval
widths (difference between upper and lower limit) for the three
different bootstrap methods: the full method shown in Figure 3.1,
the method without modelled error (i.e. leaving out Step 3 in
Figure 3.1) and the method without bootstrapping discharge (i.e.
leaving out Step 1 in Figure 3.1). (Corr=Autocorrelation, Q=
discharge, Est.=Estimate, Low.=Lower Limit, Up.=Upper Limit,
n.a. = not applicable)............................................................................. 66
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Table 3.2: Monthly sediment load estimates (in Mg per year) for the two
years of the study directly estimated without bootstrapping, and
load estimates with 95% confidence interval limits for the three
different bootstrap methods: the full method shown in Figure 3.1,
the method without modelled error (i.e. leaving out Step 3 in
Figure 3.1) and the method without bootstrapping discharge (i.e.
leaving out Step 1 in Figure 3.1). In January 2011, discharge was
zero; therefore no sediment load was transported during this
month. ................................................................................................... 67
Table 4.1: Number of observations (n), coefficient of determination (R2) and
method used for stage-discharge relationship (Q); and number of
observations and Pearson’s correlation coefficient (r2) after five-
fold cross-validation for suspended sediment concentration
predictions (SSC). Details on the linear mixed model
development can be found in Slaets et al. (2014). ................................ 91
Table 4.2: Minimum, average and maximum sediment particle size
distribution measured in the water samples collected at the
different measurement locations for the different components of
the paddy area sediment balance ........................................................... 93
Table 4.3: Sediment inputs from irrigation water and overland flow from the
37 ha upland area in the sub-watershed, and sediment export and
trapping by the 13 ha paddy area (Figure 4.1 and Appendix B).
Loads are estimated as the median of the bootstrap estimates
(Med) and therefore do not always sum up exactly within
columns, and 95% confidence intervals are shown (LL=lower
limit, UL=upper limit) in Mg per year (Mg a-1). ................................... 96
Table 4.4: Texture-specific sediment inputs from irrigation water and
overland flow from the 37 ha upland area in the sub-watershed,
and texture-specific sediment export and trapping by the 13 ha
paddy area (Figure 4.1 and Appendix B). Percentages express
proportions compared to total inputs (100%) of that textural class.
(nd = not determined) ............................................................................ 98
Table 5.1: Model fit for the nutrient concentration predictions. Shown are
number of observations (n) and squared Pearson’s correlation
coefficient (r2) after five-fold cross-validation for sediment-
associated Organic Carbon (OC), sediment-associated Nitrogen
(N) and Total Nitrogen (TN) concentration predictions for the
different measurement locations in Figure 5.1.................................... 117
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