Soil moisture dynamics in the tropical montane forest of northern thailand

SOIL MOISTURE DYNAMICS IN THE TROPICAL MONTANE FOREST OF NORTHERN THAILAND QUEK SEE LENG (B Soc Sci (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SOCIAL SCIENCES (GEOGRAPHY) DEPARTMENT OF GEOGRAPHY NATIONAL UNIVERSITY OF SINGAPORE 2009/2010 ABSTRACT This thesis examines the soil moisture states of the tropical montane mainland forest of the Mae Sa catchment in Chiang Mai, Northern Thailand The objectives of this thesis are three-fold First, this thesis describes the temporal trends of soil moisture states in the upper decimeters of three forest types They are the dry dipterocarp forest, the mixed evergreen forest and the pine forest Second, it investigates the hypothesis that soil moisture variability changes in relation to the dominant moisture states, that is, the wet and dry seasons Emphasis is given to examining moisture variability between seasonal changes Finally, soil moisture between two land use/cover types, the aforementioned forest types and a rubber plantation, are compared and described Fieldwork was undertaken between January and October of 2009 in the three common forest types of the Mae Sa catchment The temporal trends of the soils moisture states in the upper decimeters are described and the findings suggest that similar patterns exist across all three forest types Results indicate that soil moisture is more variable during the latter half of the year, that is, the months spanning the wet season and the drying down inter-season thereafter This finding of the forest is compared with that of a juvenile rubber plantation and found to be consistent although moisture levels were lower at the rubber plantation The combinative use of high temporal frequency dataset with high spatial frequency dataset was also explored and discussed to be complementary measurements that will yield insights to better understanding of catchment hydrology Keywords: soil moisture, Mae Sa, Chiang Mai, tropical montane forest, precipitation, land use land cover i ACKNOWLEDGEMENTS My gratitude must go out to the following people who have been of invaluable help in one way or another at various stages of this academic sojourn Beginning at corridor of AS2, I would like to thank the faculty and staff of the Department of Geography at NUS  Assistant Professor Wang Yi-Chen, for investing painstaking hours in a student with rouge ideas  Ms Wong Lai Wa, Mr Lee Choon Yong, Mrs Chong Mui Gek and Mr Tow Fui, for their kind advice where administration procedures and equipment loans were concerned  Professor Henry Yeung, Professor David Higgitt and Miss Pauline Lee, for being excellent and friendly co-ordinators of Graduate Studies in the Department  Miss Chang Tzu-Yin and Mr Huan Vu Duc for their help and provision of maps used in this thesis As my research took me to Chiang Mai, there was never quite a dull moment with these people around me Even the painstaking fieldwork became senselessly enjoyable as I dug those holes, which yielded some 1200 samples  Alan D Ziegler, for demonstrating and inspiring much delight in fieldwork and for his relentless impatience that drove me to go further  Phii Ya and her family, for taking me into their home each time I was in Chiang Mai for fieldwork Her care and friendship is most appreciated  The physical geographers who were on Field Studies 2009, especially Bernice, Angel, Tim, Erin, Valerie and Lawrence for their company at Pong Khrai and for ensuring sanity during my fieldwork in June 2009 Back home, friends who know me best have always rallied around me They have often asked in various ways how my thesis was coming along, and more importantly provided me with the much needed breather from research work  My reliable and efficient proof-readers, Cutie Loong, James Ong, Ruth Wen and Serene Foo, who took time to go through this research ii  Dr and Mrs Chia Hwee-Pin, Teaching Leaders of the current BSF Farrer Park Young Adults class, for their gracious and persistent prayers  Caleb cell group, for the good cheer and encouragement that they bring every Friday night as we gather over food and fellowship They are a company that I‟m grateful for  Camp Committee and especially the Foursomes – Hilary Lim, Sherwin Lew and Rachel Yong, for being committed and faithfully efficient in putting together the BPMC Radicals 2009 „Who am I?‟ camp These young minds have led me to a busy but refreshing final months of 2009  My dear friends Cheryl Chen, Pearlyn Chen, Evangeline Hu, James Ong, Jason Leong, Ruth Wen as well as Stuart and Rita Ong, for their friendship, prayers and emails They would give time to journey with me and shower me with love Their steadfast love for God and the times we co-laboured for His Kingdom have been consistent bright spots in my time as a research student They continue to spur me on to my work well Without a doubt, writing this thesis would have been nigh possible if not for the unwavering support of my family – Mr and Mrs Quek Yew Hock, See Hong and See Yee With his expertise, Dad‟s timely and practical help put in place all the formatting of this thesis Mom was ever so understanding and encouraging when I was hard pressed for time During the arduous last lap, my siblings‟ delectable company kept me sane and made that season more enjoyable Finally, my utmost love and praise goes to the LORD, my God He is my fortress and deliverer, the Shepherd of my soul whose enduring love spans every bit of my life I have witnessed His sovereign hand in every circumstance of this twoyear academic undertaking He has indeed placed me in the cleft of a rock, strengthened and upheld me with His righteous right hand All glory and honour belongs to Him Quek See Leng February 2010 iii TABLE OF CONTENTS Abstract i Acknowledgements ii Table of Contents iv List of Figures by Page viii List of Tables by Page xi List of Plates by Page xiv Chapter INTRODUCTION 1.1 Ecohydrological Effects of Rapid Economic Development 1.2 Significance of Understanding Soil Moisture Content in Montane Mainland Southeast Asia 1.3 Hypothesis and Objectives 11 1.4 Organization of Thesis 13 Chapter LITERATURE REVIEW 14 2.1 The Role of Soil Moisture 14 2.2 Spatial and Temporal Influences 22 2.2.1 Spatial Influences 22 2.2.2 Temporal Influences 26 2.3 Perspectives in Scales 28 iv Chapter STUDY AREA AND METHODOLOGY 31 3.1 Study Sites in Chiang Mai Province, Thailand 31 3.1.1 Dry Dipterocarp Forest 34 3.1.2 Mixed Evergreen Forest 36 3.1.3 Pine Forest 38 3.1.4 Juvenile Rubber Plantation 39 3.2 Sampling Strategies 41 3.2.1 Classification of Seasons and Resultant Sampling Periods 41 3.2.2 Sampling Scheme in Grids 43 3.2.3 Soil Cores 44 3.2.4 Secondary Parameters 45 3.3 Instrumentation 46 3.3.1 Campbell Scientific CR 616 Water Content Reflectometer 47 3.3.2 Delta-T ThetaProbe Type ML2x 48 3.4 Laboratory Work 50 3.4.1 Computing Volumetric Soil Moisture Content 50 3.4.2 Particle Size 51 3.5 Data Analysis of Soil Moisture Datasets 53 3.5.1 Overview of Soil Moisture Datasets 53 3.5.2 Statistical Tools and Testing 54 v Chapter RESULTS AND FINDINGS 56 4.1 Periodical Trends from 2004 to 2009 57 4.1.1 Annual Trends 58 4.1.2 Monthly Trends and Comparison at Depths 59 4.1.3 Higher Variability in Surface Soil Moisture than at Depth 63 4.1.4 Overall Trends and Comparison 71 4.2 Understanding Temporal Variations of Soil Moisture at the Upper Decimetres 73 4.2.1 General Trends of Soil Moisture Variation 73 4.2.2 Temporal Variability at 0-5 cm 76 4.2.3 Temporal Variability at 30 cm 78 4.2.4 Temporal Variability at 100 cm 80 4.2.5 Overall Temporal Variability in the Forest 82 4.3 Potential Extrapolation of the Time Series 84 4.3.1 Degree of Spatial Representation of Sensors at 0-5 cm 85 4.3.2 Degree of Spatial Representation of Sensors at 100 cm 88 4.3.3 Overall Variability 90 4.4 Temporal Variability between Two Land Use/Cover Types: Forest and Rubber Plantation 92 4.5 Summary 97 Chapter DISCUSSION 100 5.1 Seasonality in Soil Moisture 100 5.1.1 Soil Moisture Trends in the Transitional Phases 101 5.1.2 Temporal Changes between the Preferred States of Soil Moisture 103 5.1.3 Implications and Prospects in Preferred States of Soil Moisture 107 vi 5.2 Land Use/Cover and Soil Moisture: Variability between Forests and Plantation 111 5.3 Soil Moisture Measurements: Sensors and Field Samples 114 5.3.1 Influencing Factors 114 5.3.2 Prospects in Complementary Measurements 116 Chapter SUMMARY AND CONCLUSION 119 REFERENCES 125 vii LIST OF FIGURES BY PAGE Figure 1.1 Boundaries of the Montane Mainland Southeast Asia, as delineated by shaded area, spanning Cambodia, Laos, Myanmar, Thailand, Vietnam and Yunnan Province, China (Adapted from Fox and Vogler, 2005) Figure 2.1 Soil moisture as a key variable in modulating complex dynamics of the interplay between climate, soil and moisture Level of analysis is defined by scale of interest in the interplay (Adapted from Porporato and Rodriguez-Iturbe, 2002) 18 Figure 2.2 The scale triplet (Western et al., 2002, after Blöschl and Sivapalan, 1995) 29 Figure 2.3 The effect of changing each component of the scale triplet (a) Original data, (b) the effect of increasing support, (c) the effect of increasing spacing, and (d) the effect of decreasing extent (Western et al., 2002) 30 Figure 3.1 Location of Mae Sa catchment in relation to Chiang Mai, Thailand 32 Figure 3.2 Climate stations in the Catchment, established in 2004 as part of the Mae Sa Experimental Catchment project (Adapted from Wang et al., 2010) Three study sites at the dry dipterocarp forest, mixed evergreen forest and the pine forest consist of both rain gauges and other climate monitoring systems Other rain gauges in the Mae Sa Experimental Catchment indicated by smaller black circles 34 Figure 3.3 Precipitation within the Mae Sa catchment with wet season from June to September and dry season from December to January Sampling conducted at the inflexions of different seasons and inter-seasons as indicated by the dotted lines – mid-February for the dry season, late April for the wetting up inter-season, mid-June for the wet season and late September for the drying down inter-season 42 viii Figure 3.4 Grid defining sampling points at 10m intervals, with locations of climate station as indicated by „D‟ at grid B3 for the dry dipterocarp forest, „P‟ at the grid C2 for the pine forest, „M‟ and „R‟ at the grid C1 for the mixed evergreen and the juvenile rubber plantation respectively 44 Figure 4.1 Soil moisture trends between June 2004 and September 2009 plotted using hourly averages of readings taken at 20-minute intervals by the CR 616 sensors Readings over the three forest sites were averaged Steady cycles of fluctuations most pronounced at 0-5 cm, driest in March and wettest in September 59 Figure 4.2 Soil moisture and precipitation illustrated as a monthly averages computed from hourly averages from 2004 to 2009 Soil moisture increases from mid-April onwards and peaks in September 61 Figure 4.3 Mean, minimum and maximum of the five-year average of soil moisture from 2004 to 2009 for depths of 0-5cm, 100cm and 200cm 62 Figure 4.4 Monthly averages of soil moisture of the three montane mainland forest types plotted using hourly averages of readings taken at 20minute intervals by the CR 616 sensors over 2004 to 2009 Between the three depths of 0-5 cm, 100 cm and 200 cm, seasonal changes most pronounced at surface soil moisture (0 cm) of the dry dipterocarp forest 64 Figure 4.5 Soil moisture at the dry dipterocarp forest plotted with hourly averages from 2004 to 2009 Most pronounced increment at 0-5 cm as precipitation increased while soil moisture values remained stable at 100 cm and 200 cm 66 Figure 4.6 Soil moisture at the mixed evergreen forest plotted with hourly averages from 2004 to 2009 Soils driest at 100 cm while moisture levels at 200 cm are high; almost similar to 0-5 cm Soils at 100 cm and 200 cm are driest in April, up to two months after the dry season By then, surface soil moisture had increased as expected due to the precipitation 68 Figure 4.7 Soil moisture at the pine forest plotted with hourly averages from 2004 to 2009 Surface soil moisture increased as expected due to the precipitation Moisture values at 100 cm are persistently high 70 ix Chapter Summary and Conclusion more variable in the latter half of the year between June and December, spanning the wet season and the drying down inter-season The exception was that of soil moisture at 100 cm depth which was most variable during February, the dry season The difference was explained by the slower response time to precipitation At 100 cm depth, the lag time between precipitation and a general sustained change in soil moisture values was found to be one month Hence, having accounted for the lag time, the observation of soil moisture variability being lowest at the dry season is actually a reflection of variability during the stage of the 100 cm soils drying down c As the moisture levels transited between the dry and the wet states, precipitation was observed to be the dominant control in the influence of moisture levels As the soil moisture transited out of the dry state into the initial wetting up in response to the first instances of precipitation, the relatively dried-out soils reflected incidental wetting up based on antecedent precipitation As expected, surface soils were most sensitive to precipitation, with a lag time of about four days On the other hand, at the transition out of the wet state into the initial drying, soil moisture was more responsive to the first instances of the lack of precipitation, which took place within a day d It was discussed that local factors such as soil texture and topographic activity may not have exerted significant influence until the wet state, during which, areas of depression in the hillslopes would act as points of 121 Chapter Summary and Conclusion moisture confluence It was noteworthy that the hillslopes selected in this study were of similar nature; that is, topography of the three study sites were all of smooth, gentle gradients and held as control However, given the transitional differences between the dry and the wet states, further attention is needed to investigate and to track the soil moisture change This was concluded to be especially important for the montane mainland forests because the transitional stages That is, the inter-seasons span a period of two months at each juncture, thus totalling at least one-third of the year, unlike that in the temperate region which over occurs over one month The comparison and description of the variability of soil moisture between two land use/cover types – forests and juvenile rubber plantation a Moisture levels were found to be different between the forests and the juvenile rubber plantation Moisture levels at the rubber plantation were of a much lower range than at the forests Nonetheless, the lower moisture ranges were coupled with a similar minimum-maximum range as compared to the forests The difference was about 0.2 cm3/cm3 in the dry season and 0.16 cm3/cm3 during the wet season However, soil moisture values from the two land use/cover seem to undergo similar transitions with different response times to the change in precipitation levels In 122 Chapter Summary and Conclusion accordance with the different soil type at the rubber plantation, this may hint at a stronger localized control such as soil texture b The change in land use/cover from forests to rubber plantations has been economically driven At such unprecedented rates, the understanding of the hydrological implications of growing non-native rubber in montane mainland Southeast Asia may elucidate how to better manage land cover conversion In specificity, usage of water at root zone levels of rubber is cause for a separate investigation on its own to ascertain the change in soil moisture levels over the seasons The increasing breadth of this knowledge would offer insights as to how soil moisture varies at depth across seasons, and in turn, lend valuable perspectives to the hydrological effects of interspersing rubber plantations in the midst of montane mainland Southeast Asia This thesis has documented the temporal variation of soil moisture across seasonal changes at the depths of 0-5 cm, 30 cm, 100 cm and 200 cm It has also examined the degree of soil moisture variability between seasons and inter-seasons of the southwest monsoon Contributing factors that might explain the variability at different seasonal inflexions were also discussed The practical outworkings of these findings are multifaceted First, and at the onset, the new findings can be applied as a launch pad to the further understanding of montane mainland Southeast Asia This is of particular importance in the face of the changing land covers in Thailand There has been little policy or legal framework in 123 Chapter Summary and Conclusion Thailand that allows for the preservation of such fallow-related secondary forest cover such as the mixed evergreen forest (Schmidt-Vogt, 2001) Second, in light of the rapid and economically-driven land cover change from forest to rubber plantations, this data could be useful in understanding the hydrological implications of interspersing rubber plantations in the midst of montane mainland These rubber trees are non-native to Thailand and were transplanted from South America The trees are likely to change the hydrological cycle Although soil moisture tends to increase with depth, it was found that the soil moisture decreased with depth at rubber plantations Further investigation is needed to ascertain the degree of strain that the increasing extent of rubber plantations could have on the hydrological cycle Third, since the wet and dry moisture conditions may result in spatial patterns of soil moisture being controlled by different processes, the implication is that a confluence of factors is now required to determine soil wetness 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Thailand The objectives of this thesis are three-fold... 2.3 The effect of changing each component of the scale triplet (a) Original data, (b) the effect of increasing support, (c) the effect of increasing spacing, and (d) the effect of decreasing