Transpiration in various riparian woodland species of the Okavango Delta, Botswana by Aobakwe Kathleen Lubinda A dissertation submitted to the University of Botswana, Okavango Research Institute (Maun, Botswana) in accordance with the requirements of the degree of Master of Philosophy in Natural Resource Management Supervisor: Dr M. Murray-Hudson October 2014 1 Contents Acknowledgements ...................................................................................................................................... iv LIST OF TABLES ........................................................................................................................................ v LIST OF FIGURES ..................................................................................................................................... vi Abstract ....................................................................................................................................................... vii Thesis Outline ............................................................................................................................................ viii 1 CHAPTER 1 ......................................................................................................................................... 1 1.1 GENERAL INTRODUCTION ..................................................................................................... 1 1.1.1 Wetlands ............................................................................................................................... 1 1.1.2 Hydrology of wetlands .......................................................................................................... 1 1.1.3 The Okavango Delta ............................................................................................................. 3 1.1.4 Problem statement ................................................................................................................. 8 1.1.5 Research Questions .................................................................................................................... 10 1.1.6 Methodology .............................................................................................................................. 12 1.1.7 Approach .................................................................................................................................... 13 2 CHAPTER 2 ....................................................................................................................................... 15 2.1 INTRODUCTION ...................................................................................................................... 15 2.2 QUESTIONS .................................................................................................................................... 17 2.3 MATERIALS AND METHODS ...................................................................................................... 17 2.3.1 Study site .................................................................................................................................... 17 2.3.2 Sample trees ............................................................................................................................... 20 2.3.3 Tree water use ............................................................................................................................ 22 2.3.4 Canopy cover ............................................................................................................................. 25 2.3.5 Statistical analyses ..................................................................................................................... 26 2.3.6 Up-scaling to plot-level transpiration......................................................................................... 26 2.4 RESULTS ......................................................................................................................................... 27 2.4.1 Diel variations in sap flow ......................................................................................................... 27 2.4.1 Relationship between tree cross-sectional area and sap flow .................................................... 28 2.4.2 Differences in sap flux density between species ........................................................................ 30 2.4.3 Estimating plot-level transpiration ............................................................................................. 35 2.5 DISCUSSION ................................................................................................................................... 37 i 2.5.1 Differences in sap flow between tree sizes ................................................................................ 37 2.5.2 Differences in sap flow between species ................................................................................... 41 2.5.3 Changes in sap flow with season ............................................................................................... 44 2.5.4 Estimating plot-level transpiration ............................................................................................. 47 2.6 Summary and Conclusions................................................................................................................ 49 3 CHAPTER 3 ....................................................................................................................................... 50 3.1 INTRODUCTION ............................................................................................................................ 50 3.2 QUESTIONS .................................................................................................................................... 51 3.3 MATERIALS AND METHODS ................................................................................................ 52 3.3.2 Weather variables ....................................................................................................................... 52 3.3.2 Soil moisture .............................................................................................................................. 52 3.4 RESULTS ......................................................................................................................................... 53 3.4.1 Weather variables ....................................................................................................................... 53 3.4.2 Soil moisture .............................................................................................................................. 58 3.5 DISCUSSION ................................................................................................................................... 59 3.5.1 Weather variables ....................................................................................................................... 59 3.5.2 Soil moisture .............................................................................................................................. 63 3.6 Summary and conclusions ................................................................................................................ 65 4 CHAPTER 4 ....................................................................................................................................... 67 4.1 INTRODUCTION ............................................................................................................................ 67 4.2 QUESTIONS .................................................................................................................................... 69 4.3 MATERIALS AND METHODS ...................................................................................................... 70 4.3.1 Diel groundwater fluctuations .................................................................................................... 70 4.3.2 Scaling up ET to plot level ......................................................................................................... 71 4.4 RESULTS ......................................................................................................................................... 71 4.4.1 Water table depths ...................................................................................................................... 71 4.4.2 Diel groundwater fluctuations .................................................................................................... 73 4.4.3 Comparisons between groundwater level and SFD ................................................................... 76 4.4.4 Plot transpiration ........................................................................................................................ 77 4.5 DISCUSSION ................................................................................................................................... 78 4.5.1 Water table depths ...................................................................................................................... 78 4.5.2 Diel groundwater fluctuations .................................................................................................... 79 ii 4.5.3 Comparisons between groundwater levels and sap flux density ................................................ 80 4.5.4 Plot transpiration ........................................................................................................................ 82 4.6 Summary and conclusions ................................................................................................................ 83 5 CHAPTER 5 ....................................................................................................................................... 84 5.1 INTRODUCTION ............................................................................................................................ 84 5.2 SYNTHESIS ..................................................................................................................................... 85 5.2.1 Quantification of water fluxes through the common riparian trees ........................................... 85 5.2.2 Sap flow variations in various riparian trees .............................................................................. 86 5.2.3 Environmental controls on transpiration .................................................................................... 92 5.2.4 Relationship between groundwater fluctuations and sap flow ................................................... 93 5.2.5 Estimating system-level transpiration ........................................................................................ 96 5.2 RECOMMENDATION FOR FUTURE RESEARCH .................................................................................. 99 5.3 CONCLUSION ............................................................................................................................... 100 REFERENCES .............................................................................................................................................. 101 iii Acknowledgements I would like to express my sincere gratitude to BMBF – The German Ministry for Education and Research for sponsoring this work through The Future Okavango project. My deep gratitude is also extended to my supervisor, Dr Mike Murray-Hudson for his tremendous assistance in making this work what it is, his guidance and encouragement. I would also like to offer my special thanks to three very important people who urged me on, Mrs Violet and Mr Mmoloki Lubinda and Mr General Warren Makgasa. When days were dark, when I felt overwhelmed these people were always supporting me. For that I will forever be grateful. To my colleagues, who are now more like big brothers and sisters I never had, I want to say thank you. I have learnt a lot from you and made great friends in Cornelia Gwatidzo, Kgalalelo Thito and Dimpho Matlhola. iv LIST OF TABLES Table 2-1 Sample trees and their locations ................................................................................................. 21 Table 2-2 Study periods and their corresponding seasons* ........................................................................ 25 Table 2-3 Species and their corresponding cross-sectional areas ............................................................... 28 Table 2-4 Comparison of sap flux density ± standard deviation measured in different species at different seasons ........................................................................................................................................................ 31 Table 2-5 Up-scaled transpiration in Maun, Nxaraga and Seronga during low, medium and high water levels ........................................................................................................................................................... 35 Table 3-1 Mean soil moisture content (± standard deviation) in three sites in the Okavango Delta in different seasons.......................................................................................................................................... 58 Table 4-1 Diurnal groundwater fluctuations (cm) ...................................................................................... 76 Table 4-2 Seasonal plot transpiration estimated from Maun, Nxaraga and Seronga .................................. 77 Table 5-1 Seasonal variation in SFD from different species ...................................................................... 87 v LIST OF FIGURES Figure 1-1 Schema of a "typical" island in the Okavango Delta .................................................................. 5 Figure 1-2 Conceptual island model (adapted from Bauer et al., 2006) ....................................................... 7 Figure 1-3: The Okavango Delta with the three study sites shown ............................................................ 12 Figure 2-1 Images of the study sites ........................................................................................................... 20 Figure 2-2 Images of the studied species .................................................................................................... 22 Figure 2-3 Sap flow sampling design illustration ....................................................................................... 24 Figure 2-4 Example of a diel sap flow curve from an individual of Diospyros mespiliformis showing variations in sap flow with time of day ....................................................................................................... 28 Figure 2-5 Relationship between cross-sectional area and sap flux density ............................................... 30 Figure 2-6 Sap flux densities in Group 1 species ........................................................................................ 32 Figure 2-7 Sap flux densities in Group 2 species ........................................................................................ 33 Figure 2-8 Comparison of sap flux density in an evergreen and a deciduous tree measured in three different seasons. ......................................................................................................................................... 34 Figure 2-9 Sap flux densities in different individuals of Diospyros mespiliformis .................................... 34 Figure 2-10 Canopy cover measured in different seasons in Maun (A), Nxaraga (B) and Seronga (C) .... 36 Figure 3-1 Relationship between SFD and air temperature ........................................................................ 53 Figure 3-2 Correlations between air temperature and SFD in Maun (A), Nxaraga (B) and Seronga (C) during the three seasons .............................................................................................................................. 55 Figure 3-3 Relationship between SFD from an individual of Diospyros mespiliformis and relative humidity in Maun during July-August 2012 ............................................................................................... 56 Figure 3-4 Correlations between SFD and relative humidity in Maun, Nxaraga and Seronga during the three seasons ............................................................................................................................................... 57 Figure 4-1 Static water levels in Maun, Nxaraga and Seronga during February-April 2013 ..................... 73 Figure 4-2 Examples of diel groundwater level fluctuations in Maun (A), Seronga (C) during July-August 2012 and Nxaraga (B) during November-December 2012 ......................................................................... 75 Figure 4-3 Comparisons between sap flow and groundwater level at different times of the day ............... 76 Figure 4-4 Relationship between groundwater level and SFD in Seronga and Maun ................................ 77 Figure 5-1 Total sap flow in L/yr/m2 at each study site .............................................................................. 85 Figure 5-2 Annual estimated percentage of water losses incurred in the Okavango Delta ........................ 96 vi Abstract In arid and semi-arid areas, evapotranspiration (ET) by phreatophytes is a principal groundwater sink and a significant component of wetland water budgets. Understanding and accurately quantifying ET is necessary for water resources management, especially under an uncertain climatic future. In the Okavango Delta ET consumes about 96% of the total input, mainly from islands which are fringed by riparian woodlands. ET measurements, however, have historically been difficult to make and hence ET tends to have been calculated as the remainder term from the water balance. Using the compensation heat pulse velocity method, sap flow measurements, which are widely used in ecophysiological field studies, were used in this study to estimate tree transpiration in the distal, mid and upper Delta during low, medium and high water levels. These were related to air temperature, relative humidity and soil moisture and groundwater levels. The relationship between sap flow and tree size was also explored. Transpiration for the whole Delta was estimated from sap flow measurements and ET from groundwater fluctuations. Sap flow varied between and within species in different seasons because of environmental factors and plant characteristics as seasons changed. High temperatures and low relative humidity led to increases in sap flow. Relationships between sap flow and soil moisture were unclear whilst groundwater significantly responded to sap flow, declining with increased sap flow and increasing when sap flow slowed down. ET from the whole Delta (with riparian woodland covering an area of 1.19E+09 m2) was estimated to be 4.06E+09 L/year from sap flow measurements. This is equivalent to 27% of the total annual input into the Delta. Groundwater fluctuations gave estimates of 8.47E+09 L/year (56%), almost double the sap flow estimates. Keywords: Sap flow, Compensation heat pulse velocity method, Transpiration vii Thesis Outline Background on wetlands water balance, specifically focusing on evapotranspiration (ET) from riparian phreatophytes is given in Chapter 1. The problem statement, research questions and description of the study area are also given in this chapter. In Chapter 2 sap flow variations in different common riparian species of the Okavango Delta are investigated. Variations were investigated in different seasons and from different parts of the Delta. Relationships between sap flow and air temperature, relative humidity and soil moisture are investigated in Chapter 3. In Chapter 4, sap flow is related to diel fluctuations in groundwater. The findings of the whole study are synthesized in Chapter 5. viii 1 CHAPTER 1 1.1 GENERAL INTRODUCTION 1.1.1 Wetlands Wetlands are lands which are permanently wet or filled with water creating a saturated environment, resulting in anaerobic conditions, in which only animals and plants adapted to the saturated conditions survive (Cowardin et al. 1979; Ellery and Ellery 1997). Although wetlands cover only a small proportion of the earth (about 6%) they provide a number of important ecosystem services (Cherry 2012). Among other services, wetlands provide a unique habitat for a variety of flora and fauna, recharge groundwater, support a rich biodiversity, provide food and goods for humans, act as sinks and sources for materials and improve water quality (Mitsch and Gosselink 2000; Butchart 2000; Reddy and DeLaune 2004; Cherry 2012). 1.1.2 Hydrology of wetlands The hydrology of a wetland is the main component that sets the wetland ecosystem apart from other ecosystems as it creates unique physiochemical conditions (Mitsch and Gosselink 2000). Hydrologic pathways act as transport mechanisms transporting energy and nutrients to and from wetlands. These pathways include precipitation, tides, flooding rivers, groundwater flow and surface runoff (Mitsch and Gosselink 2000). To establish response models between ecological and hydrological factors of wetlands is a relatively new field in wetland research, which can provide a scientific basis for wetland protection, assessment and management (Wang, Wang, and 1