The long-term impact of Acacia mearnsii trees on evaporation, streamflow and groundwater resources TT 505/11 The long-term impact long-term of The TT 505/11 AD Clulow, CS Everson & MB Gush Acacia mearnsii Acacia trees on evaporation, streamflow and groundwater resources and groundwater streamflow on evaporation, trees TT 505/11 THE LONG-TERM IMPACT OF ACACIA MEARNSII TREES ON EVAPORATION, STREAMFLOW AND GROUNDWATER RESOURCES Report to the WATER RESEARCH COMMISSION by A.D. Clulow1, C.S. Everson1, M.B. Gush2 1UKZN, School of Bioresources Engineering and Environmental Hydrology, Pietermaritzburg 2CSIR Natural Resources and the Environment, Pietermaritzburg WRC Report No. TT 505/11 November 2011 i Obtainable from Water Research Commission Private Bag X03 Gezina, 0031 [email protected] The publication of this report emanates from a project titled The long-term impact of Acacia mearnsii trees on evaporation, streamflow and groundwater resources (WRC Project No. K5/1682). DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. ISBN 978-1-4312-0020-3 Printed in the Republic of South Africa ii ACKNOWLEDGEMENTS The research in this project was funded by the Water Research Commission, for whose assistance we are sincerely grateful. The Department of Water Affairs and Forestry are thanked for funding and assistance with drilling boreholes at the site. Mondi are thanked for allowing access to their catchment and for contributions in kind (labour, management operations and data). We also wish to acknowledge contributions made by members of the steering committee: Dr R Dube Water Research Commission (Chairman) Prof J Annandale University of Pretoria Dr M Moodley AquaGreen Consulting Mr J Bosch CSIR, Natural Resources and the Environment Prof P Roberts Forest Industries Association Mr P Gardiner Mondi Forests Mr JM Nel University of the Western Cape Dr A van der Mescht Mangosuthu Technikon Mr AG October University of the Western Cape Prof MJ Savage University of KwaZulu-Natal The following people from the Council of Scientific and Industrial Research (CSIR) provided valuable technical assistance to the trial: Joshua Xaba, Lucas Ngidi, Vivek Naiken and Lelethu Sinuka. Franscois Fourie performed the soil resistivity survey. Bioresources Engineering and Environmental Hydrology at the University of KwaZulu-Natal is acknowledged for their support through assistance from post-graduate students. iii iv TABLE OF CONTENTS ACKNOWLEDGEMENTS .............................................................................. iii TABLE OF CONTENTS .................................................................................. v LIST OF FIGURES ....................................................................................... viii LIST OF TABLES .......................................................................................... xii LIST OF ABBREVIATIONS AND SYMBOLS .............................................. xiii EXECUTIVE SUMMARY ............................................................................... xv 1. INTRODUCTION ................................................................................... 1 1.1 Background ................................................................................ 1 1.2 Experimental Approach .............................................................. 2 1.3 Project Objectives ....................................................................... 3 1.4 History of Research and Funding at Two Streams ..................... 3 2. THE STUDY AREA ............................................................................... 5 2.1 Study Site ................................................................................... 6 3. MATERIALS AND METHODS .............................................................. 8 3.1 Tree Growth ............................................................................... 8 3.2 Rainfall ....................................................................................... 9 3.3 General Climatic Variables ......................................................... 9 3.4 Evaporation Measurement Using Scintillometry ....................... 13 3.5 Computing Total Evaporation ................................................... 19 3.6 Streamflow ............................................................................... 21 3.7 Electrical Resistivity Tomography Study ................................... 22 3.7.1 General description of resistivity method .................... 22 3.7.2 Resistivity data collection and processing ................... 24 3.8 Groundwater Monitoring ........................................................... 25 3.9 Soil Water Dynamics ................................................................ 26 v 3.9.1 Time domain reflectometry .......................................... 26 3.9.2 Probe design ............................................................... 26 3.9.3 Sensor installation ....................................................... 27 3.9.4 Soil physical properties ............................................... 29 3.9.5 Soil physical properties ............................................... 31 3.9.6 Soil survey .................................................................. 31 3.10 Water Balance Modelling .......................................................... 31 3.11 Hydrological Modelling ............................................................. 32 3.11.1 Description of the WAVES model ............................... 32 3.11.1.1 Recent modifications to WAVES ................... 37 3.11.1.2 Suitability for use at Two Streams ................. 37 3.11.2 Description of the ACRU model .................................. 38 3.11.2.1 Suitability for use at Two Streams ................. 40 3.11.3 WAVES catchment configuration ................................ 40 3.11.3.1 Climatic data .................................................. 40 3.11.3.2 Soils information ............................................ 41 3.11.3.3 Land-cover and model setup ......................... 41 3.11.4 ACRU catchment configuration ................................... 41 3.11.4.1 Rainfall and streamflow data ......................... 43 3.11.4.2 Additional climatic data .................................. 44 3.11.4.3 Soils information ............................................ 45 3.11.4.4 Land-cover and model setup ......................... 45 4. RESULTS AND DISCUSSION ........................................................... 46 4.1 Tree Growth ............................................................................. 46 4.1.1 Tree height .................................................................. 46 4.1.2 Leaf area index (LAI) .................................................. 48 4.1.3 Roots .......................................................................... 51 4.2 Rainfall Monitoring .................................................................... 52 4.3 Water-use of Black Wattle ........................................................ 54 4.3.1 Large aperture scintillometer estimates of total evaporation ................................................................. 54 4.3.2 Priestley-Taylor verification ......................................... 61 vi 4.3.3 Reference evaporation and crop factor for Black Wattle .................................................................................... 63 4.4 Streamflow Gauging ................................................................. 67 4.5 Electrical Resistivity Tomography Survey ................................. 70 4.6 Groundwater ............................... Error! Bookmark not defined. 4.7 Soil Water ................................................................................. 73 4.8 Monthly Water Balance ............................................................ 77 4.9 Hydrological Modelling ............................................................. 80 4.9.1 Soil water modelling .................................................... 80 4.9.2 Streamflow modelling .................................................. 84 5. FINAL DISCUSSION AND CONCLUSION ......................................... 86 6. REFERENCES .................................................................................... 91 Appendix A. The two weekly intervals used to calculate effective height of the LAS beam above the canopy. ................................... 99 Appendix B. Borehole logs. ..................................................................... 102 vii LIST OF FIGURES Figure 2.1. Location of the Seven Oaks district in the KwaZulu-Natal midlands. .................................................................................... 5 Figure 2.2. Location of instruments in the Two Streams catchment. The large aperture scintillometer (LAS) transect is shown with the transmitter on the west and receiver on the east of the catchment. The automatic weather station (AWS) site included the area where the soil water measurements took place. The five borehole sites were labelled according to their position in the plantation. ............................................................................. 7 Figure 3.1. Original automatic weather station at Two Streams. ................ 11 Figure 3.2. The automatic weather station