Effect of antecedent soil moisture on infiltration and preferential flow in texture contrast soils By Marcus Hardie. BSc.(Hons), MSc.(App) Submitted in fulfilment of the requirements for the Degree of Doctor of philosophy School of Agricultural Science The University of Tasmania (September 2011) Statement of originality and copyright This thesis does not contain material which has been accepted for a degree or diploma by the University or any other institution, except by way of background information and duly acknowledged in the thesis, and to the best of the candidate’s knowledge and belief no material previously published or written by another person except where due acknowledgement is made in the text of the thesis, nor does the thesis contain any material that infringes copyright. The research associated with this thesis abides by the international and Australian codes on human and animal experimentation, the guidelines by the Australian Government’s Office of the Gene Technology Regulator and the rulings of the safety, Ethics and Institutional Biosafety Committees of the University. Signed Marcus Hardie This thesis may be made available for loan. Copying of any part of this thesis is prohibited for two years from the date this statement was signed; after that time limited copying is permitted in accordance with the Copyright Act 1968. Page iii Effect of antecedent soil moisture on preferential flow in texture contrast soils Abstract Preferential flow has been shown to be both common and widespread in agricultural soils, however the processes and mechanisms responsible for preferential flow in texture contrast soils have not previously been investigated. Through a combination of dye tracer experiments, soil morphology, in situ soil moisture monitoring, infiltration studies, and soil water modelling, the effect of antecedent soil moisture on the occurrence, type and depth of preferential flow was investigated in a series of texture contrast soil profiles. Preferential flow was dominated by hydrophobicity induced finger flow in the A1 horizon and bypass flow through shrinkage cracks in the subsoil. Differences between sites in horizon thickness, chemical properties, presence / absence of an A2 horizon, and abundance of sand infills resulted in little variation in preferential flow. At low antecedent soil moisture, dye tracer infiltrated to 85 - 119 cm depth, infiltration bypassed up to 99 % of the soil matrix, and wetting front velocities were estimated up to 12 000 mm hr-1. Perched water tables and subsurface lateral flow did not develop due to the abundance of subsoil shrinkage cracks. At high antecedent soil moisture dye tracer infiltrated to 24 – 40 cm depth. While water repellence had been overcome or leached from the A1 horizon, infiltration of new water was impeded by difficulty displacing existing soil water further down the soil profile. This resulted in wetting front instability and lateral flow through the A1 horizon rather than within the A2 horizon or along the upper surface of the B horizon as reported in the literature. Occurrence of preferential flow was not related to rainfall intensity or rainfall magnitude. Rather, preferential flow was significantly more likely to occur when antecedent soil moisture was below approximately 50 - 60 % PAWC. Water repellence had a profound effect on the development of preferential flow, however the relationship between water repellence and antecedent soil moisture was not straightforward. Potential water repellence varied seasonally in relation to rainfall history in which water repellence was not re-established after rainfall unless input of new hydrophobic substances occurred. Ability to model and predict the frequency and magnitude of preferential flow was limited by poor model performance, attributed to parameter uncertainty and inability to simulate water repellence and lateral flow. Results indicate that in agricultural landscapes which contain texture contrast soils, shallow groundwater and waterways may be at risk of contamination by preferential transport of agrochemicals at low antecedent soil moisture. Page v Effect of antecedent soil moisture on preferential flow in texture contrast soils Precipitation Pasture Sand Infill Bypass flow Clay columns Water table Figure i Depiction of my thesis topic by my son Jacob Hardie aged 5 years old. Figure demonstrates preferential flow through sand infills and bypass flow to a watertable. Acknowledgements There are many people I wish to thank and acknowledge. It would not have been possible to conduct or complete my PhD without your support and input. Firstly I wish to acknowledge Rob Clarke, Shaun Lisson and the many people in DPIPWE including Kim Evens, Greg Pinkard, Peter Voller and Declan McDonald who made possible my secondment to TIAR, and generously allowed me the time and space to conduct my studies. I am indebted to Bill Cotching and Ted Lefroy for providing funding through the Landscape Logic project. I hope and trust I have delivered on your expectations. I am genuinely indebted to you guys for putting your trust in me, it would not have been possible to have conducted this work without your support. I wish to acknowledge and thank my supervisory team, Shan Lisson, Bill Cotching and Richard Doyle. Thanks heaps guys, I know I caused you guys a fair deal or stress and torment over the last few years, all that I can say is that I am grateful, you all made an amazing contribution to this thesis in so many different ways. I also wish to thank my poor long suffering family, having someone doing a PhD in a family is a strain on everyone especially my poor long suffering wife. I hope I didn’t make life too difficult for you all. Boys you will now get your father back, sorry for my absences and grumpiness over the last 4 years. I wish to thank the many people who assisted me with field work. Firstly I have to thank Kathrin Mattern who made such an amazing contribution to my PhD, you were a great friend and support, I am indebted to you and hope to see you back here in Tassie. I also wish to thank Marek Matuszek (the animal) for all that digging, sorry for dropping the shade tent on your head. Sam Rees thanks heaps mate, you were great support and really enjoyed spending time in the field with you, I hope you submit soon. Thanks also to a number of people who helped in the field or lab, Jorge De Carvalho Martins, Suresh Panta and Garth Oliver. I am deeply indebted to Brent Clothier and his team, Markus Deurer and Steve Green who allowed me come over to their island and pester them for a week about my PhD. I sincerely thank you guys for your time, patience and most of all inspiration and insights. It was great working with you, I look forward to working with you again. Thanks also for technical support and advice from Hamish Cresswell, the crucially critical Greg Holtz, and Freeman Cook. To Jane, Sally and everyone at TIAR thanks again. Page vii Effect of antecedent soil moisture on preferential flow in texture contrast soils Publications arising from this thesis Hardie, M., Cotching, W.E., Doyle, R. and Lisson, S., (in press) Influence of climate, soil water and leaching on seasonal variations in potential water repellence. Hydrological Processes. Hardie, M., Doyle, R., Cotching, W., Mattern, K., Lisson, S. (in press) Temporal variation in the hydraulic conductivity of a series of texture-contrast soils resulting from changes in antecedent soil moisture. Hydrological Processes. Hardie, M., Cotching, W., Doyle, R., Holz, G., Lisson, S. (2011) Effect of antecedent soil moisture on preferential flow in a texture contrast soil. Journal of Hydrology. 398 (3-4) 191-201. Hardie, M., Doyle, R., Cotching, W., Lisson, S. (2010) Development of preferential flow below a soil moisture threshold. World Congress of Soil Science Conference, Brisbane, Australia. Poster and short manuscript. Hardie, M., Cotching, W., Doyle, R., Lisson, S. (2010) Effect of leaching on hydrophobicity and infiltration into a texture contrast soil. World Congress of Soil Science Conference, Brisbane, Australia. Poster and short manuscript. Hardie, M., Doyle, R., Cotching, W., Lisson, S. (2009) Effect of antecedent soil moisture on preferential flow in a texture contrast soil. Current Activities in Tasmanian Soil Science Workshop, Launceston, Tasmania. Extended Abstract & Presentation. Hardie, M., Doyle, R., Cotching, W., Lisson, S. (2008) Preferential flow in a texture contrast soil. ASSI Biannual Conference, Palmerston North, New Zealand. Extended Abstract & Presentation. Page ix Effect of antecedent soil moisture on preferential flow in texture contrast soils Contents Statement of originality and copyright .............................................................................................. iii Abstract ............................................................................................................................................... v Acknowledgements ........................................................................................................................... vii Publications arising from this thesis .................................................................................................. ix List of figures ................................................................................................................................... xvii List of tables .................................................................................................................................... xxv 1.0 Introduction ..........................................................................................................................