Sourcing Contributing Areas to River Flow in Ellen Brook, Western Australia, using Environmental Indicators and Mixing Models Alice Micenko Supervisor: A/Prof. Keith Smettem Dissertation submitted in partial fulfilment of the requirements for Bachelor of Engineering (Environmental), University of Western Australia Acknowledgements i Acknowledgements There are a number of people whose input and support was essential to this project. At the top of the list is my supervisor, Associate Professor Keith Smettem. Thank you Keith for imparting a small portion of your very extensive knowledge about rivers and catchment behaviour to me. I have learnt a lot from working with you this year. I must thank Robin Smith, Wayne Tyson and Christian Zammit from the Department of Environment who provided data and maps for the project. I would also like to express my gratitude to Dianne Krikke and Elizabeth Halladin for their help with locating and understanding the laboratory equipment that I needed. To my parents, family and friends: thanks for putting up with my somewhat erratic behaviour this year, and for listening to me complain about the Ellen Brook. You have been amazing. I promise I will return to normal next year! I also strongly appreciate all the help and encouragement I received from the staff and students at the Centre for Water Research. Individual thanks go to Michael Evans, Jacinta Hewett and Ross Perrigo for their input into the project and dissertation. Finally, a special thank you and congratulations go to all the final year students who worked through this year with me. I couldn’t have done it alone. The mark of a successful man is one that has spent an entire day on the bank of a river without feeling guilty about it. – Now we can! Sourcing Contributing Areas To River Flow in Ellen Brook, Western Australia, Using Environmental Indicators and Mixing Models Abstract ii Abstract The total flow in a stream is variable in both space and time, and is derived from a number of different sources. In ungauged catchments, environmental indicators, also known as environmental tracers, can be used to separate the contribution to total flow made by each of these sources. In order for environmental indicators to be used for this purpose, they must display conservative or predictable behaviour and significant spatial variation throughout the study area. Australia has a large number of ungauged rural catchments and there is a need for a simple and inexpensive method of determining contributing areas in these catchments. The study site for this project, the Ellen Brook catchment, located 20km north-east of Perth, Western Australia, was chosen for the benefits that may be derived from a better understanding of streamflow generation processes in the catchment, and because it can easily be divided into discrete regions that display substantially different soil and water properties. The two catchment regions used for the purpose of this study are the Swan Coastal Plain area in the west and the Darling and Dandaragan Plateau area to the east. Examination of a range of water parameters, including pH, temperature, colour intensity, dissolved oxygen, phosphorous and electrical conductivity, was undertaken at 9 sites throughout the catchment with the aim of identifying those parameters most suitable for use as environmental indicators of water source and flow contribution. Electrical conductivity and colour intensity measured through spectrophotometry were found to be the most strongly applicable tracers within the Ellen Brook catchment. Significant water signature differences between the two regions of the catchment were observed, with high colour and low salinity on the Swan Coastal Plain and little or no colour but high salinity in streams emanating from the Darling and Dandaragan Plateaus. Electrical conductivity and colour levels in the main Ellen Brook channel immediately downstream from these two source areas indicated that between 60% and 80% of the flow at the catchment is contributed by the Swan Coastal Plain region, with the remainder being derived from the Darling and Dandaragan Plateaus. The proportional contribution by the Swan Coastal Plain region was found to increase towards the end of the winter season. This is thought to be due to a rapid response time and subsequent soil drying on the Plateaus. Sourcing Contributing Areas To River Flow in Ellen Brook, Western Australia, Using Environmental Indicators and Mixing Models Glossary iii Glossary Baseflow See “Old” water Conservative substance A substance that does not undergo reactions or decay in a water body, and is transported at the velocity of the water body. Contributing Areas The geographic regions of a catchment that contribute runoff to a stream. Contributing areas can vary with time (see Variable Source Area Runoff Generation). Environmental tracer Any naturally occurring substance that can be used to follow the progression of water through a catchment. Generally conservative and additive in nature. Ephemeral stream A stream that flows only after periods of significant rainfall. In south-west Western Australia, ephemeral streams flow mainly in Winter and Spring. Hortonian Runoff Generation Runoff generation that occurs when the rate of rainfall exceeds the infiltration capacity of the soil. Also known as Infiltration Excess Runoff Generation. Humus Substances derived from the decomposition of plant and animal material. Contains organic carbon. Hydrograph A graph illustrating the changes in flow at one point in a stream over time. Hydrographs can depict discharge, stream height or stream velocity. Sourcing Contributing Areas To River Flow in Ellen Brook, Western Australia, Using Environmental Indicators and Mixing Models Glossary iv Infiltration capacity The maximum rate at which precipitation water can enter a given soil under a given set of conditions. Interflow Rapidly moving subsurface water with a travel time between those of baseflow and surface flow, derived from rainfall that infiltrates the soil but does not become part of the main groundwater body. Also known as throughflow or shallow groundwater flow. Lateritic Derived from the weathering of aluminium and iron oxides and hydroxides. Lateritic soils are typically red, yellow or brown in colour due to the presence of iron. ‘New’ Water Water that is added to a catchment during a precipitation event. ‘Old’ Water Water that exists in a catchment prior to a precipitation event. The term ‘old’ water usually refers to that part of streamflow that is derived from groundwater. Also known as pre-event water or baseflow. Overland flow Runoff generated by rainfall travelling along the surface of the ground to a stream channel or other collection point. Also known as surface runoff. Palusplains Plains or flats that experience seasonal waterlogging. Runoff In a steady-state catchment, the portion of rainfall that does not undergo evapotranspiration. For the purpose of this report runoff is defined as the portion of rainfall that enters a stream through baseflow, interflow or overland flow. Saturation Excess Runoff Generation Runoff that occurs via overland flow when the soil becomes saturated so that no more rainfall can enter. Also known as the Dunne Saturation Excess Runoff Generation. Sourcing Contributing Areas To River Flow in Ellen Brook, Western Australia, Using Environmental Indicators and Mixing Models Glossary v Source area The physical area in which the water comprising streamflow originates. Spectrophotometry Technique whereby the light absorbance of a sample at different points on the electromagnetic spectrum is measured. Surface runoff See “Overland Flow” Variable Source Area Runoff Generation Theory The runoff generation theory that states that the areas of a catchment contributing to flow expand and contract as the extent of soil saturation changes. Water signature The distinct combination of physical and chemical properties that allow a particular water body or source to be identified. Sourcing Contributing Areas To River Flow in Ellen Brook, Western Australia, Using Environmental Indicators and Mixing Models Table of Contents vi Table of Contents Acknowledgements...........................................................................................i Abstract............................................................................................................ ii Glossary.......................................................................................................... iii 1 Introduction to the Project.........................................................................1 2 Literature Review......................................................................................3 2.1 Introduction to Runoff Pathways ................................................................. 3 2.2 Traditional hydrograph separation............................................................... 5 2.3 Use of Environmental Tracers for Hydrograph Separation.......................... 6 2.4 Principles of tracer hydrology ...................................................................... 7 2.5 Previous use of tracers................................................................................ 9 2.5.1 Typical characteristics of previous tracer studies ................................ 9 2.5.2 Success of previous tracer studies.................................................... 10 2.5.3 Use of tracers to identify changes in flow pathways.......................... 11 2.5.4 Use of tracers for spatial separation