Isotopic and chemical tracers used to identify inputs to streamflow during a rain event in the NSW Southern Highlands

Scarff, SA; Hughes, CE; Morrison, TN

Isotopic and chemical tracers used to identify inputs to streamflow during a rain event in the NSW Southern Highlands

Date

2009-12-03

Authors

Scarff, SA

Hughes, CE

Morrison, TN

Publisher

Australasian Environmental Isotope Conference

Abstract

Isotopic and chemical tracers were used to study flow pathways in a small agricultural catchment at the headwaters of Kellys Creek in the Southern Highlands of NSW, Australia. The catchment is a cattle grazing area with clay soils overlying shale and basalt. Farm dams intercept approximately 59% of the catchment. Sampling was conducted prior to and during a rain event in June 2008. Stream, rain, shallow groundwater, spring and dam waters were collected and analysed. Dissolved organic carbon, 222Rn, δ2H, δ18O and major ions were measured for use in hydrogeochemical analysis and hydrograph separations were conducted to identify the sources of stream flow during and after a rain event [1]. Hydrograph separations have been undertaken in many studies of small to medium sized, and mostly forested catchments [2; 3], the majority indicating that pre-event water supplies at least 50% of streamflow at peak discharge [2]. Much lower pre-event water contributions have been found for studies in Mediterranean agricultural catchments [4; 5]. Both two and three component hydrograph separations, using δ2H, δ18O and Cl¯, were conducted for the June rain event. The precipitation and stream discharge are shown in Fig. 1. Rainfall during this period totalled 97 mm. Following an initial wetting up period (1 in Fig. 1) streamflow generally increased quickly in response to rainfall, indicating a fast response time in the catchment. The ratio of discharge to rainfall increased throughout the period as indicated by the increase in peak discharge with respect to rainfall from 2 to 4 (Fig. 1), suggesting that surface runoff is a dominant factor in the catchments hydrology. The increase in catchment response to new rainfall as the period progresses indicates that soil and dam storage, which initially formed a buffer to store rainfall and runoff, gradually fill up and those parts of the catchment begin to contribute to discharge. Peak 4 can be explained by the filling and subsequent discharge from farm dams in the catchment. The two component hydrograph separations, using δ2H and δ18O, found the majority of stormflow was event water, sourced from precipitation runoff. The three component hydrograph separation using δ18O/δ2H found event water was the primary contributor (51.1 %) to stormflow, followed by the dam component (37.3 %), then pre-event water (11.6 %) [1]. Alternatively, the δ18O/Cl- hydrograph separation, found a lower event water contribution (31.7 %), with almost half the discharge being sourced from dams (48.1 %), followed by pre-event water (20.2 %) [1]. Both hydrograph separations show the largest contributions of event water at the peak flow (Peak 4, Fig. 1) and the largest contribution from pre-event water at Peak 2, (Fig. 1). Although the hydrograph separations differ slightly in terms of which component contributes the majority of flow, it was concluded that significant proportions of stormflow was sourced from farm dams and event waters. Pre-event water was found to have a lower stormflow contribution, but was dominant during the first flush and increased post event. Low deuterium excess was a useful tool to distinguish evaporated pre-event waters (e.g. dam or stream water) from event waters. Transit time distribution modelling is being conducted to better characterise the rainfall input function given the time-varying isotope composition of the rainfall.

Keywords

Tracer techniques, Agriculture, Rain, Streams, New South Wales, Australia, Dams, Deuterium

Citation

Scaff, S. A., Hughes, C. E., & Morrison, T. N. (2009). Isotopic and chemical tracers used to identify inputs to streamflow during a rain event in the NSW Southern Highlands. Presentation to the 10th Australasian Environmental Isotope Conference and 3rd Australasian Hydrogeology Research Conference, Resources and Chemistry Precinct, Curtin University Perth, Western Australia, 1st – 3rd December 2009, (pp. 77).