Transit times from rainfall to baseflow in headwater catchments estimated using tritium: the Ovens River, Australia

I Cartwright,U Morgenstern

doi:10.5194/hess-19-3771-2015

Abstract

Abstract. Headwater streams contribute a significant proportion of the total flow to many river systems, especially during summer low-flow periods. However, despite their importance, the time taken for water to travel through headwater catchments and into the streams (the transit time) is poorly understood. Here, 3H activities of stream water are used to define transit times of water contributing to streams from the upper reaches of the Ovens River in south-east Australia at varying flow conditions. 3H activities of the stream water varied from 1.63 to 2.45 TU, which are below the average 3H activity of modern local rainfall (2.85 to 2.99 TU). The highest 3H activities were recorded following higher winter flows and the lowest 3H activities were recorded at summer low-flow conditions. Variations of major ion concentrations and 3H activities with streamflow imply that different stores of water from within the catchment (e.g. from the soil or regolith) are mobilised during rainfall events rather than there being simple dilution of an older groundwater component by event water. Mean transit times calculated using an exponential-piston flow model range from 4 to 30 years and are higher at summer low-flow conditions. Mean transit times calculated using other flow models (e.g. exponential flow or dispersion) are similar. There are broad correlations between 3H activities and the percentage of rainfall exported from each catchment and between 3H activities and Na and Cl concentrations that allow first-order estimates of mean transit times in adjacent catchments or at different times in these catchments to be made. Water from the upper Ovens River has similar mean transit times to the headwater streams implying there is no significant input of old water from the alluvial gravels. The observation that the water contributing to the headwater streams in the Ovens catchment has a mean transit time of years to decades implies that these streams are buffered against rainfall variations on timescales of a few years. However, impacts of any changes to land use in these catchments may take years to decades to manifest themselves in changes to streamflow or water quality.

Highlights

Documenting the timescales over which rainfall is transmitted through catchments to streams is critical for understanding catchment hydrology and for the protection and management of river systems
It is likely that precipitation in the whole region is between 1170 and 1420 mm yr−1, which are the annual totals at Bright in the north of the catchment and the Victorian Alps to the south of the Ovens catchment
Δ18O and δ2H values of stream water define arrays with slopes of 4–6 (Table 1, Fig. 6) that most likely reflects a combination of instream evaporation, especially in February 2014, and possibly the altitude effect where stream water derived from rainfall at higher altitudes has lower δ18O and δ2H values

Summary

Introduction

Documenting the timescales over which rainfall is transmitted through catchments to streams (the transit time) is critical for understanding catchment hydrology and for the protection and management of river systems. Likewise the factors that control variations in transit times between catchments are not well documented. Especially in arid or semi-arid regions, are commonly sustained by groundwater inflows during lowflow periods (Winter, 1999; Sophocleous, 2002). Where the lower and middle reaches of rivers are developed on alluvial sediments, these sediments provide a ready source of groundwater to sustain the river during low-flow periods. River systems in limestone terrains are likewise sustained by drainage through karst systems. Headwater catchments that are developed on indurated or crystalline rocks may not be linked to well-developed groundwater sys-

Results

Discussion

Conclusion