Sources and mean transit times of intermittent streamflow in semi-arid headwater catchments

Abstract

Determining the sources and mean transit times (MTTs) of water that generates streamflows is important for understanding and managing headwater catchments. The sources and especially the MTTs of water that contributes to streamflow in seasonally intermittent streams are far less studied than for perennial streams. Here we use major ions, dissolved organic carbon, stable isotopes, radon and tritium to quantify the sources and MTTs of intermittent streamflows in three headwater catchments (Banool, McGill and Plantation) from a semi-arid area in southeast Australia. At the start of streamflows, the MTTs of stream water varied from 25 to 42 years in the 1.51 km2 Banool catchment, 3 to 4 years in the 3.38 km2 McGill catchment and 9 to 14 years in the 3.41 km2 Plantation catchment. Winter rainfall increased the relative contribution of younger waters in all three catchments. During higher winter streamflows, the MTTs of stream water reduced to ≤5 years in the Banool catchment and <1 year in the McGill and Plantation catchments. The sources of streamflow also differed between the catchments. Regional groundwater, which close to the stream has a residence time of several hundred years, dominated in the Banool catchment, whereas younger water (residence times of up to 9 years) stored in the riparian zone was the main source in the McGill and Plantation catchments. The differences in MTTs between the catchments may reflect land-use differences, especially the presence of plantation forests in the McGill and Plantation catchments. Overall, due to being less well-connected to the regional groundwater, the MTTs of these intermittent streams are far shorter than those reported for perennial headwater streams in southeast Australia. The short MTTs indicate that these intermittent streams are vulnerable to short-term variations in rainfall.