Mean transit times help understand the volume of catchment water required to sustain streamflow in contrasting southeast Australian rivers

Streams may be connected to a large store of water, such as regional groundwater, and/or sustained by smaller near-river stores (such as riparian groundwater). Documenting the sources of water in streams is important for understanding catchment water balances, protecting riverine environments from pollution, and predicting the efforts of near-river pumping. Additionally, streams connected to large water stores will be buffered against the impacts of short-term climate variability (such as droughts that last a few years). Many techniques that document groundwater-stream water interaction allow the location and fluxes of baseflow to be determined but do not constrain from where in the catchment the baseflow is derived. The mean transit time (MTT) represents the time taken for water to migrate from where it is recharged in the catchment to where it discharges into the stream. Estimating the MTTs of stream water allows the volume (V) of the water store that sustains streamflow (Q) to be estimated (V=Q×MTT). This study compares the water stores sustaining streamflow in contrasting rivers in southeast Australia based on tritium MTTs calculated using lumped parameter models. Perennial streams (Oven, Yarra, Latrobe, and Gellibrand Catchments) have long MTTs (4 to 179 years) that are higher at low streamflows. By contrast, the MTTs of similar size intermittent streams (Deep Creek, Wimmera, and Gatum Catchments) range from <1 to 35 years (and are mostly less than 20 years). The estimated volumes of the catchment contributing to streamflow are 3 to 5 orders of magnitude smaller than those in comparable perennial streams. These differences reflect the limited connection between the intermittent streams and the deeper regional groundwater system compared with the perennial streams, especially at low flows. Rather, intermittent streams may be sustained mainly by smaller younger reservoirs in the riparian zone. These intermittent streams will be more susceptible to short-term climate variabilities and changes to flow regimes may have significant impacts on water supplies and the health of the riverine system. Intermittent streams are globally distributed in a range of environments, especially in semi-arid areas. Climate change and water stress have resulted in many perennial streams gradually becoming intermittent and this trend is expected to increase. In southeast Australia, around 30% of catchments have not recovered following multiple drought years between 1996 and 2010 (the Millennium Drought), and streamflow has kept declining. The increased intermittence fundamentally changes the catchment water balance, specifically making regional groundwater less important, and increases the reliance of these streams on more vulnerable small young water stores.