Spatial aggregation of time-variant stream water ages in urbanizing catchments

We calibrated an integrated flow–tracer model to simulate spatially distributed isotope time series in stream water in a 7.9-km² catchment with an urban area of 13%. The model used flux tracking to estimate the time-varying age of stream water at the outlet and both urbanized (1.7 km²) and non-urban (4.5 km²) sub-catchments over a 2.5-year period. This included extended wet and dry spells where precipitation equated to >10-year return periods. Modelling indicated that stream water draining the most urbanized tributary was youngest with a mean transit time (MTT) of 171 days compared with 456 days in the non-urban tributary. For the larger catchment, the MTT was 280 days. Here, the response of urban contributing areas dominated smaller and more moderate runoff events, but rural contributions dominated during the wettest periods, giving a bi-modal distribution of water ages. Whilst the approach needs refining for sub-daily time steps, it provides a basis for projecting the effects of urbanization on stream water transit times and their spatial aggregation. This offers a novel approach for understanding the cumulative impacts of urbanization on stream water quantity and quality, which can contribute to more sustainable management.