Assessing urbanization impacts on catchment transit times

Stable isotopes have potential for assessing the hydrologic impacts of urbanization, although it is unclear whether established methods of isotope modeling translate to such disturbed environments. We tested two transit time modeling approaches (using a gamma distribution and a two-parallel linear reservoir (TPLR) model) in a rapidly urbanizing catchment. Isotopic variability in precipitation was damped in streams with attenuation inversely correlated with urban cover. The models captured this reasonably well, although the TPLR better represented the integrated dual response of urban and nonurban areas with reduced uncertainty. Percent urban cover influenced the shape of the catchment transit time distribution. Total urban cover reduced the mean transit time to <10days compared with 1year and 2-3years with 63% and 13% urbanization, respectively, while it was at >4years for nonurban sites.