Assessing urbanization impacts on catchment transit times

Abstract

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 <10 days compared with ~1 year and ~2–3 years with 63% and 13% urbanization, respectively, while it was at >4 years for nonurban sites.

Hydrological transit times in nested urban and agricultural watersheds in the Greater Toronto Area, Canada

“…Estimated MTTs were 2.8 times longer for R1. Relatively short MTTs observed at R5 reflect the integration of flow contributions from both agricultural and urban areas of the watershed, despite the much larger contribution from agricultural areas, similar to the observations of Soulsby et al () in the urbanizing Burn of Bennie watershed in Scotland. Interestingly, MTTs appear to “stabilize” at R3, changing very little downstream to R5.…”

“…A similar lack of a clear, best fit model was also observed in the application of the TPLR and Gamma models to a mixed use, urbanizing watershed in Scotland (Soulsby et al, 2014). There may be two explanations for this absence.…”

“…They also present avenues for further refinement of our understanding of urban transit times, namely, how to improve estimates of MTTs and better understand factors controlling these MTTs with future urban MTT studies. First, as demonstrated by the work of Soulsby et al (, ), sampling should be conducted across at least a few years in order to account for the nonstationarity of TTDs and ensure high‐tracer recovery. The latter is particularly important in paired or nested watershed studies where the relatively undisturbed watershed may have longer MTTs.…”

“…Shorter transit times in urban watersheds have been observed in a series of nested watersheds with mixed land uses in the Aberdeen, U.K. area, which exhibited mean transit times (MTTs) ranging from more than 4 years for non-urban subwatersheds, to less than 10 days in a 63% urbanized subwatershed (Soulsby, Birkel, & Tetzlaff, 2014). Further work in the same region also found a large difference in MTTs between the most urbanized subwatershed (171 days) and the non-urban subwatershed (456 days; Soulsby, Birkel, Geris, & Tetzlaff, 2015).…”

Assessing the effects of catchment‐scale urban green infrastructure retrofits on hydrograph characteristics

“…Similarly, on Parkhaven, where no underdrains were installed in the green infrastructure, lag times did not significantly change. Ultimately, infiltrated water may emerge in the stream as baseflow, days, weeks or months after the storm (Hamel et al ., ; Soulsby et al ., ). As a result of the green infrastructure, Klusner and Hetzel became more dissimilar in the timing of run‐off delivery to the receiving stream.…”