Enhancing urban runoff modelling using water stable isotopes and ages in complex catchments

Abstract

Increased urbanization, coupled with the projected impacts of climatic change, mandates further evaluation of the impact of urban development on water flow paths to guide sustainable land-use planning. Though the general urbanization impacts of increased storm runoff peaks and reduced baseflows are well known; how the complex, non-stationary interaction of the dominant water fluxes within dynamic urban water stores sustain streamflow regimes over longer periods of time is less well quantified. In particular, there is a challenge in how hydrological modelling should integrate the juxtaposition of rapid and slower flow pathways of the urban 'karst' landscape and different approaches need evaluation. In this context, we utilized hydrological and water stable isotope datasets within a modelling framework that combined the commonly used Hydrologic Engineering Center Hydrological Modelling System (HEC-HMS) urban runoff model along with a simple hydrological tracer module and transit time modelling to evaluate the spatial and temporal variation of water flow paths and ages within a heavily urbanized 217 km 2 catchment in Berlin, Germany. Deeper groundwater was the primary flow component in the upper reaches of the catchment within fewer urbanized regions, while the addition of wastewater effluent in the mid-reaches of the catchment was the dominant water supply to sustain baseflow in the lower main stem stream, with additional direct storm runoff and shallow subsurface contributions in the more urbanized lower reaches. Water ages from each modelling approach mirrored flow contributions and water age mixing potential in subsurface storage; with older average water and lower young water contributions in less urbanized sub-catchments and younger average water and higher young water contributions in more urbanized regions. The results from the first step towards more integrated tracer-aided hydrologic modelling tools for similar peri-urban catchments, given the potential limitations of simpler model frameworks. The results have broader implications for assessing the uncertainty in evaluating urban impacts on hydrological function under environmental change.

Integrated monitoring and modeling to disentangle the complex spatio-temporal dynamics of urbanized streams under drought stress

“…As reported by Smith et al ( 2023 ) for the neighboring Panke catchment, Berlin experiences a warm, temperate maritime climate ( Cfb in the Köppen classification). The average annual precipitation in the Wuhle catchment (2011–2020) was 590 mm, with frequent low-intensity winter precipitation and intense summer convective rainfall events yielding over 80 mm.…”

“…In this study, our third specific research aim was to use exploratory modeling as a learning framework and to integrate our insights about the catchment’s response. In this regard, and despite its recent successful application in the Panke catchment in Berlin (Smith et al, 2023 ), we have seen that successfully calibrating HEC-HMS to the Wuhle proved to be difficult in a way that both storm flow and base flow components could be adequately captured. This likely reflects uncertainties over catchment size and any related interactions between the upper AQ2 aquifer that is hydraulically connected to the Wuhle stream network and the larger regional aquifer system (Limberg & Thierbach, 2002 ).…”

“…Model calibration used a Latin Hypercube Sampling (LHS) design to generate a near-random sample of 23 parameter values for each of the 11 sub-catchments from within plausible ranges based on prior work in the neighboring Panke catchment by Smith et al ( 2023 ) (Table S2). A total of 150.000 simulations were carried out on Cirrus, a Linux-based High-Performance Computing Cluster at the Humboldt University in Berlin.…”

“…Mixing of stable isotopes (δ 2 H and δ 18 O) and water ages were computed for the fluxes and storages from calibrated HEC-HMS results. Additional effects of passive storage on damping the isotopic dynamics were incorporated using an amount-weighted approach (as in Smith et al, 2023 ), assuming a complete and uniform mixing for each time step as with other isotopic modeling approaches (Ala-Aho et al, 2017 ; Kuppel et al, 2018 ). At the end of each time step, one time step was added to each average water age in storage to account for the aging of water.…”

“…At the end of each time step, one time step was added to each average water age in storage to account for the aging of water. Because in-stream evaporative fractionation is negligible in Berlin (see results for the Panke stream by Smith et al, 2023 ), these effects were not considered within the isotope mixing module.…”