Variations of urban water balances considering subsurface sewer fluxes: a hydrologic modeling study

• Reponses of sewer-mediated fluxes to environmental factors is evaluated by modeling. • Sewer-mediated fluxes are more sensitive to water table depth and pipe defect size. • Natural fluxes are more sensitive to soil profiles and trench backfill materials. • Native soil backfill reduces sewer-mediated fluxes and restores natural flow. Urban water balances are strongly shaped by the complex interplay between infiltrated stormwater, subsurface soil, and buried infrastructures. Sewer-mediated subsurface fluxes such as inflow and infiltration (I&I) and exfiltration can significantly alter the path and fate of stormwater. However, these processes are often overlooked in hydrologic analysis; the relative contribution of rainfall and groundwater on sewer-mediated fluxes and the combined and relative influence of interacting environmental and structural factors on these fluxes remain largely underexplored. In this study, we developed an integrated hydrologic model using HYDRUS-2D to represent the detailed geometry of a defective sewer pipe with a colmation layer within a trench-scale domain. We validated it with prior data and applied it on various hypothetical scenarios to evaluate how does sewer-mediated subsurface fluxes respond to variations in environmental and structural factors such as soil type, water table depth, pipe defect size, and trench backfill materials. We also performed a feature importance statistical analysis among different factors to further quantify the relative importance of these factors. Findings reveal that the rainwater partitioning and the resulting water balances exhibited notable variations across different native soil types and water table depths. Comparatively, sewer-mediated fluxes are more sensitive to water table depth and pipe defect sizes; while natural fluxes of surface runoff and evapotranspiration are more sensitive to surrounding native soil and trench backfill soil types. Backfilling trenches with native soils (particularly clayey soils) instead of granular imported soils reduced sewer fluxes and enhanced natural outflows. This study can inform catchment-scale hydrologic models to identify hotspots for operation and maintenance in urban sewers and evaluate the impact of sewer-mediated fluxes on watershed hydrology and streamflow regimes.