Effect of a two-stage tandem rain garden in regulating stormwater runoff under climate change scenarios in Xi'an, China

Study region Xi'an, a semi-humid monsoon city in northwestern China, where a field-scale two-stage tandem rain garden receives rooftop runoff from a university office building catchment. Study focus We evaluated whether a modular DRAINMOD model could reproduce monitored inflow and drainage from the tandem system, assessed long-term runoff reduction under contrasting climate projections, and identified implications for storage and drainage management. Continuous simulations used the 1951–2004 historical baseline and bias-corrected daily precipitation and air temperature from 19 CMIP6 GCMs for 2024–2100 under SSP1–2.6, SSP2–4.5, and SSP5–8.5. New hydrological insights Across the modeled 400–1000 mm annual-rainfall gradient, the fitted sensitivities show that a 1 °C increase offsets the effect of 58.3 mm additional rainfall on runoff-reduction rate. Under representative future conditions, mean runoff reduction stayed above 40%, but the least-favorable wet scenarios declined by 1.07–5.54% by late century, indicating uneven resilience across the rainfall spectrum. This divergence arises because rainfall intensification overwhelms the forebay's capture capacity, whereas higher temperatures accelerate evaporative storage recovery between events. These findings quantify, for this study area, the trade-off between greater rainfall input and enhanced evaporative recovery, identifying storm clustering as the decisive factor governing whether the system retains its runoff-reduction function under future climate.