Spatial Implementation Feasibility of Low-Impact Development for Sustainable Urban Stormwater Management: A Dual-Boundary Framework Linking Hydrologic Demand and Physical Spatial Capacity

Hydrologic effectiveness does not necessarily imply spatial implementability for low-impact development (LID) in space-constrained built-up areas. Existing LID planning and multi-objective optimization studies focus on runoff-control performance, construction costs, and spatial configuration, but rarely assess whether the scale required to meet a stormwater-control target can be accommodated by retrofit space. Here, we develop a dual-boundary framework that compares the hydrologic requirement lower bound Smin, defined as a probabilistic planning bound based on the 95% posterior-coverage criterion, with the spatial capacity upper bound Smax, representing the maximum proportion of subcatchment area available for potential LID retrofitting under built-environment constraints. Smax is quantified using a stepwise spatial deduction mechanism (SSDM), whereas Smin is inferred from Storm Water Management Model (SWMM)-generated samples using a hierarchical Bayesian network (HBN). In a built-up area of Hohhot, mean Smax and Smin were 42.37% and 27.44%, yet 17% of the area exhibited Smin > Smax. For performance-matched Pareto solutions, the mean runoff-reduction matching error was 0.52%, while the weighted spatial exceedance ratio reached 1.70%, with potentially infeasible investment exposure averaging CNY 1.32 million. Clipping allocations above Smax reduced runoff-control performance by 1.42% on average. Under the built-environment and rainfall conditions examined, these results show that aggregate capacity surplus can conceal local demand–capacity mismatch and that part of the simulated hydrologic performance may depend on LID areas exceeding local spatial capacity. By jointly assessing hydrologic demand and spatial capacity, the dual-boundary framework provides a practical basis for incorporating spatial implementation feasibility into LID planning, thereby supporting more sustainable urban stormwater management.