AbstractLow impact development (LID) provides a new way to address the problems of urban nonpoint source pollution and storm flood management. Implementation of LID measures is challenging due to the variation of characteristics in different watersheds, such as budget, size, number, location, and combination. Traditional bioretention soil media (BSM) is defined as a mixture in which the mass ratio of local river sand, soil, and wood chips is 65∶30∶5. A certain proportion of modifiers with better adsorption effects was added to the traditional BSM to prepare an improved BSM. The media infiltration capacity, ponding depth, and discharge ratio (catchment area/facility area) were key design parameters affecting watershed management of urban hydrologics in loess regions. Therefore, a numerical calculation method of ponding depth and discharge ratio was proposed in this study to evaluate the effects of media modification on the system. Under the design condition of a 60-min rainfall duration and two-year recurrence interval, a limitation of the ponding depth and duration to 15 cm and 20 min, respectively, should result in the discharge ratios of planting soil, BSM+10% water treatment residuals (WTR), and BSM+10% fly ash to be ≤5∶1, ≤20∶1, and ≤10∶1, respectively. The sensitive parameters were identified and verified through a pilot test and soil physical model HYDRUS simulation, in which the calibrated and validated model can be used to predict the thickness of modified media under different runoff volume and pollutant reduction design objectives.