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;Q<sub>p</sub> :Peak flow in units of volume per time (e.g. m<sup>3</sup>/s)
;R :Ratio of catchment impervious area (A<sub>i</sub>) to the practice footprint area (A<sub>p</sub>). ''syn.'' I/P ratio. For [[green roofs]] and often [[permeable paving]] systems that do not receive drainage from external impervious areas this will be 0. For [[bioretention]], [[infiltration trenches]] and [[infiltration chambers]] receiving pavement runoff, it is recommended to be kept between 15 to 20.
;R<sub>a</sub> :Existing rise of swale gradient (length, usually m)