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;A<sub>c</sub> :Area of the catchment, whole contributing drainage area (usually in m<sup>2</sup>).
;A<sub>c</sub> :Area of the catchment, whole contributing drainage area (usually in m<sup>2</sup>).
;A<sub>i</sub> :Area of impervious cover within the catchment (usually in m<sup>2</sup>).
;A<sub>p</sub> :Area of the practice e.g. footprint area of a bioretention cell (usually in m<sup>2</sup>)
;A<sub>p</sub> :Area of the practice e.g. footprint area of a bioretention cell (usually in m<sup>2</sup>)
;d :Depth e.g. of an infiltration practice (units of length, usually mm or m)
;D :Duration of the design storm event (in units of time, usually hrs)
;d<sub>p</sub> :Surface ponding depth of an infiltration practice (unit of length, usually mm or cm)
;d<sub>m</sub> :Filter media depth of a bioretention, stormwater planter or soil cell practice (unit of length, usually mm or cm)
;d<sub>s</sub> :Storage reservoir depth of an infiltration practice (unit of length, usually mm or cm)
;d<sub>T</sub> :Total depth of an infiltration practice (unit of length, usually cm or m)
;f :Infiltration rate, measured (usually mm/hr)
;f':Design infiltration rate (length per time, usually mm/hr) after application of safety correction factor (SCF)
;f :Infiltration rate (usually mm/hr)
;h :Height (units of length)
;h :height (units of length)
;i :Intensity of the design storm event (in units of depth/time, usually mm/hr)
;i :Thornthwaite monthly heat index (-)
;i :Thornthwaite monthly heat index (-)
;L :Length, e.g. of a reach, channel or swale (usually in m)
;L :Length, e.g. of a reach, channel or swale (usually in m)
;m :run of side slopes in channel cross sections (length, but usually expressed as a ratio in relation to rise. e.g. side slopes = 1:3, so m = 3)
;m :Run of side slopes in channel cross sections (length, but usually expressed as a ratio in relation to rise. e.g. side slopes = 1:3, so m = 3)
;n :Porosity, applies to aggregate and other void-forming materials, filter media and native soils (unit is vol/vol, so dimensionless)
;n :Manning's coefficient (unitless) Note: the symbol n is also used for porosity so it is important to be sure which term is implied.
;n :Manning's coefficient (unitless) Note: the symbol n is also used for porosity so it is important to be sure which term is implied.
;N :Number of days in the month
;N :Number of days in the month
;Q<sub>p</sub> :Peak flow in units of volume per time (e.g. m<sup>3</sup>/s)
;Q<sub>p</sub> :Peak flow in units of volume per time (e.g. m<sup>3</sup>/s)
;R :Ratio of catchment (A<sub>c</sub>) to the practice footprint area (A<sub>p</sub>). ''syn.'' I/P ratio. For [[green roofs]] (and often permeable paving systems) this will be 1:1. In [[bioretention]] it may be 15 or 20 to 1.
;R :Ratio of catchment impervious area (A<sub>i</sub>) to the practice permeable footprint area (A<sub>p</sub>). ''syn.'' I/P ratio. For [[green roofs]] and often [[permeable pavements|permeable pavement]] 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)
;R<sub>a</sub> :Existing rise of swale gradient (length, usually m)
;SCF :Safety correction factor (dimensionless)
;SCF :Safety correction factor (dimensionless)
;t :time (usually in hrs)
;t :Drainage time (usually in hrs)
;T :Temperature (°C)
;T :Temperature (°C)
;V :Volume (usually in m<sup>3</sup>)
;V :Volume (usually in m<sup>3</sup>)
;n :Porosity, applies to aggregate and other void-forming materials, filter media and native soils (unit is vol/vol, so dimensionless)
;x :Perimeter of infiltration practice or swale x-section (m)
;y :Depth of water (L, usually m or sometimes mm)
;y :Depth of water (L, usually m or sometimes mm)