Stormwater Quality Calculations

I am working with a few clients to develop a Stormwater Retrofit Plan.  We are trying to reduce phosphorous being discharged in a TMDL watershed.  One of the Retrofits being
considered is the repair/rebuilding of roadside swales.  The problem is
estimating the current phosphorous contribution from the swales.  A
formula has been proposed to make this estimate.  However, the source of
the formula is not known.  I was wondering if anyone has any
information on the background of the formula.  This is the
formula:

"The following calculations will be used to determine the amount of Phosphorous that is reduced at each site.  This
calculation will be completed during the planning and design process to
determine the current amount of Phosphorous.  The calculation will need
to be completed before implementation as well as after in order to find
the change in Phosphorous levels.  These calculations were developed by
Westchester County and were approved for use by New York State
Department of Environmental Conservation.  

Pload = (0.4536) ((R-1)(A)(L)(BD)(Ptest))/((RP)(1,000,000))

where:

Pload = TP Loading (kg/yr)

R       = enlargement ratio based on watershed impervious cover

A       = existing channel area (SQFT)

L       = channel or stream length(FT)

BD     = bulk density (LB/CF)

Ptest  = phosphorus level in soil (mg/kg)

RP     = relaxation period (years)

0.4536 and 1,000,000 are conversion factors

A and L are to be based on field survey
BD will be assumed to be 95 LB/CF based on typ soil conditions

RP is set at an assumed value of 67 years, based on channel enlargement theory

R is based on site impervious cover and channel enlargement empirical data

Ptest will use standard published values (recommend using 300 mg/kg)
 
R as a function of impervious cover is empirically derived to be:
y = 0.0012(x)(x) + 0.0239(x) + 1

where y is enlargement ratio, x is imperviousness (%)"

Any information on the source of this formula would be useful.

Thanks