Watersheds

As an alternative to using the P8 interface, watershed data can be imported from or exported to an external Excel file . 

For additional guidance in selecting coefficients for impervious areas, see SLAMM calibrations.

For detailed description of runoff algorithms, see Simulation Methods.

Runoff Curve Numbers

The specified SCS Curve Number (CN) reflects an area-weighted-average of the pervious areas, which generally reflect land cover and soil hydrologic group.

This is a change from previous P8 versions (<=3.2), which assumed that the specified CN value also reflected indirectly connected impervious areas (see below).  When input files from previous versions are read, the indirectly connected fraction is set to 0, so simulation results should not change relative to previous versions.  If a distinction between pervious and indirect impervious areas is desired, the specified CN values should be revised to reflect only the pervious portions.

The following table lists typical CN values as a function of land use, hydrologic condition, and soil group:

Land Use	Hydrologic Condition	A	B	C	D
Grassed Areas	Good (>75% Cover)	39	61	74	80
	Fair	49	69	79	84
	Poor (<50% Cover)	68	79	86	89
Meadow / Idle	Good	30	58	71	78
Woods	Good (thick forest)	25	55	70	77
	Fair	36	60	73	79
	Poor (thin, no mulch)	45	66	77	83
Construction Site	Newly Graded	81	89	93	95
Impervious	Not Connected (Draining to Pervious Areas)	98	98	98	98

Source: USDA, SCS (1977)

See comparison of P8 and SLAMM simulations of runoff from pervious areas.

Indirectly Connected Impervious Fraction
 

Indirectly connected impervious areas are assumed to drain onto pervious areas, as opposed to a curb, storm sewer, or other stormwater conveyance facility.  The Curve Number used in the simulation is an area-weighted average of the specified Curve Number for pervious areas and CN= 98 for the indirectly connected impervious areas.

A conservative analysis would ignore the distinction between direct and indirect impervious areas and assume that all impervious areas are directly connected. That might be appropriate, for example, when the pervious portion accounts for a small percentage of the watershed or is in close proximity to storm drainage systems.

The indirectly connected impervious fraction is assigned a value of 0.0 in files imported from P8 Versions <=3.2.

The following data are derived from Wisconsin urban watershed data (Panuska, 1998 ): 

 

Land Use Category	Indirect Impervious	Direct Impervious	Pervious
Regional Mall	0	86	14
Heavy Industrial	2	80	18
Strip Mall	0	75	25
Light Industrial	0	79	31
Institutional	0	41	59
HD Residential	0	51	49
MD Residential	13	24	63
Airport	25	9	66
Park	6	8	86
LD Residential	10	6	84

 

Directly Connected Impervious Fractions

The directly connected impervious fractions (swept, unswept) are assumed to reflect areas that drain directly to a curb, storm sewer, or other stormwater conveyance system.

For a given precipitation regime, runoff and particle loading simulations, as well as BMP designs, are typically sensitive to connected impervious fractions, so they should be defined based upon site-specific information for each watershed if possible.

The following table lists impervious fractions as a function of GIS land use classifications, as estimated for the original P8 prototype watershed (Hunt-Potowomut in Rhode Island, Walker, 1990).  These fractions may vary depending on region and definition of the GIS land use categories are defined. 

GIS Land Use	Mean	Range
111 Residential High Density >8 Units/acre	.44	.32-.60
113 Residential Medium Dens. 1-3.9 Units/ac	.27	.20-.38
114 Residential Med-Low Dens. .5-.9 Units/ac	.25	.06-.79
115 Residential Low Density .2-.49 Units/ac	.14	.10-.18
116 Residential Rural Density <.2 Units/ac	.05	.03-.06
120 Commercial	.62	.44-.92
131 Industrial Heavy	.81	.74-.93
132 Industrial Medium	.77	.59-1.0
141 Transportation Roads, Interch., Service	.41	.23-.60
180 Institutional Educ., Health, Prisons, Milit.	.47	.30-.77

Impervious Fractions vs. Land Use Classifications (USDA, 1985)

Residential Lot Size (acres)        1/8    1/4     1/3    1/2     1
Impervious Fraction                  .65    .38     .30     .25    .20
Industrial                                .72
Commercial & Business              .85

Depression Storage

This watershed variable refers to impervious portion of watershed only. Kidd (1978) presents the following equation, based upon data from Holland, United Kingdom, & United States:

Depression Storage (in) = .03 Slope ^ -0.49

where Slope = average watershed slope (%)

Based upon this equation:

Slope % Depression Storage (in)
.5             .042
 1             .030
 2             .021
 3             .018
 4             .015
 5             .014

Simulations of particle removal efficiency are typically insensitive to depression storage in the above range.

Directly-Connected Impervious Area Runoff Coefficient

In P8 Version 1.1, runoff from impervious areas equaled precipitation in excess of depression storage (i.e. a runoff coefficient of 1.0 was assumed). This assumption is appropriate to generate conservative BMP designs in absence of site-specific calibration data.  In subsequent versions, an impervious runoff coefficient (0 to 1) can be specified.  If the coefficient is less than 1.0, the remainder of excess rainfall & snowmelt is assumed to infiltrate can be routed to an aquifer device if desired.

Value Description

1.0     Recommended to produce conservative BMP designs

0.9     'Simple Method' Schuler (1987)

The specified runoff coefficients apply to storms with cumulative rainfall + snowmelt less than or equal to a  "breakpoint" specified on the P8  'Edit General '  screen..   A runoff coefficient of 1.0 is applied when the cumulative rainfall + snowmelt exceeds the breakpoint.  The default value (0.8 inches) is consistent with SLAMM calibrations.  The breakpoint is automatically set to 0.8 inches when an input file created in Versions <=3.2 is read.  Users should manually reset the breakpoint value to 1.0 if compatibility with previous versions is desired.

SLAMM Small-Storm Hydrology Calibrations

The following coefficients for directly-connected impervious areas have been calibrated to runoff vs. rainfall relationships utilized in the Source Loading Area Management Model (SLAMM, PV & Assoc, 2007 ).   These calibrations assume that the rainfall breakpoint is set at the default value of 0.8 inches (General Inputs screen).  Above that value, a runoff coefficient of 1.0 is assumed.

Impervious Category	Depression Storage (inches)	Runoff Coef.
Flat Roofs	0.080	0.907
Pitched Roofs	0.030	1.000
Impervious	0.002	0.973
Unpaved	0.200	0.962
Smooth-Textured Streets	0.022	0.701
Intermediate-Textured Streets	0.025	0.644
Rough Textured Streets	0.030	0.654
Typical Urban Source Area	0.022	0.8

See Walker (2007) for derivation of these values.  See additional calibrations to SLAMM Standard Land Use Files. 

Street Sweeping Parameters & Particle Load Factors

Each watershed has 2 types of impervious surfaces (swept vs. not swept) (e.g., streets/parking areas vs. rooftops)

The sweeping season is specified in YYMM format.

The 'Sweeping Efficiency Scale Factor' (normally = 1) is multiplied by 'Sweeper Efficiency' values specified on the Particle input screen.  This factor can be adjusted upwards to reflect tight parking controls adjust downwards to reflect weak or no parking controls

The default sweeping efficiencies specified in the NURP50 particle file apply to vacuum sweeping only.  Studies in Wisconsin indicate that removal efficiencies for broom sweepers are <=0% for particle sizes corresponding to the NURP settling velocity distribution.    Accordingly, it recommended that the street sweeping routine not be used for broom sweepers.

Modeling Construction Sites

Approximate estimates of sediment yield from construction sites can be developed using pervious area curve numbers typical of newly graded areas (USDA,SAS, 1977):

Hydrologic  Soil Group      A     B    C     D
Curve Number               81    89   93   95

The particle file (BARESOIL.P8P) contains coefficients for estimating suspended solids concentration in runoff from construction sites. These have been calibrated to give an average total suspended solids concentration of ~10,000 ppm (vs. ~100 ppm for stabilized urban watersheds) (Schueler, 1987) using the Providence rainfall file (ignoring snowmelt).