United States Office of Water EPA-823-R-99-003

Environmental Protection 4305 March 1999

Agency

BASINS Technical Note 3

NPSM/HSPF Simulation Module Matrix
Page 1 of 9

Technical Note 3

NPSM/HSPF Simulation Module Matrix

March 31, 1999

The Nonpoint Source Model (NPSM), the GIS and Windows interface to the Hydrologic

Simulation Program Fortran (HSPF), is a flexible watershed simulation model that permits users

to simulate a large number of pollutants over a range of simplistic to complex formulations. The

HSPF program has a modular structure in which pervious and impervious land segments define

the first two modules, and free flowing reaches and reservoirs define the third. Each module

contains specialized algorithms, or sections, for simulating the environmental fate processes

important to a particular pollutant or group of pollutants (e.g. the pesticide section simulates

absorption/desorption and volatilization). While all modules and sections can be selected, the

typical model simulation requires only a limited selection. Additionally, however, some sections

are dependent on output from other modules and NPSM/HSPF will not function with an improper

combination of sections selected. Also, by starting simple and gradually building the model’s

complexity, a user increases their ability to trap errors and will thus reduce the time required to

build a functioning, complex NPSM/HSPF simulation. Finally, some sections need not be

simulated as long as the user can supply a time-series of data for the output from that section. 

This technical note, then, is meant to answer the following two key questions: 1)What is the

minimum necessary set of modules and sections for modeling my pollutant of concern?; and 2) In

what order should I add modules such that I can slowly build the complexity of my model?

The matrix (see Figure 1, below) shows the hydrologic process or pollutant to be

simulated along the top, and the HSPF modules and sections along the left. A set of notes, at the

end, provides additional details about the conditions under which a section is required,

recommended, or optional. To read the matrix, select the process or pollutant you wish to model,

and read down that column to determine which HSPF sections you must simulate as well as those

which are recommended or optional. For example, to simulate pesticides, reading down the

column in the PERLND block, shows ATEMP and SNOW as optional (only necessary if snow is

significant in your watershed), PWATER and SEDMNT as required, PSTEMP as only necessary

if first-order adsorption-desorption is selected, and MSTLAY (soil moisture storage and fluxes)

and PEST (by definition) as required. Reading down the pesticide column in the RCHRES

module, HYDR and ADCALC are required, HTRCH is recommended, SEDTRN is

recommended to account for reduced photolysis due to sediment shading (and is required if the

pesticide is sediment associated) and GQUAL is required. While NPSM/HSPF does not simulate

pesticide application or fate and transport on impervious land segments, you still need to model

the simulation elements that will effect quantities in the receiving water body: i.e. ATEMP and

SNOW (if simulated in the PERLND module), IWATER (for hydrology), IWTGAS (for

temperature if HTRCH selected in RCHRES module) and SOLIDS (if SEDTRN selected in

RCHRES). 
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Snow

Basic Hydrology

Soil Erosion

Soil TemperatureWater Temperature

Dissolved Oxygen

Carbon Dioxide

General WQ

Pesticides

NitrogenPhosphorus

Tracers

Snow

Basic HydrologySolids

Water TemperatureDissolved Oxygen

Carbon Dioxide

General WQ

HSPF Section Pervious Land (PERLND) Impervious Land (IMPLND)

Required

Recommended

SNOW

PWATER

SEDMNT

PSTEMP

PWTGAS

PQUAL

Optional

MSTLAY

PEST

NITR

PHOS

TRACER

Adjusted Air TempAdjusted Air Temp

IWATER

SOLIDS

IWTGAS

IQUAL

PERLND IMPLND

ATEMP

ATEMP

SNOW

1 - Recommended module sections are required unless a timeseries of observed data or estimated values are available in place of the variables they simulate.

2 - Activate only if constituent being modeled is associated with sediment. 

3 - ATEMP is recommended to adjust for any elevation differences between the observation site and the watershed.

4 - PSTEMP is required only if the first-order adsorption-desorption is selected instead of the equilibrium Freundlich approach. 

2

3 3 3 3 3 3 3

4

2

1

3 3 3 3

Note, each module (Pervious Land [PERLND], Impervious Land [IMPLND], and Free Flowing

Reach or Reservoir [RCHRES]) is essentially its own separate matrix since they can each be run

independently of the others (e.g. running general water quality constituent simulations in a

RCHRES segment does not require a constituent load from either a PERLND or IMPLND;

similarly, modeling nutrient washoff in a PERLND does not require you to route the runoff to a

RCHRES segment). 
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HSPF Section Free Flowing Reach or Reservoir (RCHRES)

Required

Recommended

ADCALC

CONS

HTRCH

SEDTRN

GQUAL

Optional

OXRX

NUTRX

PLANK

PHCARB

Flow Routing

RCHRES

HYDR

RQUAL

1 - Recommended module sections are required unless a timeseries of observed data or estimated values are available in place

 of the variables they simulate.

2 - Required only if using Toffaleti or Colby method for sand transport.

3 - Required only if constituent being modeled is sediment-associated; recommended if photolysis is considered to account for

 sediment shading.

4 - Required only if NH3/PO4 association with sediment is considered - otherwise, highly recommended.

5 - Required only if NUTRX is also active and sediment association is considered - otherwise, highly recommended.

6 - Consideration of entire nutrient and biological cycle is highly recommended.

Conservative

Water Temperature

Sediment

General WQ

Pesticides

Dissolved Oxygen

BOD

Inorganic Phosphorus

Inorganic Nitrogen

Benthic Algae

Phytoplankton

Organic C, N & P

Zooplankton

pH

Carbon Dioxide

Total Inorganic Carbon

6

6 6 6 6

6

3 3 5 5 4 4 4 4 4 4 4 4 4

2

1

Figure 1. Matrix of HSPF Sections Required vs. Pollutants and Processes Modeled

Figure 1. Matrix of HSPF Sections Required vs. Pollutants and Processes Modeled (cont.)
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Table Notes:

Key

Q: “required”

R: “recommended”

O: “optional”

PERLND

Adjusted Air Temp

ATEMP Q by definition; used to adjust for air temperature differences between

meteorologic station and site due to elevation differences (HSPF uses

lapse rate that varies between 0.0035 and 0.005 degrees-F/ft)

Snow

ATEMP R can be bypassed, in HSPF by adjusting gage temperature directly 

(required in NPSM if SNOW simulated).

SNOW Q by definition

Basic Hydrology

ATEMP O results used only if SNOW is simulated (required in NPSM if SNOW

simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q by definition

Soil Erosion

ATEMP O results used only if SNOW is simulated (required in NPSM if SNOW

simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q surface quantities SURO and SURS used in SEDMNT routine

SEDMNT Q by definition

Soil Temperature

ATEMP R can be bypassed, in HSPF by adjusting gage temperature directly

PSTEMP Q by definition
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Water Temperature, Dissolved Oxygen, Dissolved CO2

ATEMP R results used only if SNOW or PSTEMP simulated

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q runoff components SURO, IFWO AGWO are the key simulation

elements

PSTEMP R observed/estimated soil temps may be input instead

PWTGAS Q by definition

General WQ

ATEMP O results used only if SNOW is simulated

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q surface quantities SURO and SURS used in SEDMNT routine

SEDMNT Q only needed if water quality constituent is sediment-associated

PQUAL Q by definition

Pesticides

ATEMP R results used only if SNOW or PSTEMP simulated (required in NPSM

if SNOW simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q soil moisture fluxes/storages are the key simulation elements

SEDMNT Q soil erosion must be simulated

PSTEMP R observed/estimated soil temps may be input directly in HSPF

MSTLAY Q leaching factors must be simulated

PEST Q by definition

Nitrogen

ATEMP R results used only if SNOW or PSTEMP simulated (required in NPSM

if SNOW simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q soil moisture fluxes/storages must be simulated

SEDMNT Q soil erosion must be simulated

PSTEMP R observed/estimated soil temps may be input directly in HSPF

MSTLAY Q leaching factors must be simulated

NITR Q by definition
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Phosphorus

ATEMP R results used only if SNOW or PSTEMP simulated (required in NPSM

if SNOW simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q soil moisture fluxes/storages must be simulated

SEDMNT Q soil erosion must be simulated

PSTEMP R observed/estimated soil temps may be input directly in HSPF

MSTLAY Q leaching rate must be simulated

PHOS Q by definition

Tracer

ATEMP O results used only if SNOW is simulated

SNOW O only needed in regions where snow is significant hydrologic cycle

component

PWATER Q soil moisture fluxes/storages must be simulated

MSTLAY Q leaching rate must be simulated

TRACER Q by definition
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IMPLND

Adjusted Air Temp

ATEMP Q by definition; used to adjust for air temperature differences between

meteorologic station and site due to elevation differences

Snow

ATEMP R can be bypassed, in HSPF by adjusting gage temperature directly 

(required in NPSM if SNOW simulated).

SNOW Q by definition

Basic Hydrology

ATEMP O results used only if SNOW is simulated (required in NPSM if SNOW

simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

IWATER Q by definition

Solids

ATEMP O results used only if SNOW is simulated (required in NPSM if SNOW

simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

IWATER Q surface runoff must be simulated

SOLIDS Q by definition

Water Temperature, Dissolved Oxygen, Dissolved CO2

ATEMP R can be bypassed, in HSPF by adjusting gage temperature directly 

(required in NPSM if SNOW simulated).

SNOW O only needed in regions where snow is significant hydrologic cycle

component

IWATER Q surface runoff must be simulated

IWTGAS Q by definition

General WQ

ATEMP O results used only if SNOW is simulated (required in NPSM if SNOW

simulated)

SNOW O only needed in regions where snow is significant hydrologic cycle

component

IWATER Q surface runoff must be simulated

SOLIDS Q only needed if water quality constituent is sediment-associated

IQUAL Q by definition
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RCHRES

Flow Routing

HYDR Q by definition

Conservative Substance

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

CONS Q by definition

Water Temperature

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

HTRCH Q by definition

Sediment Transport

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

HTRCH R required only if Toffaletti or Colby methods are used for sand, can be

bypassed by inputting water temperature directly (in HSPF, but not in

NPSM)

SEDTRN Q by definition

General WQ, Pesticides

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

HTRCH R can be bypassed by inputting water temperature directly

SEDTRN R only required if sediment-associated, otherwise is optional;

recommended if photolysis is considered (to account for sediment

shading); can be bypassed by inputting sediment concentration directly

in HSPF

GQUAL Q by definition
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Dissolved Oxygen, BOD

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

HTRCH R can be bypassed by inputting water temperature directly

SEDTRN O only if NUTRX used

OXRX Q by definition

NUTRX O nutrients usually important

PLANK O plankton usually important

Inorganic Phosphorus, Inorganic Nitrogen

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

HTRCH R can be bypassed by inputting water temperature directly

SEDTRN Q phosphate and ammonia adsorption usually important

OXRX Q DO and BOD must be simulated

NUTRX Q by definition

PLANK O plankton usually important

Benthic Algae, Phytoplankton, Organic C, N, &P, Zooplankton

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

HTRCH R can be bypassed by inputting water temperature directly

SEDTRN R can be bypassed by inputting sediment concentration directly (unless

already needed by NUTRX)

OXRX Q DO and BOD must be simulated

NUTRX Q nutrients must be simulated

PLANK Q by definition

pH, Carbon Dioxide, Tot Inorganic Carbon

HYDR Q flow routing must be simulated

ADCALC Q transport factors must be simulated

CONS R can be bypassed by inputting alkalinity concentration directly

HTRCH R can be bypassed by inputting water temperature directly

SEDTRN R only if needed by NUTRX

OXRX Q DO and BOD must be simulated

NUTRX Q nutrients must be simulated

PLANK Q plankton CO2 fluxes must be simulated

PHCARB Q by definition