BASINS Technical Note 6Back to BASINS Technical Notes
BASINS Technical Note 6
Estimating Hydrology and Hydraulic Parameters for HSPF
This technical note provides BASINS users with guidance in how to estimate the input parameters in the ATEMP, SNOW, PWATER, IWATER, HYDR, and ADCALC sections of the Hydrological Simulation Program Fortran (HSPF) watershed model. For each input parameter, this guidance includes a parameter definition, the units used in HSPF, and how the input value may be determined (e.g. initialize with reported values, estimate, measure, and/or calibrate). Where possible, the note discusses how to estimate initial values using the data and tools included with BASINS. Also discussed, where appropriate, is the physical basis of each parameter and the corresponding algorithms as described in the HSPF Users Manual (Bicknell, et al, 1997) and earlier literature sources. In addition to the guidance provided below, model users are directed to other sources, including the ARM Model User Manual (Donigian and Davis, 1978), as well as studies of agricultural BMP representation with HSPF (Donigian and Crawford, 1973;Donigian et al, 1983; and Casman, 1989).
Summary tables are attached that provide 'typical' and 'possible' (i.e. maximum 'expected') ranges for the parameters discussed below, based on both the parameter guidance and experience with HSPF over the past two decades on watersheds across the U. S. and abroad (Donigian, 1998). The overarching principal in parameter estimation should be that the estimated values must be realistic, i.e. make 'physical' sense, and must reflect conditions on the watershed. If the values estimated by the model user and/or derived from the guidance below, do not agree with the value ranges in the summary table, the user should question and re-examine the estimation procedures. The estimated values may still be appropriate, but the user needs to confirm that the parameter values reflect unusual conditions on the watershed.
Another source of parameter information is the HSPF Parameter Database (HSPFParm) (US EPA, 1999). Developed by AQUA TERRA Consultants, under contract to the EPA, HSPFParm consists of parameter values from previous applications of HSPF across North America assimilated into a single database, and with a customized graphical user interface for viewing and exporting the data. The pilot HSPFParm Database contains parameter values for model applications in over 40 watersheds in 14 states. The parameter values, contained in the database, characterize a broad variety of physical settings, land use practices, and water quality constituents. The database has been provided with a simplified interactive interface that enables modelers to access and explore the HSPF parameter values developed and calibrated in various watersheds across the United States, and to assess the relevance of the parameters to their own watershed setting.
The parameter guidance below is listed in order of the parameter tables required by each module section (i.e. ATEMP, SNOW, PWATER, IWATER, HYDR, and ADCALC) in the HSPF UCI (users control input) file, and the parameters are grouped as required in each UCI table.
Air Temperature (ATEMP) Parameters
The ATEMP section variables
are used by both the PERLND and IMPLND modules. This section is
not required for basic hydrology unless SNOW is being simulated.
ELDATElevation difference (feet), (measure). ELDAT is the difference in elevation between the air temperature gage and the mean elevation of the associated pervious land segment (PLS). ELDAT is equal to the PLS elevation minus the gage elevation and can be either positive (PLS higher) or negative (gage elevation higher). ELDAT is used to adjust the gage air temperature to the PLS using a lapse rate; see Section 4.2(1).1 in the HSPF User Manual (Bicknell et al., 1997) for additional information.
Use of BASINS Data/Tools:
AIRTMP Initial air temperature (degrees F), (measure/ estimate). Air temperature at start of simulation period.
Use of BASINS Data/Tools:
Snow (SNOW) Parameters
The SNOW section variables
are used in both the PERLND and IMPLND modules. Note: flag CSNOFG in PWATER
and IWATER must be selected for snow melt to be taken into account.
The ICE-FLAG table contains
only the flag to simulate ice formation in the snow pack. A value of 0
means ice formation is not simulated; 1 means that frozen water or frozen
ground at the bottom of the snowpack is computed daily based on air temperature
and inches of melt in the snowpack. This option is appropriate in regions
where frozen ground conditions are observed.
LATLatitude (degrees), (measure). LAT is positive for the northern hemisphere, negative for the southern hemisphere. LAT is used in the calculation of snow surface albedo to identify summer and winter time periods.
Use of BASINS Data/Tools:
MELEVMean elevation (feet), (measure). MELEV is used to compute the convective heat flux from the atmosphere to the snowpack.
Use of BASINS Data/Tools:
SHADEFraction of the land segment shaded from solar radiation by trees or slope (unitless), (measure/estimate).SHADE controls short wave solar radiation that reaches the snowpack. SHADE is also used in the calculation of long wave radiation, based on Stefan's law of black body radiation (Bicknell et al, 1997).
of BASINS Data/Tools:
SNOWCFFactor by which recorded precipitation is multiplied during snow events to account for poor gage catch efficiency (unitless), (estimate). Snow catch in a gage is affected by wind speed, instrument characteristics (e.g. snow shields), and gage exposure/location. SNOWCF is normally 1.0 or greater. Crawford (1999) states that physically realistic values of SNOWCF are in the range of 1.0 to 1.5. Larson and Peck, 1974 present data relating snow catch factor (SCF - a SNOWCF analogue) to windspeed. For unshielded gages, they show a parabolic curve for SCF with a value of 1.0 at 0.0 mph, 1.3 at 5 mph, and 1.8 at 10.0 mph. They also show a plot of the ratio of gage catch to true catch (the inverse variable) as roughly straight line plots, both starting from 1.0 at 0.0 mph. The shielded gage is roughly 0.75 at 10 mph, and the unshielded is roughly 0.55 at 10 mph. Only 200 out of 3500 weather stations in the U.S. are shielded; shielding of weather stations began in the mid 1940's (Doty, 2000).
snowpack depth (water equivalent) at which the entire land segment is
covered with snow (inches), (estimate).
COVIND is a function of topography and climatic conditions. The ARM Model
User Manual (Donigian and Davis, 1978, p. 65, MPACK variable) says typical
values are in the range of 1.0 to 6.0 inches. For mostly flat topography
and where snow is a common occurrence, values will be near the low end
of this range since complete coverage of the segment will result from
low snowfall amounts; whereas in mountainous watersheds the COVIND values
will be near the high end of the range reflecting the need for greater
snowfall to achieve complete coverage. Note that COVIND is in water equivalent,
so that a value of 1.0 inches corresponds to approximately 10 inches of
RDCSNDensity of new snow relative to water when air temperature is at or below 0 degrees F (unitless ratio), (estimate, then calibrate). The expected values are from 0.10 to 0.20. RDCSN can be calibrated by comparing field snow density data from snow courses with model snowpack (PACK) and density results (RDENPF). RDCSN is automatically adjusted in the model when the air temperature is above 0 degrees F. The snow density/ temperature functional relationship is described by Bicknell et al., (1997).
TSNOWWet bulb air temperature below which precipitation occurs as snow under saturated conditions (degrees F), (estimate, then calibrate). Values of 31 to 33 degrees F are often used (Donigian and Davis, 1978). When the air temperature exceeds the value of TSNOW, precipitation is modeled as rain, not snow. TSNOW can vary between 30 and 40 degrees F; the initial estimate should be at or near 32 degrees F. Crawford (1999) states that TSNOW is the most obvious parameter to change to increase or reduce snow accumulation. However, changing TSNOW is effective only when significant snowfall occurs at or near 32 degrees F. If typical winter air temperatures are below 30 degrees F then TSNOW will have little or no effect.
SNOEVPFactor to adjust evaporation (sublimation) from the snowpack (unitless), (estimate, then calibrate). Values near 0.1 are expected (Donigian and Davis, 1978, p. 65, EVAPSN variable). Evaporation from the snowpack occurs only when the vapor pressure of the air is less than that at the snow surface (Bicknell et al, 1997). Evaporation occurs only from the frozen content of the snowpack and is adjusted based on SNOEVP, wind movement, and the fraction of the land segment covered by the snowpack. Snow evaporation is not large in most watersheds, but can be important where windy, low humidity conditions are common (Crawford, 1999).
CCFACTFactor to adjust the rate of heat transfer from the atmosphere to the snowpack, due to condensation and convection, to match field conditions (unitless), (estimate, then calibrate). CCFACT is a function of climatic conditions. Typical values are near 1.0, although a range of 0.5 to 2.0 has been observed (Donigian and Davis, 1978, p. 64, CCFAC variable). CCFACT is used in conjunction with wind movement and air temperature to compute heat transfer from the snowpack to the land surface.
MWATERMaximum liquid water holding capacity in the snowpack (in/in), (estimate, then calibrate). MWATER is a function of the mass of ice layers; the size, shape, and spacing of snow crystals; and the degree of channelization and honeycombing of the snowpack to allow liquid water accumulation. Experimental values range from 0.01 to 0.05, with 0.03 a common value (Donigian and Davis, 1978, p. 64, WC).
rate of snowmelt by ground heat (in/day), (estimate, then calibrate).
MGMELT is the rate of melt when the snowpack temperature is at 32 degrees
F. A standard value is 0.01 in/day. Areas with deep frost penetration
and/or frozen ground have MGMELT values approaching zero (Donigian and
Davis, 1978, p. 64, DGM).
Pack-snow Initial quantity of snow (water equivalent) in the snowpack (inches), (estimate). If the simulation starts at the beginning of the water year (October 1) Pack-snow is usually set to zero, except in arctic climates.
Pack-ice Initial quantity of ice (water equivalent) in the snowpack (inches), (estimate). If the simulation starts at the beginning of the water year (October 1) Pack-ice is usually set to zero, except in arctic climates.
Pack-watr Initial quantity of liquid water in the snowpack (inches), (estimate). If the simulation starts at the beginning of the water year (October 1) Pack-watr is usually set to zero, except in arctic climates.
RDENPF Initial density of the frozen contents (Pack-snow and Pack-ice) of the snowpack relative to water (unitless ratio), (estimate). If the simulation starts at the beginning of the water year (October 1) RDENPF is usually set to 0.01 (minimum value), except in arctic climates; otherwise see RDCSN.
DULL Initial index to the dullness of the snowpack surface, from which the initial albedo is estimated (unitless), (estimate). If the simulation starts at the beginning of the water year (October 1) DULL is usually set to zero (for perfectly reflectable snow), except in arctic climates. DULL ranges from zero to 800 and is an empirical index. It's increased by one for each hour in which new snow does not fall.
PAKTMP Initial mean
temperature of the frozen contents of the snowpack (degrees F), (estimate).
If the simulation starts at the beginning of the water year (October 1)
PAKTMP is usually set to 32 degrees F, except in arctic climates.
COVINX Initial snowpack depth (water equivalent) required for the entire land segment to be covered with snow (inches), (estimate). If the simulation starts at the beginning of the water year (October 1) COVINX is usually set to its efault value of 0.01, except in arctic climates; otherwise see COVIND, SNOW-PARM1.
XLNMLT Initial increment to ice storage in the snowpack (inches), (estimate). XLNMLT represents an equivalent heat deficit that must be overcome before snowmelt is released from the pack; otherwise some portion of the potential melt will freeze and become pack ice. For most simulations, XLNMLT can usually be set to zero, except possibly in arctic climates, because the values will be recalculated based on current (usually hourly) air temperatures. XLNMLT is used only if ICE-FLAG is set to 1.
SKYCLR Initial fraction of sky assumed to be clear (unitless), (estimate). Unless a storm is in progress at the start of simulation period, set SKYCLR to 1.0 (no clouds).
Use of BASINS
Pervious Land Hydrology (PWATER) Parameters
The PWAT-PARM1 table includes flags to indicate the selected simulation algorithm option, or the selection of monthly variability versus constant values for selected parameters. Where flags indicate monthly variability, the corresponding monthly values must be provided in Monthly Input Parameters (see below following the PWAT_PARM4 Table section). That section also provides guidance on which parameters are normally specified as monthly values.
CSNOFG Flag to use snow simulation data; must be checked (CSNOFG=1) if SNOW is simulated.
RTOPFG Flag to select overland flow routing method; choose either the method used in predecessor models (HSPX, ARM, and NPS) or the alternative method as described in the HSPF User Manual. Recommendation: Set RTOPFG=1; This method, used in the predecessor models is more commonly used, and has been subjected to more widespread application.
UZFG Flag to select upper zone inflow computation method; choose either the method used in predecessor models (HSPX, ARM, and NPS) or the more exact numerical solution to the integral of inflow to upper zone, i.e the alternative method. Recommendation: Set UZFG=1; This method, used in the predecessor models, is more commonly used, and has been subjected to more widespread application.
VCSFG Flag to select constant or monthly-variable interception storage capacity, CEPSC. Monthly value can be varied to represent seasonal changes in foliage cover; monthly values are commonly used for agricultural, and sometimes deciduous forest land areas.
VUZFG Flag to select constant or monthly-variable upper zone nominal soil moisture storage, UZSN. Monthly values are commonly used for agricultural areas to reflect the timing of cropping and tillage practices.
VMNFG Flag to select constant or monthly-variable Manning=s n for overland flow plane, NSUR. Monthly values are commonly used for agricultural, and sometimes deciduous forest land areas.
VIFWFG Flag to select constant or monthly-variable interflow inflow parameter, INTFW. Monthly values are not often used.
VIRCFG Flag to select constant or monthly varied interflow recession parameter, IRC. Monthly values are not often used.
VLEFG Flag to select constant or monthly varied lower zone ET
parameter, LZETP. Monthly values are commonly used for agricultural, and
sometimes deciduous forest land areas.
FOREST Fraction of land covered by forest (unitless) (measure/estimate). FOREST is the fraction of the land segment which is covered by forest which will continue to transpire in winter (i.e. coniferous). This is only relevant if snow is being considered (i.e., CSNOFG=1 in PWATER-PARM1).
Use of BASINS Data/Tools:
LZSN Lower zone nominal soil moisture storage (inches), (estimate, then calibrate). LZSN is related to both precipitation patterns and soil characteristics in the region. The ARM Model User Manual (Donigian and Davis, 1978, p. 56, LZSN variable) includes a mapping of calibrated LZSN values across the country based on almost 60 applications of earlier models derived from the Stanford-based hydrology algorithms. LaRoche et al (1996) shows values of 5 inches to 14 inches, which is consistent with the 'possible' range of 2 inches to 15 inches shown in the Summary Table. Viessman, et al, 1989, provide initial estimates for LZSN in the Stanford Watershed Model (SWM-IV, predecessor model to HSPF) as one-quarter of the mean annual rainfall plus four inches for arid and semiarid regions, or one-eighth annual mean rainfall plus 4 inches for coastal, humid, or subhumid climates. These formulae tend to give values somewhat higher than are typically seen as final calibrated values; since LZSN will be adjusted through calibration, initial estimates obtained through these formulae may be reasonable starting values.
Index to mean soil infiltration rate (in/hr); (estimate, then calibrate).
In HSPF, INFILT is the parameter that effectively controls the overall
division of the available moisture from precipitation (after interception)
into surface and subsurface flow and storage components. Thus, high values
of INFILT will produce more water in the lower zone and groundwater, and
result in higher baseflow to the stream; low values of INFILT will produce
more upper zone and interflow storage water, and thus result in greater
direct overland flow and interflow. LaRoche et al (1996) shows a range
of INFILT values used from 0.004 in/hr to 0.23 in/hr, consistent with
the 'typical' range of 0.01 to 0.25 in/hr in the Summary Table. Fontaine
and Jacomino (1997) show sediment and sediment associated transport to
be sensitive to the INFILT parameter since it controls the amount of direct
overland flow transporting the sediment. Since INFILT is not a maximum
rate nor an infiltration capacity term, it's values are normally much
less than published infiltration rates, percolation rates (from soil percolation
tests), or permeability rates from the literature. In any case, initial
values are adjusted in the calibration process.
An alternate estimation method that has not been validated, is derived from the premise that the combination of infiltration and interflow in HSPF represents the infiltration commonly modeled in the literature (e.g. Viessman et al, 1989, Chapter 4). With this assumption, the value of 2.0*INFILT*INTFW should approximate the average measured soil infiltration rate at saturation, or mean permeability.
Use of BASINS Data/Tools:
Use of Soil Hydrologic Group/INFILT Table:
Use of Alternative 2.0*INFILT*INTFW Method:
LSUR Length of assumed overland flow plane (ft) (estimate/measure). LSUR approximates the average length of travel for water to reach the stream reach, or any drainage path such as small streams, swales, ditches, etc. that quickly deliver the water to the stream or waterbody. LSUR is often assumed to vary with slope such that flat slopes have larger LSUR values and vice versa; typical values range from 200 feet to 500 feet for slopes ranging from 15% to 1 %. It is also often estimated from topographic data by dividing the watershed area by twice the length of all streams, gullies, ditches, etc that move the water to the stream. That is, a representative straight-line reach with length, L, bisecting a representative square areal segment of the watershed, will produce two overland flow planes of width ½ L. However, LSUR values derived from topographic data are often too large (i.e. overestimated) when the data is of insufficient resolution to display the many small streams and drainage ways. Users should make sure that values calculated from GIS or topographic data are consistent with the ranges shown in the Summary Table.
Use of BASINS Data/Tools:
SLSUR Average slope of assumed overland flow path (unitless) (estimate/measure). Average SLSUR values for each land use being simulated can often be estimated directly with GIS capabilities. Graphical techniques include imposing a grid pattern on the watershed and calculating slope values for each grid point for each land use.
Use of BASINS Data/Tools:
KVARY Groundwater recession flow parameter used to describenon-linear groundwater recession rate (/inches) (initialize with reported values, then calibrate as needed). KVARY is usually one of the last PWATER parameters to be adjusted; it is used when the observed groundwater recession demonstrates a seasonal variability with a faster recession (i.e. higher slope and lower AGWRC values) during wet periods, and the opposite during dry periods. LaRoche, et al, 1996 reported an extremely high 'optimized' value of 0.66 mm-1 or (17 in-1) (much higher than any other applications) while Chen, et al, 1995 reported a calibrated value of 0.14 mm-1 (or 3.6 in-1). Value ranges are shown in the Summary Table. Users should start with a value of 0.0 for KVARY, and then adjust (i.e. increase) if seasonal variations are evident. Plotting daily flows with a logarithmic scale helps to elucidate the slope of the flow recession.
Groundwater recession rate, or ratio of current groundwater discharge
to that from 24 hours earlier (when KVARY is zero) (/day) (estimate,
then calibrate). The overall watershed recession rate is a complex
function of watershed conditions, including climate, topography, soils,
and land use. Hydrograph separation techniques (see any hydrology or water
resources textbook) can be used to estimate the recession rate from observed
daily flow data (such as plotting on a logarithmic scale, as noted above);
estimated values will likely need to be adjusted through calibration.
Value ranges are shown in the Summary Table. LaRoche, et al, 1996 reported
an optimized value of 0.99; Chen, et al, 1995 reported values that varied
with land use type, ranging from 0.971 for grassland and clearings to
0.996 for high density forest; Fontaine and Jacomino, 1997 reported a
calibrated value of 0.99. This experience reflects normal practice of
using higher values for forests than open, grassland, cropland and urban
PETMAX Temperature below which ET will be reduced to 50% of that in the input time series (deg F), unless it=s been reduced to a lesser value from adjustments made in the SNOW routine (where ET is reduced based on the percent areal snow coverage and fraction of coniferous forest). PETMAX represents a temperature threshold where plant transpiration, which is part of ET, is reduced due to low temperatures (initialize with reported values, then calibrate as needed). It is only used if SNOW is being simulated because it requires air temperature as input (also a requirement of the SNOW module), and the required low temperatures will usually only occur in areas of frequent snowfall. Use the default of 40oF as an initial value, which can be adjusted a few degrees if required.
PETMIN Temperature at and below which ET will be zero (deg F). PETMIN represents the temperature threshold where plant transpiration is effectively suspended, i.e. set to zero, due to temperatures approaching freezing (initialize with reported values, then calibrate as needed). Like PETMAX, this parameter is used only if SNOW is being simulated because it requires air temperature as input (also a requirement of the SNOW module), and the required low temperatures will usually only occur in areas of frequent snowfall. Use the default of 35oF as an initial value, which can be adjusted a few degrees if required.
INFEXP Exponent that determines how much a deviation from nominal lower zone storage affects the infiltration rate (HSPF Manual, p. 60) (initialize with reported values, then calibrate as needed). Variations of the Stanford approach have used a POWER variable for this parameter; various values of POWER are included in Donigian and Davis (1978, p. 58). However, the vast majority of HSPF applications have used the default value of 2.0 for this exponent. Use the default value of 2.0, and adjust only if supported by local data and conditions.
INFILD Ratio of maximum and mean soil infiltration capacities (initialize with reported value). In the Stanford approach, this parameter has always been set to 2.0, so that the maximum infiltration rate is twice the mean (i.e. input) value; when HSPF was developed, the INFILD parameter was included to allow investigation of this assumption. However, there has been very little research to support using a value other than 2.0. Use the default value of 2.0, and adjust only if supported by local data and conditions.
Use of BASINS Data/Tools:
DEEPFR The fraction of infiltrating water which is lost to deep aquifers (i.e. inactive groundwater), with the remaining fraction (i.e. 1-DEEPFR) assigned to active groundwater storage that contributes baseflow to the stream (estimate, then calibrate). It is also used to represent any other losses that may not be measured at the flow gage used for calibration, such as flow around or under the gage site. This accounts for one of only three major losses from the PWATER water balance (i.e. in addition to ET, and lateral and stream outflows). Watershed areas at high elevations, or in the upland portion of the watershed, are likely to lose more water to deep groundwater (i.e. groundwater that does not discharge within the area of the watershed), than areas at lower elevations or closer to the gage (see discussion and figures in Freeze and Cherry, 1979, section 6.1). DEEPFR should be set to 0.0 initially or estimated based on groundwater studies, and then calibrated, in conjunction with adjustments to ET parameters, to achieve a satisfactory annual water balance.
BASETP ET by riparian vegetation as active groundwater enters streambed; specified as a fraction of potential ET, which is fulfilled only as outflow exists (estimate, then calibrate). Typical and possible value ranges are shown in the Summary Table. If significant riparian vegetation is present in the watershed then non-zero values of BASETP should be used. Adjustments to BASETP will be visible in changes in the low-flow simulation, and will effect the annual water balance. If riparian vegetation is significant, start with a BASETP value of 0.03 and adjust to obtain a reasonable low-flow simulation in conjunction with a satisfactory annual water balance.
Fraction of model segment (i.e. pervious land segment) that is subject
to direct evaporation from groundwater storage, e.g. wetlands or marsh
areas, where the groundwater surface is at or near the land surface, or
in areas with phreatophytic vegetation drawing directly from groundwater.
This is represented in the model as the fraction of remaining potential
ET (i.e. after base ET, interception ET, and upper zone ET are satisfied),
that can be met from active groundwater storage (estimate, then
calibrate). If wetlands are represented as a separate PLS (pervious
land segment), then AGWETP should be 0.0 for all other land uses, and
a high value (0.3 to 0.7) should be used for the wetlands PLS. If wetlands
are not separated out as a PLS, identify the fraction of the model segment
that meets the conditions of wetlands/marshes or phreatophytic vegetation
and use that fraction for an initial value of AGWETP. Like BASETP, adjustments
to AGWETP will be visible in changes in the low-flow simulation, and will
effect the annual water balance. Follow above guidance for an initial
value of AGWETP, and then adjust to obtain a reasonable low-flow simulation
in conjunction with a satisfactory annual water balance.
Amount of rainfall, in inches, which is retained by vegetation, never
reaches the land surface, and is eventually evaporated (estimate,
then calibrate). Typical guidance for CEPSC for selected land
surfaces is provided in Donigian and Davis (1978, p. 54, variable EPXM)
Donigian et al (1983) provide more detail guidance for agricultural conditions, including residue cover for agricultural BMPs. As part of an annual water balance, Viessman, et al. 1989 note that 10-20% of precipitation during growing season is intercepted and as much as 25% of total annual precipitation is intercepted under dense closed forest stands; crops and grasses exhibit a wide range of interception rates - between 7% and 60% of total rainfall. Users should compare the annual interception evaporation (CEPE) with the total rainfall available (PREC in the WDM file), and then adjust the CEPSC values accordingly. (See Monthly Input Values below).
UZSN Nominal upper zone soil moisture storage (inches) (estimate, then calibrate). UZSN is related to land surface characteristics, topography, and LZSN. For agricultural conditions, tillage and other practices, UZSN may change over the course of the growing season. Increasing UZSN value increases the amount of water retained in the upper zone and available for ET, and thereby decreases the dynamic behavior of the surface and reduces direct overland flow; decreasing UZSN has the opposite effect. Donigian and Davis (1978, p. 54) provide initial estimates for UZSN as 0.06 of LZSN, for steep slopes, limited vegetation, low depression storage; 0.08 LZSN for moderate slopes, moderate vegetation, and moderate depression storage; 0.14 LZSN for heavy vegetal or forest cover, soils subject to cracking, high depression storage, very mild slopes. Donigian et al., (1983) include detailed guidance for UZSN for agricultural conditions. LaRoche shows values ranging from 0.016 in to 0.75 in. Fontaine and Jacomino showed average daily stream flow was relatively insensitive to this value but sediment and sediment associated contaminant outflow was sensitive; this is consistent with experience with UZSN having an impact on direct overland flow, but little impact on the annual water balance (except for extremely small watersheds with no baseflow). Typical and possible value ranges are shown in the Summary Table.
n for overland flow plane (estimate). Manning=s
n values for overland flow are considerably higher than the more common
published values for flow through a channel, where values range from a
low of about 0.011 for smooth concrete, to as high as 0.050-0.1 for flow
through unmaintained channels (Hwang and Hita, 1987). Donigian and Davis
(1978, p. 61, variable NN) and Donigian et al (1983) have tabulated the
following values for different land surface conditions:
For agricultural conditions, monthly values are often used to reflect the seasonal changes in land surfaces conditions depending on cropping and tillage practices. Additional tabulations of Manning's n values for different types of surface cover can be found in: Weltz, et al, 1992; Engman, 1986; and Mays, 1999. Manning's n values are not often calibrated since they have a relatively small impact on both peak flows and volumes as long as they are within the normal ranges shown above. Also, calibration requires data on just overland flow from very small watersheds, which is not normally available except at research plots and possibly urban sites.
INTFW Coefficient that determines the amount of water which enters the ground from surface detention storage and becomes interflow, as opposed to direct overland flow and upper zone storage (estimate, then calibrate). Interflow can have an important influence on storm hydrographs, particularly when vertical percolation is retarded by a shallow, less permeable soil layer. INTFW affects the timing of runoff by effecting the division of water between interflow and surface processes. Increasing INTFW increases the amount of interflow and decreases direct overland flow, thereby reducing peak flows while maintaining the same volume. Thus it affects the shape of the hydrograph, by shifting and delaying the flow to later in time. Likewise, decreasing INTFW has the opposite effect. Base flow is not affected by INTFW. Rather, once total storm volumes are calibrated, INTFW can be used to raise or lower the peaks to better match the observed hydrograph. Typical and possible value ranges are shown in the Summary Table.
IRC Interflow recession coefficient (estimate, then calibrate). IRC is analogous to the groundwater recession parameter, AGWRC, i.e. it is the ratio of the current daily interflow discharge to the interflow discharge on the previous day. Whereas INTFW affects the volume of interflow, IRC affects the rate at which interflow is discharged from storage. Thus it also affects the hydrograph shape in the 'falling' or recession region of the curve between the peak storm flow and baseflow. The maximum value range is 0.3 - 0.85, with lower values on steeper slopes; values near the high end of the range will make interflow behave more like baseflow, while low values will make interflow behave more like overland flow. IRC should be adjusted based on whether simulated storm peaks recede faster/slower than measured, once AGWRC has been calibrated. Typical and possible value ranges are shown in the Summary Table.
to lower zone evapotranspiration (unitless) (estimate, then calibrate).
LZETP is a coefficient to define the ET opportunity; it affects evapotranspiration
from the lower zone which represents the primary soil moisture storage
and root zone of the soil profile. LZETP behaves much like a 'crop coefficient'
with values mostly in the range of 0.2 to 0.7; as such it is primarily
a function of vegetation; Typical and possible value ranges are shown
in the Summary Table, and the following ranges for different vegetation
are expected for the 'maximum' value during the year:
Monthly Input Parameter Tables:
CEPS, SURS, IFWS, UZS, LZS, AGWS, are initial values for storage of water in interception, surface ponding, interflow, the upper zone, lower zone, and active groundwater, respectively, and GWVS is the initial index to groundwater slope. All these storages pertain to the first interval of the simulation period.
The surface related storages (i.e. CEPS, SURS, IFWS) are highly dynamic, and will reach a dynamic equilibrium within a few days, at most. These state variables can be left blank, or set to 0.0 unless an individual storm is being simulated. The soil storages (i.e. UZS, LZS, and AGWS, and the GWVS) are much less dynamic, so their beginning values can impact the simulation for a period of months to a few years. If possible, users should allow as long a startup time period as possible (i.e. set the simulation period to begin prior to the period you=ll use for comparison against monitoring data or other use); as noted each of these storages should reach a dynamic equilibrium within a few years of simulation. UZS and LZS should be set equal to UZSN and LZSN respectively, unless it is known that the starting date is during a particularly wet or dry period; starting values can be increased or decreased if wet or dry conditions were evident prior to the simulation period. AGWS is a bit more problematic. If far too high or too low, baseflow will be excessive or skewed low for several months or years, depending on AGWRC and KVARY. Improper values of GWVS can also cause simulation accuracy problems again for lengths of time depending on values of AGWRC and KVARY. However, since when KVARY is set to 0.0 seasonal recession is not represented and GWVS is not calculated. To avoid problems, then, AGWS should be set to 1.0 inch and GWVS to 0.0 for initial simulation runs.
If the simulation period is limited in duration, you can check and reset
these state variables to values observed for the same period in subsequent
years with similar climatic conditions. However, if major calibration
changes are made to the parameters controlling these storages (e.g. UZSN,
LZSN, INFILT), then the initial conditions should be checked
and adjusted during the calibration process. The values for AGWS
and GWVS should be checked and adjusted as noted above, which assuming
a yearly cycle of groundwater storage, can be compared to values during
similar seasons in the simulation period. If the initial simulated baseflow
(before the first significant rainfall) is much different from the initial
observed streamflow, then further adjustments can be made to raise or
lower the flow rates.
Impervious Land Hydrology (IWATER) Parameters
The IWAT-PARM1 table includes a number of flag variables to indicate either the selection of a simulation algorithm option, or whether the parameter will be treated as a constant or be varied monthly. As with PWAT-PARM1, where flags indicate monthly variability, corresponding monthly values must be provided in Monthly Input Parameter tables (see below following IWAT-PARM3 section).
CSNOFG Flag to use snow simulation data; must be checked (CSNOFG=1) if SNOW module is run.
RTOPFG Flag to select overland flow routing method. If RTOPFG=0, a new routing algorithm is used. RTOPFG=1 results in the use of the method used by predecessor models (HSPX, ARM, and NPS). Recommendation: set RTOPFG=1; this method is more commonly used and has been subjected to more widespread application.
VRSFG Flag to select constant or monthly-variable retention storage capacity, RETSC. Monthly values are not often used.
VNNFG Flag to select constant or monthly-variable Manning's n for overland flow plane, NSUR. Monthly values are not often used.
Flag to determine if lateral surface inflow to the impervious land
segment will be subject to retention storage (RTLIFG=1). This flag only
has an impact if the another land segment drains to the impervious land
segment; otherwise lateral surface inflow is nonexistent. This feature
is not commonly used in most HSPF applications.
LSURLength of assumed overland flow plane (feet), (measure/estimate). See PWAT-PARM2/ LSUR discussion. For impervious areas, LSUR reflects the overland flow length on directly connected, or effective impervious area (EIA), and is usually in the range of 50 to 150 feet, although longer lengths may apply in commercial or industrial regions of large metropolitan areas. Impervious surfaces that drain to pervious land, rather than to a reach, are considered part of the pervious land segment and not part of the EIA.
SLSURAverage slope of the assumed overland flow path (unitless), (measure/estimate). See PWAT-PARM2 / SLSUR discussion.
NSURManning's n for overland flow plane (estimate). See PWAT-PARM4 / NSUR discussion. Recommendation: set NSUR within the range of 0.05 to 0.10 for paved roads and parking lots.
(interception) storage of the impervious surface (inches) (estimate).
RETSC is the impervious equivalent to the interception storage variable
(CEPSC) used for pervious land segments. RETSC is the depth of water that
collects on the impervious surface before any runoff occurs. A study of
five urban watersheds in the Puget Sound region conducted by the U.S.
Geological Survey (Dinicola, 1990) found that a value of 0.10 for RETSC
was appropriate. If parking lots and rooftops are designed for detention
storage, larger values up to 0.5 inches may be reasonable.
The following two parameters are used only if SNOW is being simulated.
PETMAXTemperature below which ET will be reduced by 50% of that in the input time series (degree F), (estimate, then calibrate). See PWAT-PARM3 /PETMAX discussion.
at and below which ET will be set to zero (degree F), (estimate,
See PWAT-PARM3 /PETMIN discussion.
Monthly Input Parameter Tables:
Flow Routing (HYDR and ADCALC) Parameters
HSPF computes streamflow through a stream reach or reservoir based on two assumptions: (1) there is a fixed relationship between depth, volume, and discharge, and (2) discharge is a function of volume (see ODGTFG discussion for exception). This means that flow reversals and backwater effects in an upstream reach are not simulated. Routing is computed using storage routing or kinematic wave routing. Momentum is not considered in the routing computations.
VCONFG Flag to select constant or monthly-variable factors to adjust the outflow from the FTABLE discharge column, i.e. use the MON-CONVF table. Monthly values are not often used.
AUX1FG Flag to compute stream channel depth, stage, surface area, average depth (volume/surface area), top width (surface area/length), and hydraulic radius. AUX1FG must be set to 1 if precipitation and evaporation fluxes are calculated for the stream reach, or water quality is simulated for the reach.
AUX2FG Flag to compute average cross section area (volume/length) and average velocity (discharge/average cross sectional area). AUX2FG must be set to 1 if oxygen in the stream reach is simulated. If AUX2FG=1 then AUX1FG must equal 1.
AUX3FG Flag to compute bed shear velocity and bed shear stress. These values are used to calculate sediment deposition and scour (inorganic and organic) for a stream reach. If AUX3FG=1 then AUX2FG and AUX1FG must equal 1.
ODFVFG For use when the stream reach outflow demand is a function of volume. The value is the column in the appropriate FTABLE that contains stream reach discharge values. A maximum of five discharge exits can be specified for a single stream reach. For a stream reach with a single exit, the appropriate value of ODFVFG(1) is usually 4, since the FTABLE includes columns for depth, surface area, and volume in columns 1, 2, and 3, respectively. With two exits (e.g. natural exit plus a side diversion) ODFVFG(1)=4 and ODFVFG(2)=5.
If the value of ODFVFG is less than zero, then the absolute value is the column in the COLIND time series which designates which FTABLE column to use. The COLIND time series is used to vary the FTABLE column used in the computation of discharge on a seasonal or daily basis. For example, if ODFVFG=-1 then the model checks the appropriate COLIND value for that time step and if the COLIND value equals 6 then column 6 in the FTABLE is used in the outflow calculations for that particular stream reach. In addition, a fractional value for COLIND can be used to interpolate between columns in the FTABLE, e.g. a value of 4.3 can be used to interpolate between columns 4 and 5, with 30% of the difference between columns 4 and 5 being added to column 4. This option can be used to simulate different rule curves and to vary from one set to another.
ODGTFG Flag to specify that the stream reach outflow demand is a function of time. The value is the number corresponding to the OUTDGT subscript number specified in the External Sources Block. The OUTDGT number links the reach outflow demand to a flow time series data set in the WDM. A maximum of five discharge exits can be specified for a single stream reach. For a stream reach with a single exit, the appropriate value of ODGTFG(1) is usually 1 (the corresponding operation number of OUTDGT in External Sources will then be 1). With two exits (e.g. natural exit plus a side diversion) ODFVFG(1)=1 and ODFVFG(2)=2, corresponding to subscripts 1 and 2 of OUTDGT.
FUNCT Flag to combine ODFVFG and ODGTFG functions, if appropriate.
For each of a maximum of 5 possible exits the value of FUNCT can be 1
(select the smaller of the ODFVFG and ODGTFG values), 2 (larger of ODFVFG
and ODGTFG), or 3 (sum of ODFVFG and ODGTFG).
FTBDSN When the FTABLE is stored in the WDM file, this is the Data Set Number (DSN) in the WDM file for that reach's FTABLE. When the FTABLE is stored in the FTABLES block of the UCI file, FTBDSN is set to zero.
FTABNO When FTBDSN is zero, this is the Id number of the FTABLE as included in the FTABLES block of the UCI file. When FTBDSN > 0, this is the indicator to identify a particular FTABLE data set in the FTBDSN DSN.
LEN Length of the stream reach (miles), (measure). Length is used in the computation of auxiliary 1, 2, and 3 parameter values.
Use of BASINS Data/Tools:
DELTH Change in elevation from the upstream end of the stream reach to the downstream end (feet), (measure). DELTH is used if reaeration is computed using the Tsivoglou-Wallace equation in the OXRX Block or if sandload transport capacity is computed using either the Toffaleti or Colby method in the SEDTRN Block.
Use of BASINS Data/Tools:
STCOR Stage correction to convert RCHRES depth (DEP) to an equivalent stage or elevation (feet), (measure). STAGE (elevation) = DEP (depth) + STCOR. This parameter is relevant only if you want to compare reach depth reported as stage to modeled reach stage.
KS Weighting factor for hydraulic routing (unitless), (initialize with reported value). KS is a weighting factor applied in the computation of the stream reach outflow. The outflow for any given time step is KS times the outflow at the beginning of the time step plus the complement (1-KS) times the outflow at the end of the time step. Increasing KS from 0.0 to 1.0 increases the likelihood of model instability; the value 0.5 will produce the most accurate results. Recommendation: set KS=0.50.
DB50 Median diameter of the
bed sediment (inches), (estimate/measure). DB50 is used
to calculate (1) the bed shear stress if the stream reach is a lake, and
(2) the rate of sand transport if the Toffaleti or Colby method is used.
Note: DB50 is not connected with the sand particle diameter (D) input
in the SAND-PM table in the SEDTRN Block.
Monthly Input Parameter Table:
ADCALC calculates values for variables which are necessary to simulate longitudinal advection of dissolved or entrained constituents. These values are dependent on the volume and outflow values computed by the hydraulics section (HYDR) of the model.
CRRAT Ratio of the maximum velocity to the mean velocity in the stream channel cross section under typical flow conditions (unitless), (estimate/measure). CRRAT must be 1.0 or greater, where a value of 1.0 corresponds to completely uniform velocity (plug flow) across the stream channel. CRRAT is used to determine the relative volumes of water stored in the stream reach versus that leaving the reach, in a given time step. If CRRAT is greater than the volume:outflow ratio then the outflow is assumed to be in part, made up of water that entered the reach in that same interval. The inflow constituent concentration, then alters the outflow constituent concentration.
VOL Volume of water in the
stream reach at start of simulation (acre-feet) (estimate).
VOL is only used if section HYDR is inactive, and thus HYDR-INIT/VOL is
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HSPF ATEMP AND SNOW PARAMETERS AND VALUE RANGES