Refactoring the SWMM 5 Help File – Climate Data in SWMM5

Climate Data in SWMM5


The Climatology object in EPA SWMM describes the following climate-related variables used for computing runoff and snowmelt:

· Temperature
· Evaporation
· Wind Speed
· Snowmelt
· Areal Depletion
· Climate Adjustments
Rain Gages

Rain Gages supply precipitation data for one or more subcatchment areas in a study region. The rainfall data can be either a user-defined time series or come from an external file. Several different popular rainfall file formats currently in use are supported, as well as a standard user-defined format.

The principal input properties of rain gages include:

· rainfall data type (e.g., intensity, volume, or cumulative volume)
· recording time interval (e.g., hourly, 15-minute, etc.)
· source of rainfall data (input time series or external file)
· name of rainfall data source

See Also

Rain Gage Properties

Rainfall Files

Rainfall Files

SWMM’s rain gage objects can utilize rainfall data stored in external Rainfall Files. The program currently recognizes the following formats for storing such data:

· Hourly and fifteen minute precipitation data from over 5,500 reporting stations retrieved using NOAA’s National Climatic Data Center (NCDC) Climate Data Online service (www.ncdc.noaa.gov/cdo-web) (space delimited format only).
· The older DS-3240 and related formats used for hourly precipitation by NCDC.
· The older DS-3260 and related formats used for fifteen minute precipitation by NCDC.
· HLY03 and HLY21 formats for hourly rainfall at Canadian stations, available from Environment Canada at www.climate.weather.gc.ca.
· FIF21 format for fifteen minute rainfall at Canadian stations, also available from Environment Canada.
· a standard user-prepared format where each line of the file contains the station ID, year, month, day, hour, minute, and non-zero precipitation reading, all separated by one or more spaces.

When requesting data from NCDC’s online service, be sure to specify the TEXT format option, make sure that the data flags are included, and, for 15-minute data, select the QPCP option and not the QGAG one.

An excerpt from the user-prepared format might look as follows:

STA01 2004 6 12 00 00 0.12

STA01 2004 6 12 01 00 0.04

STA01 2004 6 22 16 00 0.07

This format can also accept multiple stations within the same file.

When a rain gage is designated as receiving its rainfall data from a file, the user must supply the name of the file and the name of the recording station referenced in the file. For the standard user-prepared format, the rainfall type (e.g., intensity or volume), recording time interval, and depth units must also be supplied as rain gage properties. For the other file types these properties are defined by their respective file format and are automatically recognized by SWMM.

Rain Gage Properties
Name User-assigned rain gage name.
X-Coordinate Horizontal location of the rain gage on the Study Area Map. If left blank then the rain gage will not appear on the map.
Y-Coordinate Vertical location of the rain gage on the Study Area Map. If left blank then the rain gage will not appear on the map.
Description Click the ellipsis button (or press Enter) to edit an optional description of the rain gage.
Tag Optional label used to categorize or classify the rain gage.
Rain Format Format in which the rain data are supplied:

INTENSITY: each rainfall value is an average rate in inches/hour (or mm/hour) over the recording interval.

VOLUME: each rainfall value is the volume of rain that fell in the recording interval (in inches or millimeters).

CUMULATIVE: each rainfall value represents the cumulative rainfall that has occurred since the start of the last series of non-zero values (in inches or millimeters).

Rain Interval Recording time interval between gage readings in decimal hours or hours:minutes format.
Snow Catch Factor Factor that corrects gage readings for snowfall.
Data Source Source of rainfall data; either TIMESERIES for user-defined time series data or FILE for an external data file.
– Series Name Name of time series with rainfall data if Data Source selection was TIMESERIES; leave blank otherwise (double-click to edit the series).
– File Name Name of external file containing rainfall data (see Rainfall Files).
– Station No. Recording gage station number.
– Rain Units Depth units (IN or MM) for rainfall values in user-prepared files (other standard file formats have fixed units depending on the format).
Climatology Editor

The Climatology Editor is used to enter values for various climate-related variables required by certain SWMM simulations. The dialog is divided into six tabbed pages, where each page provides a separate editor for the following data categories:

· Temperature
· Evaporation
· Wind Speed
· Snowmelt
· Areal Depletion
· Adjustments
Areal Depletion Page

The Areal Depletion page of the Climatology Editor dialog is used to specify points on the Areal Depletion Curves for both impervious and pervious surfaces within a project’s study area. These curves define the relation between the area that remains snow covered and snow pack depth. Each curve is defined by 10 equal increments of relative depth ratio between 0 and 0.9. (Relative depth ratio is the ratio of an area’s current snow depth to the depth at which there is 100% areal coverage).

Enter values in the data grid provided for the fraction of each area that remains snow covered at each specified relative depth ratio. Valid numbers must be between 0 and 1, and be increasing with increasing depth ratio.

Clicking the Natural Area button fills the grid with values that are typical of natural areas. Clicking the No Depletion button will fill the grid with all 1’s, indicating that no areal depletion occurs. This is the default for new projects.


Evaporation can occur for standing water on subcatchment surfaces, for subsurface water in groundwater aquifers, for water traveling through open channels, and for water held in storage units. Evaporation rates can be stated as:

· a single constant value
· a set of monthly average values
· a user-defined time series of values
· values computed from the daily temperatures contained in an external climate file
· daily values read from an external climate file.

Note that these represent potential rates. The actual amount of water evaporated will depend on the amount available.

If rates are read directly from a climate file, then a set of monthly pan coefficients should also be supplied to convert the pan evaporation data to free water-surface values.

An option is also available to allow evaporation only during periods with no precipitation.

See Also

Climate Files

Climatology Editor

Evaporation Page

The Evaporation page of the Climatology Editor dialog is used to supply potential evaporation rates, in inches/day (or mm/day), for a study area. There are five choices for specifying these rates:

· Constant Value

Use this choice if evaporation remains constant over time. Enter the value in the edit box provided.

· Time Series

Select this choice if evaporation rates will be specified in a time series. Enter or select the name of the time series in the dropdown combo box provided. Click the button to bring up the Time Series Editor for the selected series. Note that for each date specified in the time series, the evaporation rate remains constant at the value supplied for that date until the next date in the series is reached (i.e., interpolation is not used on the series).

· Directly From Climate File

This choice indicates that daily evaporation rates will be read from the same climate file that was specified for temperature. Enter values for monthly pan coefficients in the data grid provided.

· Computed from Temperatures

The Hargreaves’ method will be used to compute daily evaporation rates from the daily air temperature record contained in the external climate file specified on the Temperature page of the dialog. This method also uses the site’s latitude, which can be entered on the Snowmelt page of the dialog even if snow melt is not being simulated.

· Monthly Averages

Use this choice to supply an average rate for each month of the year. Enter the value for each month in the data grid provided. Note that rates remain constant within each month.

· Evaporate Only During Dry Periods

Select this option if evaporation can only occur during periods with no precipitation.

In addition this page allows the user to specify an optional Monthly Soil Recovery Pattern. This is a time pattern whose factors adjust the rate at which infiltration capacity is recovered during periods with no precipitation. It applies to all subcatchments for any choice of infiltration method. For example, if the normal infiltration recovery rate was 1% during a specific time period and a pattern factor of 0.8 applied to this period, then the actual recovery rate would be 0.8%. The Soil Recovery Pattern allows one to account for seasonal soil drying rates. In principle, the variation in pattern factors should mirror the variation in evaporation rates but might be influenced by other factors such as seasonal groundwater levels. The button is used to launch the Time Pattern Editor for the selected pattern.

Climate Files

SWMM can use an external Climate File that contains daily air temperature, evaporation, and wind speed data. The program currently recognizes the following formats:

· Global Historical Climatology Network – Daily (GHCN-D) files (TEXT output format) available from NOAA’s National Climatic Data Center (NCDC) Climate Data Online service at www.ncdc.noaa.gov/cdo-web.
· Older NCDC DS3200 or DS3210 files.
· Canadian climate files available from Environment Canada at www.climate.weather.gc.ca.
· A user-prepared climate file where each line contains a recording station name, the year, month, day, maximum temperature, minimum temperature, and optionally, evaporation rate, and wind speed. If no data are available for any of these items on a given date, then an asterisk should be entered as its value.

When a climate file has days with missing values, SWMM will use the value from the most recent previous day with a recorded value.

For a user-prepared climate file, the data must be in the same units as the project being analyzed. For US units, temperature is in degrees F, evaporation is in inches/day, and wind speed is in miles/hour. For metric units, temperature is in degrees C, evaporation is in mm/day, and wind speed is in km/hour.
Climate Adjustments

Climate Adjustments are optional modifications applied to the temperature, evaporation rate, and rainfall intensity that SWMM would otherwise use at each time step of a simulation. Separate sets of adjustments that vary periodically by month of the year can be assigned to these variables. They provide a simple way to examine the effects of future climate change without having to modify the original climatic time series.

In a similar manner, a set of monthly adjustments can be applied to the hydraulic conductivity used in computing rainfall infiltration on all pervious land surfaces, including those in all LID units, and exfiltration from all storage nodes and conduits. These can reflect the increase of hydraulic conductivity with increasing temperature or the effect that seasonal changes in land surface conditions, such as frozen ground, can have on infiltration capacity. They can be overridden for individual subcatchments (and their LID units) by assigning a monthly infiltration adjustment Time Pattern to a subcatchment. Monthly adjustment time patterns for depression storage and pervious surface roughness coefficient (Mannings n) can also be specified for individual subcatchments (see Subcatchment Properties).

Adjustments Page

The Adjustments page of the Climatology Editor dialog is used to supply a set of monthly adjustments applied to the temperature, evaporation rate, and rainfall that SWMM uses at each time step of a simulation:

· The monthly Temperature adjustment (plus or minus in either degrees F or C) is added to the temperature value that SWMM would otherwise use in a specific month of the year.
· The monthly Evaporation adjustment (plus or minus in either in/day or mm/day) is added to the evaporation rate value that SWMM would otherwise use in a specific month of the year.
· The monthly Rainfall adjustment is a multiplier applied to the precipitation value that SWMM would otherwise use in a specific month of the year.
· The monthly Conductivity adjustment is a multiplier applied to the saturated hydraulic conductivity associated with surface infiltration for a subcatchment, a LID unit, or seepage out of a conduit or storage unit. The multiplier is not applied when computing soil percolation rates, deep groundwater losses, or seepage from the bottom of LID units.

Leaving a monthly adjustment blank means that there is no adjustment made in that month.

Note that the same adjustment is applied for each time period within a given month and is repeated for that month in each subsequent year being simulated.

Two such curves can be supplied to SWMM, one for impervious areas and another for pervious areas.

See Also Climatology Editor


Air temperature data are used when simulating snowfall and snowmelt processes during runoff calculations. If these processes are not being simulated then temperature data are not required. Air temperature data can be supplied to SWMM from one of the following sources:

· a user-defined time series of point values (values at intermediate times are interpolated)
· an external climate file containing daily minimum and maximum values (SWMM fits a sinusoidal curve through these values depending on the day of the year).

For user-defined time series, temperatures are in degrees F for US units and degrees C for metric units. The external climate file can also be used to supply evaporation and wind speed as well.

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