Climatology

Climate-related variables used for computing runoff and snowmelt are defined in the Climatology editor.

 

Figure: The Climatology editor for SWMM models

The editor organizes climatological data into the following groups:

• Temperature
• Evaporation
• Wind Speed
• Snowmelt
• Areal Depletion
• Adjustments

Temperature

Temperature data are used when simulating snowfall and snowmelt processes during runoff calculations. If these processes are not being simulated then no temperature data are needed.

Temperature data can be supplied from one of the following sources:

• A time series from the 'Time series' editor in the current project (values at intermediate times are found by interpolation)
• An external climate file. The following file formats are recognized:
  • Global Historical Climatology Network - Daily (GHCN-D) files, in text output format, available from NOAA's National Centers for Environmental Information (NCEI) Climate Data Online service
  • Older NCEI DS3200 or DS3210 files
  • Canadian climate files available from Environment Canada
  • A user-defined climate file where each line contains the 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 using a NOAA-GHCN climate file, the unit of temperature data in the file must also be selected..

For user-defined climate files, the following units must be used in the file when the project is in SI unit system:

• Temperatures in degrees C
• Evaporation in mm/day
• Wind speed in km/hour

The following units must be used when the project is in US unit system:

• Temperatures in degrees F
• Evaporation in inches/day
• Wind speed in miles/hour.

 

Figure: The SWMM Climatology Temperature tab

Edit field	Description	Used or required by simulations	Field name in datastructure
	Radio button for temperature data type	Yes	TypeNo
No Data	No temperature data	Yes if TypeNo = 1
Time Series ID	Temperature data read from time series defined in Time Series editor	Yes if TypeNo = 2	TimeSeriesID
Climate File	Temperature data read from NCDC *.DAT file	Yes if TypeNo = 3	FileName
Start Reading File at	Start date for reading from NCDC data file	Yes if TypeNo = 3	StartDate
Temperature units for GHCN files	Units of temperature values, when using an external NOAA-GHCN climate file	Yes if using an external NOAA-GHCN climate file	TempUnit

Table: Edit fields in the Temperature tab (mss_Temperature)

Evaporation

Evaporation can occur for standing water on catchment surfaces, for subsurface water in groundwater aquifers, and from water held in storage units. Evaporation rates can be defined as:

• A single constant value
• A set of monthly average values
• User-supplied time series of daily values
• Daily values read from a NCDC climate file
• Values computed from the daily temperatures in the external climate file

If a NCDC file is used, then a set of monthly pan coefficients should also be supplied to convert the pan evaporation data to free water surface values.

 

Figure: The Climatology Evaporation tab

Edit field	Description	Used or required by simulations	Field name in datastructure
	Radio button for evaporation data type	Yes	TypeNo
Constant Value	Evaporation is set to constant value TypeNo = 1	Yes if TypeNo = 1	ConstValue
Monthly Values TypeNo = 2
Jan	Monthly evaporation value for January entered by user	Yes if TypeNo = 2	Value1
Feb	Monthly evaporation value for February entered by user	Yes if TypeNo = 2	Value2
Mar	Monthly evaporation value for March entered by user	Yes if TypeNo = 2	Value3
Apr	Monthly evaporation value for April entered by user	Yes if TypeNo = 2	Value4
May	Monthly evaporation value for May entered by user	Yes if TypeNo = 2	Value5
June	Monthly evaporation value for June entered by user	Yes if TypeNo = 2	Value6
July	Monthly evaporation value for July entered by user	Yes if TypeNo = 2	Value7
Aug	Monthly evaporation value for August entered by user	Yes if TypeNo = 2	Value8
Sept	Monthly evaporation value for September entered by user	Yes if TypeNo = 2	Value9
Oct	Monthly evaporation value for October entered by user	Yes if TypeNo = 2	Value10
Nov	Monthly evaporation value for November entered by user	Yes if TypeNo = 2	Value11
Dec	Monthly evaporation value for December entered by user	Yes if TypeNo = 2	Value12
Time Series ID	Evaporation data read from time series defined in Time Series editor	Yes if TypeNo = 3	TimeSeriesID
Directly from Climate File TypeNo = 4
Jan	Evaporation data for January read from NCDC file	Yes if TypeNo = 4	Pan1
Feb	Evaporation data for February read from NCDC file	Yes if TypeNo = 4	Pan2
Mar	Evaporation data for March read from NCDC file	Yes if TypeNo = 4	Pan3
Apr	Evaporation data for April read from NCDC file	Yes if TypeNo = 4	Pan4
May	Evaporation data for May read from NCDC file	Yes if TypeNo = 4	Pan5
June	Evaporation data for June read from NCDC file	Yes if TypeNo = 4	Pan6
July	Evaporation data for July read from NCDC file	Yes if TypeNo = 4	Pan7
Aug	Evaporation data for August read from NCDC file	Yes if TypeNo = 4	Pan8
Sept	Evaporation data for September read from NCDC file	Yes if TypeNo = 4	Pan9
Oct	Evaporation data for October read from NCDC file	Yes if TypeNo = 4	Pan10
Nov	Evaporation data for November read from NCDC file	Yes if TypeNo = 4	Pan11
Dec	Evaporation data for December read from NCDC file	Yes if TypeNo = 4	Pan12
Computed from Temperatures in the Climate Files	Evaporation calculated based on temperature in the climate file	Yes if TypeNo = 5
Evaporate Only During Dry Periods	Option for setting evaporation to occur only during dry periods	Yes	DryOnlyNo
Monthly Soil Recovery Pattern	Monthly infiltration recovery rate pattern defined in Time Patterns editor	Yes	SoilRecoveryPatternID

Table: Edit fields in the Evaporation tab (mss_Evaporation)

Wind Speed

Wind speed is an optional climatological variable that is only used for snowmelt calculations. Define either a set of monthly average speeds, or wind speed data from the same NCDC climatological file used for daily min/max temperatures.

 

Figure: The Climatology Wind Speed tab

Edit field	Description	Used or required by simulations	Field name in datastructure
	Radio button for data type	Yes	WindSpeedTypeNo
No WindSpeed Data	No Windspeed data should be used	Yes if WindSpeedTypeNo = 1
Monthly Average Wind Speed WindSpeedTypeNo = 2
Jan	Monthly evaporation value for January entered by user	Yes if WindSpeedTypeNo = 2	Value1
Feb	Monthly evaporation value for February entered by user	Yes if WindSpeedTypeNo = 2	Value2
Mar	Monthly evaporation value for March entered by user	Yes if WindSpeedTypeNo = 22	Value3
Apr	Monthly evaporation value for April entered by user	Yes if WindSpeedTypeNo = 2	Value4
May	Monthly evaporation value for May entered by user	Yes if WindSpeedTypeNo = 2	Value5
June	Monthly evaporation value for June entered by user	Yes if WindSpeedTypeNo = 2	Value6
July	Monthly evaporation value for July entered by user	Yes if WindSpeedTypeNo = 2	Value7
Aug	Monthly evaporation value for August entered by user	Yes if WindSpeedTypeNo = 2	Value8
Sept	Monthly evaporation value for September entered by user	Yes if WindSpeedTypeNo = 2	Value9
Oct	Monthly evaporation value for October entered by user	Yes if WindSpeedTypeNo = 2	Value10
Nov	Monthly evaporation value for November entered by user	Yes if WindSpeedTypeNo = 2	Value11
Dec	Monthly evaporation value for December entered by user	Yes if WindSpeedTypeNo = 2	Value12
From Climate File	WindSpeed data read from the NCDC data file which is specified on the Temperature tab page	Yes if WindSpeedTypeNo = 3

Table: Edit fields in the Wind Speed tab (mss_Temperature)

Snowmelt

Snowmelt parameters are climatological variables that apply across the entire model area when simulating snowfall and snowmelt. They include:

• The temperature at which precipitation falls as snow
• Heat exchange properties of the snow surface
• Study area elevation, latitude, and longitude correction

For additional information on Snowfall and Snowmelt processes please consult the section on Snowpacks.

 

Figure: The Snowmelt tab page on the Climatology editor

Edit field	Description	Used or required by simulations	Field name in datastructure
Dividing Temperature Between Snow and Rain (degrees F)	Temperature below which precipitation falls as snow	Yes	SnowTemp
ATI Weight	This parameter reflects to what degree heat transfer within a snowpack during non-melt periods is affected by prior air temperatures. Smaller values reflect a thicker surface layer of snow which result in reduced rates of heat transfer. Values must be between 0 and 1.The default value is 0.5.	Yes	Atiwt
Negative Melt Ratio	This is the ratio of the heat transfer coefficient of a snowpack during non-melt conditions to the coefficient during melt conditions. It must be a number between 0 and 1. The default value is 0.6.	Yes	Rnm
Elevation Above MSL (feet)	Enter the average elevation above mean sea level for the study area, in feet or meters. This value is used to provide a more accurate estimate of atmospheric pressure. The default is 0.0, which results in a pressure of 29.9 inches Hg. The effect of wind on snowmelt rates during rainfall periods is greater at higher pressures, which occur at lower elevations.	Yes	Elev
Latitude (degrees North)	Enter the latitude, in degrees North, of the study area. This number is used when computing the hours of sunrise and sunset, which in turn are used to extend min/max daily temperatures into continuous values. The default is 50 degrees North.	Yes	Lat
Longitude Correction (minutes)	This is a correction, in minutes of time, between true solar time and the time on clocks. It depends on a location's longitude (q) and the standard meridian of its time zone (SM) through the expression 4 (q-SM). This correction is used to adjust the hours of sunrise and sunset when extending daily min/max temperatures into continuous values. The default value is 0	Yes	Dtlong

Table: Edit fields in the Snowmelt tab (mss_Temperature)

Areal Depletion

Areal Depletion refers to the tendency of snow to melt nonuniformly over the surface of a catchment. As the melting process proceeds, the area covered by snow is reduced. This behaviour can be described by an Areal Depletion Curve. It plots the fraction of total area that remains snow covered against the ratio of the actual snow depth to the depth at which there is 100% snow cover. A typical ADC for a natural area is shown below.

 

Figure: Example Areal Depletion Curve

Two such curves can be supplied; one for impervious areas and another for pervious areas.

 

Figure: The SWMM Areal Depletion tab

Edit field	Description	Used or required by simulations	Field name in datastructure
Impervious
0.0	Fraction of impervious area covered by snow at interval [0.0-0.1]	Yes	AdcImPerv0
0.1	Fraction of impervious area covered by snow at interval ]0.1-0.2]	Yes	AdcImPerv1
0.2	Fraction of impervious area covered by snow at interval ]0.2-0.3]	Yes	AdcImPerv2
0.3	Fraction of impervious area covered by snow at interval ]0.3-0.4]	Yes	AdcImPerv3
0.4	Fraction of impervious area covered by snow at interval ]0.4-0.5]	Yes	AdcImPerv4
0.5	Fraction of impervious area covered by snow at interval ]0.5-0.6]	Yes	AdcImPerv5
0.6	Fraction of impervious area covered by snow at interval ]0.6-0.7]	Yes	AdcImPerv6
0.7	Fraction of impervious area covered by snow at interval ]0.7-0.8]	Yes	AdcImPerv7
0.8	Fraction of impervious area covered by snow at interval ]0.8-0.9]	Yes	AdcImPerv8
0.9	Fraction of impervious area covered by snow at interval ]0.9-1.0]	Yes	AdcImPerv9
Pervious
0.0	Fraction of pervious area covered by snow at interval [0.0-0.1]	Yes	AdcPerv0
0.1	Fraction of pervious area covered by snow at interval ]0.1-0.2]	Yes	AdcPerv1
0.2	Fraction of pervious area covered by snow at interval ]0.2-0.3]	Yes	AdcPerv2
0.3	Fraction of pervious area covered by snow at interval ]0.3-0.4]	Yes	AdcPerv3
0.4	Fraction of pervious area covered by snow at interval ]0.4-0.5]	Yes	AdcPerv4
0.5	Fraction of pervious area covered by snow at interval ]0.5-0.6]	Yes	AdcPerv5
0.6	Fraction of pervious area covered by snow at interval ]0.6-0.7]	Yes	AdcPerv6
0.7	Fraction of pervious area covered by snow at interval ]0.7-0.8]	Yes	AdcPerv7
0.8	Fraction of pervious area covered by snow at interval ]0.8-0.9]	Yes	AdcPerv8
0.9	Fraction of pervious area covered by snow at interval ]0.9-1.0]	Yes	AdcPerv9

Table: Edit fields in the Areal Depletion tab (mss_Temperature)

Adjustments

Climate Adjustments are optional modifications applied to the temperature, evaporation rate, rainfall intensity, and hydraulic conductivity 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.

Adjustments to the hydraulic conductivity are 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 catchments (and their LID units) by assigning a monthly infiltration adjustment Time Pattern to a catchment. Monthly adjustment time patterns for depression storage and pervious surface roughness coefficient (Manning n) can also be specified for individual catchments (see chapter SWMM Hydrology and Hydraulic Properties).

 

Figure: The SWMM Climatology Adjustments tab

Edit field	Description	Used or required by simulations	Field name in datastructure
Temp	Temperature adjustment (+-) for the months January-Decembers	Yes	Temp01-Temp12
Evap	Evaporation adjustment (+-) for the months January-Decembers	Yes	Evap01-Evap12
Rain	Rainfall multiplier for the months January-Decembers	Yes	Rain01-Rain12
Cond	Soild conductivity multiplier for the months January-Decembers	Yes	Con01-Con12

Table: Edit fields in the Adjustments tab (mss_Adjustment.Default_Adjustment)