xpswmm/xpstorm Resource Center xps

This page contains the following topics:

General Information

Sacramento County, California, USA has been estimating runoff flows for the design of drainage facilities since the early 1960's using charts and equations developed by George S. Nolte Consulting Civil Engineers. Use of these charts and equations has historically been referred to as the Nolte method. The City of Sacramento has used constant unit discharge rates similar to those given on the Nolte charts for small areas. 

More recently, the City and County of Sacramento have developed the Sacramento method, based on the Bureau of Reclamation urban unit hydrograph. The new method takes advantage of the advances in computer modeling techniques and the availability of current rainfall and runoff data. The Sacramento method can be used for all drainage design but is especially suited for design of large drainage facilities and master planning studies.

  1. To invoke the Sacramento Hydrology options, open the swmxp.ini file with a text editor and add the following block of text.

    SAC_FLOW_FILE = c:\xps\xpswmm\templates\SACH\Rnf_SacFlows.txt
    SACH_TABLE4_1= c:\xps\xpswmm\templates\SACH\4_1.txt
    SACH_TABLE4_4= c:\xps\xpswmm\templates\SACH\4_4.txt
    SACH_TABLE4_5= c:\xps\xpswmm\templates\SACH\4_5.txt
    SACH_TABLE4_6= c:\xps\xpswmm\templates\SACH\4_6.txt
    SACH_TABLE4_7= c:\xps\xpswmm\templates\SACH\4_7.txt
    SACH_TABLE4_8= c:\xps\xpswmm\templates\SACH\4_8.txt
    SACH_TABLE6_1= c:\xps\xpswmm\templates\SACH\6_1.txt
    SACH_TABLE5_1= c:\xps\xpswmm\templates\SACH\5_1.txt
  2. Verify that the text files containing the tabular data are located in the proper folder.  The files mentioned above can be downloaded from the resource downloads or using this link.
  3. Set the mode to Runoff.
  4. On the Configuration menu, select Job Control then Mode Properties. Click on Methods and select Sacramento Method.
  5. Note that the Runoff Mode Job Control dialog has been switched to Sacramento Hydrology. Use this dialog to specify either the Nolte Method or the Sacramento Hydrology Method. In Runoff Mode, the node data dialogs will be different depending on which Method is selected.

Job Control Options


Select Zone 1, Zone 2 or Zone 3 from the drop list. Refer to the diagram below for the Hydrologic Zones as defined by the Nolte Method. Click the image to view a larger version: 

Source: County of Sacramento Master Drainage Plan, Part 1, County-wide Hydrology, Nolte and Assoc.

Ramp Time

Enter the time in minutes for the runoff rate to reach the Nolte Method value.

Generate User Defined Hydrograph in Hydraulics

When the model is solved, xp will generate a ramped user inflow hydrograph for each runoff node. Switch to Hydraulics Mode to view.

NEW IN 2023.2  Negative Flows will now be pushed into Hydraulic Mode as User-Defined Hydrographs.  Prior releases did not adjust flows lower when the Nolte flows calculated negative flows.  This will result in lower flows in the hydraulics mode.  Refer to User-Defined Inflow to turn off negative flows if preserving the older behavior.

Nolte Method Options

Node Options

While in Runoff Mode, select the node to be edited, and select Edit Data. Alternatively, double-click a node to launch the node data dialog, and enter the data for each subcatchment.


Check if subcatchment is active


Subcatchment area, in acres

Impervious Composition

Select value from drop list

Flow – Nolte

Calculated when Update button is clicked


Add another subcatchment


Remove selected subcatchment 

Sacramento County Method Options

Design Storm

Use this dialog to specify the Recurrence Interval and the Duration of the design storm in the Sacramento Method.  Only one storm should be selected in this dialog.  Use Scenarios to simulate a batch of different design storms.

Adjustments to design storm:

  • Select Storm Area
  • Use Watershed Area 
  • Specify Storm Area, acres
  • Check to Use Frequency Factor 

Historical Storm

Use this dialog to specify an Historical Storm in the Nolte Method.

  • Click on the ellipses (…) to open a Windows Explorer window. Navigate to the file containing the storm data.
  • Enter the initial loss, in.
  • Check the box to use a lag frequency factor for 1, 2 or 3 year design.

Sacramento Method Node Options

If Sacramento Method is selected in the Job Control options, double-clicking a node or selecting Edit Data from the right-click menu will launch the dialog below. 

General subcatchment parameters


Node area, in acres

Mean Elevation

Node Elevation,  in feet

Precipitation zone

Select 1, 2 or 3 from the drop list. 

Lag: Compute from Travel Time

Use this dialog to compute the travel time as the sum of travel times for the three flow paths.

Check to include each flow path in the calculation and enter the required data:

Upstream land use. Select the land use category and enter the slope, ft/ft.

Gutter Flow. Enter the length, ft, side slope ft/ft and the longitudinal slope, ft/ft

Channel and pipe flow. Insert a row for each pipe segment. Select channel type from the drop list. Enter the length, ft, slope ft/ft, diameter, ft, design flow, cfs, and Mannings n.

Lag from Basin Characteristics

Use this dialog to enter the basic characteristics for the lag time calculation.


Enter the Length of the longest catchment, in feet.

Enter the Length along the longest catchment, in feet

Enter the subcatchment Slope, in feet. 

For the Mannings 'n' value (Basin "n") select either Compute, or Specify a value. Clicking Compute launches the Channel Land Use window: 

Use this dialog to define the distribution of land use for calculation of Mannings n. This value is used for travel time calculation in Sacramento County runoff method.

Enter the values in the Land Use rows in percent or acres. Check the appropriate radio button.

Infiltration options

Loss rate. Compute from Land Use, or Specify the rate in inches/hour. 

Impervious Area. Compute from Land Use, or Specify the percentage of the area. 

Clicking the Land Use button button launches the Land Use and Soil Group dialog: 

Use this dialog to define the distribution of land use for calculation of Loss Rate and Impervious Area. For each land use, enter the area in Soil Groups A, B, C or D as a percent or in acres.

Click on OK to return to the Sacramento County Method Hydrology dialog.