Rainfall data is the single most important group of hydrologic data required by XPSWMM. A hyetograph of rainfall intensities versus time is required for the period of the simulation. For single event simulation, the data is usually entered for a gauge directly as a synthetic design storm. For continuous simulation, an historical rainfall sequence is normally used.


Each subcatchment references a single rain gauge. The data for each rain gauge may be input manually or defined to come from an external interface file. It is allowable to use a combination of manually entered rain gauges and some to be read in from an external interface file.

Spatial variability in rainfall is handled by assigning different rainfall gauges to different subcatchments. Storm motion may significantly affect hydrographs at the catchment outlet.

 

 When the Use Simulation Start Time for Rainfall Event box in the Runoff Time Control dialog is checked, input to the start date/time boxes is disabled. When the rainfall data specifies a start time, the data in the date/time boxes is ignored.


RAIN Interface file (JK < 0)

This option allows the rainfall data to be read in from an interface file (as specified within the 'Interface Files' menu option), and is usually associated with continuous simulation.

The interface file may have been created by a previous run of the Runoff module or by the Rainfall utility. These mechanisms allow use of National Weather Service and Atmospheric Environment Service data for many stations. These time series are usually available in hourly or 15-min intervals.

Continuous simulation offers an excellent, if not the only method of obtaining the frequency of events of interest, be they related to quantity or quality. Continuous simulation, however, has the disadvantages of higher cost and the need for a continuous rainfall record.

Station Number (JK)

An integer number representing the rain station. Use the select button to read the station numbers from the interface file and allow one to be selected.

If an error occurs reading the interface file it may be because the file is not in the expected format. If the interface file was not created by XPSWMM it may need to be converted.

User Input Rainfall (JK > 0)

This option allows rainfall intensity sequences to be directly entered. This method is most appropriate in a single event simulation, where a "design storm" is used. Design events can be obtained by considering historical events or generating synthetic events. Synthetic events may be constructed by the following steps:

  1. A storm duration is chosen, either arbitrarily or to coincide with the assumed catchment time of concentration. Note that time of concentration also depends on rainfall intensity and other parameters.
  2. A return period is chosen in order to select the total storm depth for the specified duration from intensity-duration-frequency (IDF) curves.
  3. A time history, or shape, for the storm is assumed, usually on the basis of historical percentage mass curves.
  4. The continuous hyetograph thus produced is discretised into a histogram for input into the model. Some of the conceptual problems associated with this procedure for constructing synthetic hyetographs are:

IDF curves themselves may consist of components of several different storms, thus not representing the time history of any storm. IDF curves cannot be used to assign frequencies to storm volumes independent of duration. Antecedent conditions must be chosen arbitrarily.

Constant Time Intervals

If Constant Time Intervals is selected a fixed interval must be entered and a rainfall amount must be entered for each interval, starting at time zero (the Start Time entered below).

Periods of zero rainfall must be explicitly entered as zeros. The list is open-ended and may be navigated by using the arrow keys, <pg up>, <pg down>, <home> and <end>. The row number of the current cursor location is displayed as the current item at the bottom of the dialog.

 

Time 

The time interval may be entered as hours or minutes depending on the option selected.

Minutes (KTIME = 0). Enter the rainfall interval as a number of minutes.

Hours (KTIME = 1). Enter the rainfall interval as a number of hours.

Time Interval (THISTO). Number of hours or minutes between consecutive rainfall data intervals. This time interval should be at least equal to the wet computation time step (from the Time Control dialog under Job Control), or an integer multiple thereof. Information may be lost by averaging over discrete rainfall intensities.

Rainfall (RAIN) 

Each rainfall amount is an Intensity or Depth for the sequence corresponding to the Item Number.  

Intensity (KPREP = 0). Enter rain data in in./hr [mm/hr]

Depth (KPREP = 1). Enter rainfall data as a depth over the interval, in. [mm]. The start time of the top left item is the rain gauge Start Date and Start Time. The start time increases by the interval specified as the item number increases. Any items left blank will be ignored rather than treated as zero. Periods of zero rainfall must be explicitly entered as zeros. Units are in inches/hr [mm/hr] for intensity, or inches [mm] for depth.

If single event snow melt is being simulated, snowfall during an interval may be entered as a negative value, in./hr [mm/hr] or in. [mm] water equivalent.

Multiplier. This item allows the user to easily multiply the rainfall values by a given amount allowing input of dimensionless temporal patterns. All rainfall values entered will be multiplied by this amount.

Variable Time Intervals

Rainfall data over variable time intervals. If variable time intervals is selected a start time and duration is required for each rainfall amount. This option eliminates the necessity of entering zero values for periods of no rainfall. Care must be taken to ensure rainfall intervals do not overlap. The list is open-ended and may be navigated by using the arrow keys, <pg up>, <pg down>, <home> and <end>. The row number of the current cursor location is displayed as the current item at the bottom of the dialog.

 

Rainfall (RAIN)

Each rainfall amount is a cumulative or absolute depth or an Intensity or Depth for the sequence depending on the option selected. 

Cumulative Depth (KPREP = 1). This option defines rainfall data as a cumulative depth over the interval, in. [mm].

Absolute Depth. This option defines rainfall data as an absolute depth over the interval.

Intensity (KPREP = 0). This option defines rain data as an intensity, in./hr [mm/hr].

Time

The time interval may be entered as hours or minutes depending on the option selected.

Minutes (KTIME = 0). This option defines rainfall interval start times and durations in decimal minutes.

Hours (KTIME = 1). This option defines rainfall interval start times and durations in decimal hours.

Time Interval (WTHIS1, REIN1)

The start time of the rainfall interval is the Rain Gauge Start Date and Start Time (as entered in the previous dialog). This may be entered in minutes or decimal hours depending on the option chosen.

Duration ( WTHIS2, WTHIS3)

Duration of the interval of measured rainfall. This may be entered in minutes or decimal hours depending on the option chosen.

Rainfall (REIN2)

Rainfall over the specified interval in in./hr [mm/hr] for intensity OR inches [mm] for depth.

If single event snow melt is being simulated, snowfall during an interval may be entered as a negative value, in./hr [mm/hr] or in. [mm] water equivalent.

Tipping Bucket Gauge

This allows input of tipping bucket rain gauge rain data.

 

Date. Date on which rainfall value was recorded.

Time. Time at which rainfall value was recorded.

Rainfall. Total depth of rainfall since last value.

User Defined Rainfall Data

This option and the proceeding dialogs provide users the ability to choose a rainfall data file.