Rainfall:Temporally varying multiple gage rainfall
To assign rainfall data from recorded gages, toggle on Multiple-Gage Rainfall, and select a method of distributing the rainfall, by either Thiessen polygons or Inverse distance weighted. WMS does not currently support the file GSSHA requires for temporally varying multiple rain gage input. If you select View Event File, WMS will automatically put you in an editor to create or edit the necessary file. The default editor is Microsoft Notepad. The file may also be created separately from WMS with any appropriate software.
The rainfall file consists of storm event header information, a description of each rain gage, and then cards with a time and value for each rainfall data point. For long-term simulations with multiple rain events, the single-event format is repeated for each additional event.
At the top of the file is a description of the event. This is followed by the number of rain data points in the rain time series. The next line specifies the number of rain gages. This is followed by a series of four-column lines containing the easting, northing, gage name, and the time series name for each rain gage. Rain gages need not be located within the watershed of study. If a gage falls in a different UTM zone to the left of the zone the watershed is in, then negative values can be entered.
Rain gages located well outside the watershed under analysis generally provide poor rainfall estimates. The instantaneous correlation distance for convective rainfall is typically on the order of a few kilometers. Use caution when using data from rain gages further than a few kilometers from the catchment.
The remaining lines at the bottom of the rain gage file reflect the temporal variation of rainfall intensity. The number of columns per line equals the number of rain gages, each being separated by space. The number of lines in this lower portion is equal to number of rainfall periods (NRPDS).
Each rainfall file must have the following cards:
- EVENT – simple description string of the event, must be set in double quotations.
- NRGAG - number of rainfall gages to follow (integer value).
- NRPDS - number of rainfall data points to follow (integer value).
- COORD - coordinate, easting and northing of gage, one card for each gage (NRGAG), must have an identifying string in double quotations, and need not have the same number or locations of gages for different storm events when multiple events are simulated.
- GAGES, RADAR, RATES, ACCUM - rainfall data point. The number of these cards must be NRPDS. Each card specifier should be followed by Year, Month, Day, Hour, Minute, and then one value of rainfall (decimal format) for each NRGAG. The proper card name depends on the type of rainfall entered. The four types are:
- GAGES - rainfall accumulation (mm) over the last time period.
- RADAR -rainfall rate (mm×hr-1) for the last time interval.
- RATES - rainfall rate (mm×hr-1) for the next time interval.
- ACCUM - cumulative amount of rainfall up until that time period (mm).
- Some guidelines for use are:
- Specified rainfall types cannot change within a storm event.
- The time interval can be any value, but there must be a rainfall value for each NRGAG.
- Separate values with spaces or tabs.
- Names of events and gages are NOT optional and must be in double quotation marks, as shown below.
The proper format is shown below. For this example there are three gages and five rainfall data points. The GSSHA User’s Manual should be consulted for additional information on constructing rainfall files.
Within GSSHA, the values of rainfall at the gages are interpolated to the grid cells. Either Thiessen polygons or inverse distance-squared interpolation are used. For RADAR type rainfall inputs, Thiessen polygons should be selected.