CATCH Algorithm

The CATCH product algorithm takes the RAIN1 products for the specified integration time (hours) and performs the following steps:

  1. For each subcatchment, all pixels in the RAIN1 product that are within or touching the subcatchment perimeter are added together and then divided by the total number of pixels.

    Thresholded pixels are treated as 0 rainfall.

    The result is the average hourly rainfall for the subcatchment.

  2. This is repeated for each hour and each subcatchment.

  3. The sum over all hours in the integration time is taken for each subcatchment to get a total average accumulation for the time period.

    If the Issue Warn button is enabled, this sum is checked against the catch file warning threshold (adjusted by the Offset and Multiplier) and a single IRIS warning is issued for all subcatchment areas that exceed the threshold.

For each subcatchment, the final output file contains:

  • The LAT/LON of the label point, the point name and index number.

  • The hourly average accumulations over the selected integration time.

    These are used for the bar graph display.

  • The total average accumulation for the integration time.
  • The fraction of pixels in the subcatchment that had non-zero precipitation.

The output file does not contain the subcatchment boundary definitions. This has implications for display when the CATCH product is transferred to another machine. See Displaying CATCH.