Should IDF analysis use a calendar year or a water year?
In short: Use the year definition that keeps the local wet season inside one analysis year. A calendar year runs from January through December. Under the common U.S. Geological Survey convention, a water year runs from October through September and is named for the year in which it ends.
Why the year boundary matters
An annual-maximum-series analysis selects the largest event in each analysis year. If a wet season crosses December and January, a calendar-year boundary can divide that season between two years. A water-year or custom starting month can keep the complete seasonal cycle together and make the yearly maxima more representative.
How the IDF Curve Tool handles year type
The tool defaults to January as the starting month but permits any month. For example, water year 2025 under the October-start convention covers October 2024 through September 2025. The selected year type is also used when the software checks whether the first and last years are complete.
Choose the boundary from local hydrology and data coverage rather than assuming October is appropriate everywhere. Document the selected starting month so another analyst can reproduce the maximum series.
See Year Type and Maximums in the IDF tutorial for the software steps.
Use the IDF Curve Tool with a calendar, water, or custom-start year.