Rainfall intensity and total rainfall depth are different quantities. IDF curves relate average intensity to duration and frequency at a location. An unsuitable duration or another region's curve affects discharge. Owners can use this selection to understand the resulting flow; engineers check source validity, time resolution, units and consistency with the calculation method.
Converting depth to intensity
For depth P in millimetres and duration t in minutes, average intensity is i = P × 60/t in mm/h. Twenty-five millimetres in thirty minutes gives 50 mm/h, not constant rainfall each minute. Show P, t and i with units. Treating minutes as hours or entering depth directly as intensity produces a major error rather than a rounding difference.
Choosing duration and probability
IDF graphs generally place duration horizontally and intensity vertically, with separate frequency curves. Select the justified return period and a duration suitable for the method. Rational-method critical duration and hydrograph storm duration are not interchangeable. Longer durations may reduce average intensity while increasing depth. Avoid uncontrolled extrapolation outside the source range and check whether axes are linear or logarithmic.
Station and record suitability
Proximity alone is insufficient: assess elevation, terrain, regional rainfall and data quality. Record observation period, short-duration coverage and updates. Daily totals do not directly represent short-duration intensity. Justify station transfer or regional methods. For larger catchments, assess point-to-area effects and spatial variation rather than select a curve simply because its location name is familiar.
IDF curves and hyetographs differ
An IDF curve gives duration-average intensity; a hyetograph describes time distribution. Equal storm depths with different timing can produce different peak flows. Define distribution, total depth and computation interval together. Uniform application of an IDF intensity is an explicit modelling assumption. Rainfall and resulting flow return periods are not automatically equal.
Quotation and calculation checks
Collect station source, record period, curve or equation files, duration units and return periods. Compare one equation result with its graph and independently check depth-to-intensity conversion. Record extrapolation and parameter versions. The owner should understand why the rainfall was chosen and what would change with another method. Traceable inputs make later revisions easier to target.
Traceability and reporting scope
For traceable hydrological calculations, record the source, observation period, units and missing-data treatment of the rainfall or streamflow series. Keep the catchment boundary, area calculation, land use, soil information and parameters of the selected method in the same data package. Calculation sheets should show inputs and unit conversions as well as the answer. Another engineer opening the files should be able to understand where each datum came from and the step in which it was used.
It is helpful for the project owner to share the location, study boundary and available mapping at the outset. Missing information does not automatically justify completing the calculation with a single assumption: surveys, alternative data or a narrower scope may be needed. Explain why the selected method suits the catchment and study purpose. The report should present uncertainties and checking scenarios alongside the central assumptions. Hydrological discharge and hydraulic water level are different outputs; where needed, a separate hydraulic model evaluates the effect of that discharge on structures or parcel elevations.
Use the related calculation tool
Sources and related articles
Let's discuss your project's data requirements and reporting scope.
Request a project quote