Storm Duration
24 hours
The complete design rainfall depth is distributed through one day.
HEC-HMS · Input preparation
Rainfall & Design Storm
This page documents the preparation of the Hare rainfall input used in HEC-HMS: daily rainfall, annual maximum rainfall, frequency analysis, adopted return-period rainfall depths and the temporal design storm applied to the event simulations.
Working rule:
monthly rainfall used for irrigation planning is not the HEC-HMS flood
input. Flood analysis begins from the daily rainfall series
and the corresponding annual maximum rainfall record.
1. Rainfall Preparation Workflow
1
Daily rainfall series
Assemble and quality-check the daily rainfall record for the Hare watershed, including dates, units, missing values and spatial coverage.
2
Annual maximum extraction
Identify the largest daily catchment-average rainfall event in each year and build the annual-maximum series used for frequency analysis.
3
Frequency analysis
Estimate return-period rainfall depths from the annual-maximum rainfall series.
4
Temporal distribution
Convert the selected design depth into incremental time blocks that sum to the required storm rainfall.
5
HEC-HMS meteorologic model
Enter the final storm distribution using the adopted computation interval and run the design-event simulations.
2. Initial Annual Maximum Daily Rainfall, 1981–1990
The first stage of the rainfall analysis used the initial annual-maximum daily rainfall series. These values were useful for establishing the frequency-analysis procedure and early HEC-HMS rainfall preparation.
| Year | Date | Area Mean (mm/day) | Max Pixel (mm/day) |
|---|---|---|---|
| 1981 | 1981-07-30 | 41.91 | 47.53 |
| 1982 | 1982-11-28 | 36.96 | 50.24 |
| 1983 | 1983-07-25 | 44.14 | 53.13 |
| 1984 | 1984-05-29 | 58.47 | 78.82 |
| 1985 | 1985-05-23 | 38.84 | 45.39 |
| 1986 | 1986-08-10 | 42.46 | 62.06 |
| 1987 | 1987-05-21 | 44.09 | 53.02 |
| 1988 | 1988-05-19 | 44.69 | 59.12 |
| 1989 | 1989-10-20 | 50.46 | 65.59 |
| 1990 | 1990-07-21 | 52.13 | 70.87 |
3. Final Corrected Annual-Maximum Rainfall, 1981–2025
The rainfall analysis was later extended and corrected using the true Hare Weir catchment represented by the final HEC-HMS basin model. The corrected catchment area is 162.035 km², and the complete CHIRPS record contains 45 years from 1981 through 2025.
For every calendar year, the complete daily CHIRPS series was searched and the date having the greatest spatial mean rainfall over the Hare Weir catchment was selected. The result is one annual maximum daily catchment-average rainfall value for each year.
Final record
1981–2025
Record length
45 years
Mean of annual maxima
58.48 mm/day
Largest annual maximum
118.05 mm/day
Final maximum rainfall result:
the largest annual maximum in the corrected 45-year series is
118.05 mm/day, observed on
20 October 2025.
This value is especially important because it is not a single high-value pixel. It is the daily spatial average over the complete 162.035 km² Hare Weir catchment on the wettest catchment-average day found in the full 1981–2025 record.
Terminology:
CHIRPS is a calendar-day rainfall product. Therefore
118.05 mm/day is correctly described as the largest
observed daily catchment-average rainfall in the record.
It is used as evidence for development of the HEC-HMS 24-hour design
storm, but it is not a moving 24-hour maximum calculated from hourly
gauge observations.
Catchment correction:
an earlier extraction used a larger downstream catchment of roughly
250 km² extending toward Lake Abaya. The corrected 162.035 km² series
supersedes that earlier series for the final Hare Weir rainfall analysis.
4. Corrected Gumbel Rainfall Frequency Results
The final corrected 45-value annual-maximum series was used to recalculate the Gumbel frequency analysis for the true Hare Weir catchment.
| Return Period | Corrected Gumbel Rainfall (mm) |
|---|---|
| 2 years | 55.92 |
| 5 years | 69.70 |
| 10 years | 78.83 |
| 25 years | 90.36 |
| 50 years | 98.92 |
| 100 years | 107.41 |
Important observation:
the largest observed annual maximum of 118.05 mm/day
is greater than the fitted Gumbel 100-year rainfall of
107.41 mm. This difference should remain visible in the
engineering record and is a reason to interpret the fitted distribution
carefully rather than forcing the observed event to fit the curve.
5. Adopted HEC-HMS Design Rainfall
The following 24-hour rainfall depths are the values already used or prepared for the current HEC-HMS design-event simulations. They are kept here exactly as documented in the simulation-run record.
10-year P24
49.61 mm
25-year P24
54.99 mm
50-year P24
58.98 mm
100-year P24
62.95 mm
| Return Period | Adopted HEC-HMS P24 (mm) | Model Status |
|---|---|---|
| 10 years | 49.61 | Prepared for design-event simulation |
| 25 years | 54.99 | Prepared for design-event simulation |
| 50 years | 58.98 | Used in the completed reference run |
| 100 years | 62.95 | Prepared for design-event simulation |
Reference HEC-HMS run:
the completed 50-year simulation used
P24 = 58.98 mm and produced the currently documented
outlet peak of 114.4 m³/s.
Keep the records separate:
the corrected CHIRPS Gumbel values above document the final long-record
daily rainfall analysis. The adopted HEC-HMS P24 values shown here are
retained because they belong to the simulation runs already completed
and documented. A CHIRPS-based 50-year storm of 98.92 mm should only be
associated with an HEC-HMS discharge after that rainfall event is run
separately in the model.
6. From Daily Rainfall to a HEC-HMS Design Storm
HEC-HMS requires rainfall to be distributed through time rather than entered only as a single daily or 24-hour total. The selected design rainfall must therefore be converted into shorter-duration cumulative depths and then into incremental rainfall blocks.
Model Interval
15 minutes
The HEC-HMS design-event simulations use a 15-minute computation interval.
Simulation Window
72 hours
The longer window allows the full flood hydrograph and recession to pass.
Mass Balance
Σ incremental rainfall = selected storm depth
The final hyetograph must reproduce the adopted event total.
7. Design-Storm Hyetograph — HEC-HMS Rainfall Input
The figure below documents the temporal distribution of the adopted 50-year HEC-HMS design storm. The total rainfall depth is 58.98 mm over 24 hours, distributed according to the SCS Type II temporal pattern at a 15-minute model interval.
The blue bars represent incremental rainfall applied during each model time step, while the cumulative curve shows the progressive total rainfall through the storm. The sum of all incremental rainfall blocks is equal to the adopted 24-hour design depth.
Important:
this hyetograph is an input to the HEC-HMS Meteorologic Model,
not a model result. HEC-HMS uses this rainfall time series together with the
basin loss, transform and routing parameters to generate the runoff and
discharge hydrographs reported on the Results page.
8. Detailed Rainfall Records
The detailed corrected annual-maximum table, revised Gumbel calculation and corrected depth-duration analysis are retained below as the technical audit trail for the final Hare rainfall analysis.