On a 6.8 km² mixed basin last week, I ran HEC-HMS with gridded precip and kept CN fixed from land cover, only tuning Ia/S and Muskingum k to match field-noted peak timing; peaks stayed about 10–15% low, but the pipe sizing call didn’t change. For efficiency, I’m leaning toward design storms plus a small safety factor instead of chasing a perfect fit — anyone else cap calibration to two parameters when data are thin?
Same boat on a 6–7 km² mixed basin: in HMS my peaks sat about 10% low until I flipped to AMC III (CN +5) instead of cranking Ia/S, then timing held and peaks lined up. If the pipe call doesn’t change, I just run the 10–25 yr design storms with a +15% bump and cap calibration at one Muskingum k pass.
@sevans41 good point; in HMS, 1–2 min timestep bumped 6.8 km² peak about 12%; watch stability.