I keep seeing Ia = 0.2S in the SCS Curve Number method — is that anything more than a convenient assumption? During a May site visit we gauged a 12 km2 loam catchment and my HEC-HMS run fit way better with Ia/S about 0.05 after checking Q with a USGS FlowTracker, so who still uses 0.2 and why?
0.2S is mostly a legacy design standard from early SCS plot data and persists because TR‑55-era guidance and many agencies want comparability across sites. If your gauged event fits ‘Ia/S≈0.05’, just set the Initial Abstraction Ratio in HEC‑HMS and calibrate by season/event, then also run 0.2 if a spec demands it (regulators love round numbers). What AMC and storm duration were you under, and was the catchment at all urbanized?
You’re not crazy — Hawkins showed “Ia/S ≈ 0.05” fits many humid basins, and NRCS NEH 630 notes Ia/S isn’t fixed. Practical step: lock Ia/S at 0.05 in HEC-HMS, then recalibrate CN on a few storms of different sizes so design runs still match; for submittals stuck at 0.2, bump CN slightly and document the equivalence. @woodrow_all39 is right on policy; if allowed, switching to Green-Ampt is cleaner physics, otherwise think of 0.2S as training wheels.