Storm Hydrograph

OpenHydroLab · Every runoff drop's travel time to the gauge is drawn from the catchment's unit-hydrograph response — the hydrograph is the arrival-time distribution of the rain

Storm

Rain below φ soaks in (→ groundwater → baseflow); rain above φ runs off (→ direct runoff).

Catchment

Storage constant of the routing reservoirs. Large, flat, forested basins store more (large k → slow, damped response); small, steep, bare basins respond fast.
Paving cuts infiltration (φ↓) and speeds the response (k↓) — the classic influence of urbanization.

Baseflow model

Reservoir: infiltrated water fills a slow groundwater store that drains to the stream — baseflow rises gently after the storm.

Show

Aha demo: press Play. Blue drops that beat the φ-index become runoff: watch them gather into rivulets, join the tributaries, and stream down the network to the gauge — the hydrograph rises exactly as they arrive, and keeps falling long after the rain stops because distant drops are still travelling. Then set Urbanization to 80% with the rural baseline on: same storm, but the drops sprint — peak roughly doubles, lag halves.

Model

ER = max(i−φ, 0) → Nash cascade (2 linear reservoirs, S = kQ) → Q_direct; baseflow: slow store (k_gw) fed by infiltration, or uniform (constant). Drop travel times ~ the cascade's IUH (gamma, n = 2).
Effective rainfall rate
Infiltration rate
Effective k (with urban.)
Mean drop travel time 2k

Hydrograph features

Peak discharge Q_p
Time of peak
Lag time (ER centroid → peak)
Baseflow at peak
Runoff ratio ER/P

Mass balance (checks itself)

Total rainfall depth
= losses (infiltration)
+ effective rainfall
∫Q_direct dt ÷ A
Direct-runoff volume
The area under the direct-runoff hydrograph must return exactly the effective-rainfall depth — continuity. The readout integrates the computed curve numerically, so the model proves it conserves mass at any setting.