Lag Time on a Hydrograph: Meaning and Interpretation
Understanding Lag Time in Hydrology
Lag time on a hydrograph is the time interval between the center of mass of rainfall excess (effective rainfall) and the peak discharge at the watershed outlet.
Visual Representation
- X-axis: Time (hours/days).
- Y-axis: Discharge (m³/s or ft³/s).
- Hyetograph overlay: Rainfall intensity vs. time (often bars).
- Hydrograph curve: Rising limb → peak → recession limb.
- Lag time (tₗ): Horizontal distance from hyetograph centroid to hydrograph peak.
What Lag Time Reveals
| Watershed Characteristic | Effect on Lag Time |
|---|
| Large area | Longer lag |
| Steep slopes | Shorter lag |
| High drainage density | Shorter lag |
| Impervious cover (urban) | Much shorter lag |
| Dense vegetation/forests | Longer lag (interception, infiltration) |
| Soil infiltration capacity | High infiltration → longer lag |
| Channelization / storm drains | Shorter lag |
| Wet antecedent conditions | Shorter lag |
Applications
- Flood forecasting: Predict peak arrival for warnings.
- Design storms: Size culverts, bridges, detention basins.
- Urbanization impact: Compare pre/post-development lag times.
- Model calibration: Key parameter in unit hydrograph, HEC-HMS, SWMM.
Related Metrics
- Time of concentration (Tc): Time for runoff from farthest point to reach outlet. Lag ≈ 0.6 × Tc (empirical).
- Rising limb length: Rainfall start to peak.
- Basin lag vs. basin lag time: Sometimes distinguished (centroid-to-peak vs. start-to-peak).
Lag time is a fingerprint of watershed hydrologic response — shorter = flashier, more flood-prone.
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