Rising Limb: The portion of the curve where discharge increases as water from the storm reaches the river channel. A steep rising limb indicates rapid runoff, often due to impermeable surfaces or steep slopes.
Peak Discharge: The highest point on the hydrograph, representing the maximum volume of water in the river during the event. This is the moment of greatest flood risk.
Recession (Falling) Limb: The segment where discharge decreases as the storm water exits the basin. This limb is usually less steep than the rising limb because water takes longer to drain through soil and rock than it does to flow over the surface.
Lag Time: The time interval between the peak of the rainfall event and the peak discharge of the river. Shorter lag times correlate with a higher risk of 'flash' flooding.
Baseflow: The normal, day-to-day flow of the river supplied by groundwater. It responds very slowly to individual rainfall events and maintains the river's flow during dry periods.
Storm Flow (Quickflow): The additional water in the river resulting from a specific storm, composed primarily of surface runoff and throughflow (water moving through the soil).
Water Balance: The relationship between inputs (precipitation) and outputs (discharge and evapotranspiration). If precipitation exceeds the basin's storage capacity, discharge increases rapidly.
Basin Morphometry: Small, circular basins tend to have shorter lag times and higher peaks because water reaches the gauging point simultaneously from all parts of the basin. Steep relief also accelerates runoff.
Geology and Soil: Impermeable rocks (like granite) or saturated soils prevent infiltration, leading to high surface runoff and 'flashy' hydrographs. Conversely, permeable rocks (like chalk) allow for infiltration and a 'flat' response.
Land Use: Urbanization increases impermeable surfaces (concrete, tarmac), which drastically reduces lag time and increases peak discharge. Vegetation and forests intercept rainfall and promote infiltration, slowing the response.
| Feature | Flashy Hydrograph | Flat Hydrograph |
|---|---|---|
| Lag Time | Short | Long |
| Peak Discharge | High | Low |
| Rising Limb | Steep | Gentle |
| Typical Causes | Urban areas, steep slopes, impermeable rock | Forested areas, gentle slopes, permeable soil |
Check the Units: Always verify if discharge is in or , and ensure time is correctly identified as hours or days. Misreading the scale is a common source of error.
Interpret Lag Time: When asked to compare two hydrographs, focus on the lag time. A shorter lag time almost always implies a higher flood risk and more efficient drainage.
Identify the Components: Be prepared to label the baseflow and storm flow. Remember that the area under the curve represents the total volume of water that passed the gauging station.
