Storm Hydrograph

A storm hydrograph is a graphical representation that illustrates how a river’s discharge (the volume of water flowing through the channel at a specific time) responds to a rainfall event. It shows how river flow varies over time following precipitation.

On the graph, the Y-axis represents discharge, measured in cubic meters per second (cumecs), while the X-axis represents time. Rainfall is usually displayed as vertical bars, and discharge as a continuous line. When discharge exceeds the bankfull level, flooding occurs, shown as a pronounced peak on the graph. Storm hydrographs are essential tools for understanding and predicting flood risks within a drainage basin.

Key Terms

• Hydrograph: A graph showing the variation in river discharge after rainfall.
• Discharge: The volume of water flowing through a river at a given point, measured in cumecs.
• Velocity: The speed of water movement in a river, measured in meters per second.
• Lag Time: The time gap between peak rainfall and peak discharge; shorter lag times indicate higher flood risk.
• Rising Limb: The upward slope on the graph representing the rapid increase in discharge after rainfall; the steeper the limb, the faster the river response.
• Falling (or Recession) Limb: The downward slope showing discharge returning to normal levels.
• Peak Rainfall: The highest recorded rainfall during the event.
• Peak Discharge: The maximum flow rate reached by the river.
• Baseflow: The slow movement of water through soil toward the river.
• Bankfull Discharge: The discharge level at which the river completely fills its channel; any excess results in flooding.
• Stormflow: Discharge contributed by surface and subsurface flow from one rainfall event.
• Approach Segment: The section of the hydrograph showing discharge before rainfall begins.

River Response to Rainfall

Rivers do not immediately reflect rainfall input because only a small portion falls directly into the channel. The initial rise in discharge usually comes from surface runoff, which travels fastest. This is followed by throughflow and groundwater flow, which contribute more gradually to the river.

Factors Influencing Hydrograph Shape

Rock Type and Soil:
Impermeable rocks like granite and clay-rich soils restrict infiltration, increasing surface runoff and creating short lag times with higher flood potential. Permeable materials encourage infiltration, leading to longer lag times and gentler hydrographs.

Slope (Relief):
Steeper slopes accelerate water movement to rivers, shortening lag time. Gentle slopes promote infiltration and delay peak discharge.

Size of the Drainage Basin:
Larger basins generally have longer lag times since water travels farther and undergoes more infiltration and evaporation. Smaller basins experience shorter lag times and higher peaks.

Vegetation Cover:
Dense vegetation intercepts rainfall, enhances infiltration, and increases lag time, resulting in a flatter hydrograph. In contrast, deforested areas promote rapid runoff and steep hydrographs.

Drainage Basin Shape:
Circular basins allow water to converge quickly, reducing lag time and producing a high peak flow. Elongated basins slow water movement, lengthening lag time.

Drainage Density:
Basins with numerous streams (high drainage density) convey water quickly to the main river, creating short lag times and higher flood risk. Low-density basins react more slowly.

Rainfall Intensity and Duration:
Heavy, intense rainfall rapidly saturates soil, causing immediate runoff and short lag time. Light, prolonged rainfall favors infiltration, producing a more gradual hydrograph.

Land Use and Human Activities:
Urban development, deforestation, and mining decrease infiltration and increase runoff, creating steep, flashy hydrographs. Practices such as afforestation and sustainable agriculture help slow runoff and flatten the hydrograph.

Tides and Storm Surges:
High tides can block river outflow into the sea, causing water to back up and flood low-lying areas.

Temperature and Seasonal Effects:
Frozen or extremely dry soils inhibit infiltration, causing rapid runoff and shorter lag times. Snowmelt can also trigger sudden surges in discharge.

Drainage Modifications:
Engineering works such as dredging and channel widening can increase river capacity and speed up water flow, reducing the likelihood of flooding.