Geography – Hydrology, river processes and hazards | e-Consult

The characteristics of a drainage basin directly dictate how water flows through it and, consequently, the pattern of discharge and the shape of its hydrograph. A hydrograph is a graph showing the rate of flow of a river over time, and its shape provides valuable insights into the basin's hydrological behavior.

Influence of Drainage Basin Characteristics:

• Slope: Steeper slopes lead to faster runoff and a hydrograph with a steeper rise to the peak discharge and a quicker fall. Gentler slopes result in slower runoff and a hydrograph with a more gradual rise and fall.
• Vegetation Cover: Vegetation, particularly forests, intercept rainfall, reducing the amount of water reaching the ground as runoff. It also increases infiltration and reduces soil erosion. Areas with dense vegetation will have lower peak discharges and a more dampened hydrograph. Deforestation, conversely, increases runoff and peak discharges.
• Geology: The type of rock underlying the drainage basin influences infiltration rates and groundwater flow. Impermeable rock layers (e.g., granite) restrict infiltration, leading to higher surface runoff and a more pronounced peak in the hydrograph. Permeable rock layers (e.g., sandstone) allow for greater infiltration and groundwater storage, resulting in a more sustained baseflow and a less extreme hydrograph.
• Soil Type: As mentioned above, soil type significantly affects infiltration rates. Sandy soils have high infiltration rates, while clay soils have low rates. This influences the amount of surface runoff and the peak discharge.

Discharge Relationships and Hydrograph Shape:

A typical hydrograph shows a rapid rise to a peak discharge following a period of heavy rainfall, followed by a gradual fall. The time taken to reach the peak (lag time) is influenced by the size and characteristics of the drainage basin. A larger drainage basin generally has a longer lag time. The baseflow, representing the sustained discharge during dry periods, is influenced by groundwater storage and the permeability of the underlying geology. A hydrograph with a high baseflow indicates significant groundwater contribution.