Hydrographs are essential tools in hydrology, depicting how streamflow changes over time. They reveal crucial information about water movement in a watershed, from the initial rise in flow to its peak and eventual recession.

Understanding hydrograph components and types helps hydrologists analyze runoff events, predict flooding, and manage water resources. By examining factors like peak discharge and runoff volume, we can better prepare for and respond to various hydrological scenarios.

Components of a Hydrograph

Components of a hydrograph

• Rising limb represents the initial increase in streamflow due to rainfall or snowmelt (thunderstorm)
  • Begins when water reaches the stream channel through overland flow or subsurface flow
  • Slope of the rising limb indicates the rate of increase in streamflow influenced by factors such as rainfall intensity and basin characteristics (steep slopes)
• Peak is the highest point on the hydrograph representing the maximum streamflow during the runoff event (flood stage)
  • Time to peak depends on factors such as basin size, shape, and land use (urbanized areas)
  • Peak discharge is a key parameter for flood risk assessment and management
• Falling limb represents the decrease in streamflow after the peak as the basin drains (recession curve)
  • Slope of the falling limb indicates the rate of decrease in streamflow influenced by factors such as soil moisture and groundwater contribution
  • Influenced by factors such as channel storage and evapotranspiration (riparian vegetation)
• Baseflow is the sustained low flow in a stream between runoff events maintained by groundwater discharge (springs) and delayed subsurface runoff
  • Represents the minimum streamflow during dry periods (summer months)
  • Influenced by factors such as geology, aquifer characteristics, and long-term precipitation patterns

Types of hydrograph contributions

• Direct runoff is water that flows over the land surface and reaches the stream channel relatively quickly (overland flow)
  • Contributes to the rising limb and peak of the hydrograph
  • Influenced by factors such as rainfall intensity, land use (impervious surfaces), and antecedent moisture conditions
• Interflow is water that infiltrates the soil and moves laterally through the upper soil layers before reaching the stream (throughflow)
  • Contributes to the hydrograph after direct runoff but before baseflow
  • Influenced by factors such as soil properties (permeability), topography, and vegetation (root systems)
• Baseflow is water that enters the stream from groundwater discharge and delayed subsurface runoff (seepage)
  • Contributes to the lower portion of the hydrograph and sustains streamflow during dry periods
  • Influenced by factors such as geology (karst formations), aquifer characteristics, and long-term precipitation patterns

Types of Hydrographs

Characteristics of hydrograph types

• Storm hydrographs represent the streamflow response to a single rainfall event (thunderstorm)
  • Characterized by a rapid rise to peak flow followed by a gradual recession (hours to days)
  • Shape influenced by factors such as rainfall intensity, duration, and antecedent moisture conditions (dry vs. wet soils)
• Annual hydrographs represent the streamflow variation over a year (water year)
  • Characterized by seasonal patterns related to precipitation, snowmelt, and evapotranspiration (spring runoff)
  • Shape influenced by factors such as climate (monsoon), land use, and basin characteristics (elevation)
• Flood hydrographs represent the streamflow response during high flow events that exceed bankfull discharge (100-year flood)
  • Characterized by a rapid rise to peak flow and a gradual recession, often with a higher peak than storm hydrographs
  • Shape influenced by factors such as rainfall intensity, duration, antecedent moisture conditions, and basin characteristics (floodplain storage)

Interpretation of runoff events

• Timing of runoff events:
  1. Time to peak: Duration from the start of the rising limb to the peak discharge (hours)
  2. Lag time: Duration between the center of mass of rainfall and the peak discharge (hours)
  3. Time of concentration: Duration for water to travel from the most hydraulically distant point in the basin to the outlet (minutes to hours)
• Magnitude of runoff events:
  • Peak discharge: The maximum streamflow during the runoff event ($m^3/s$)
  • Runoff volume: The total volume of water discharged during the runoff event, represented by the area under the hydrograph ($m^3$)
  • Runoff depth: The depth of water that would cover the basin if the runoff volume were uniformly distributed over the basin area ($mm$)