8.1 Components and types of hydrographs

Components of a Hydrograph

A hydrograph is a graph showing how streamflow (discharge) changes over time at a specific point in a river or stream. It's one of the most fundamental tools in hydrology because it captures the entire story of a runoff event: how quickly water arrives, how much arrives, and how long it takes to drain away.

By reading a hydrograph, you can analyze flood risk, estimate runoff volumes, and understand how a watershed responds to rainfall or snowmelt. The shape of a hydrograph tells you a lot about the basin it came from.

Structural Components

Every hydrograph has four main components. Picture a curve that rises from a baseline, hits a peak, then gradually falls back down.

• Rising limb — the upward portion of the curve showing the initial increase in streamflow after rainfall or snowmelt begins. It starts when water first reaches the stream channel through overland flow or subsurface pathways. The steepness of the rising limb reflects how fast discharge is increasing, which depends on rainfall intensity and basin characteristics like slope gradient. A steep, rocky catchment with intense rain produces a very steep rising limb.
• Peak discharge — the highest point on the hydrograph, representing maximum streamflow during the event. This is the single most important number for flood risk assessment. How quickly the hydrograph reaches its peak (the time to peak) depends on basin size, shape, and land use. Urbanized areas with lots of impervious surface tend to produce earlier, sharper peaks than forested basins of the same size.
• Falling limb (recession limb) — the downward portion after the peak, showing streamflow decreasing as the basin drains. This decline is more gradual than the rise because water stored in the soil, channel, and floodplain continues feeding the stream even after rain stops. Factors like channel storage, soil moisture release, and evapotranspiration (especially from riparian vegetation) all shape the recession curve.
• Baseflow — the sustained, low-level flow in a stream between runoff events, maintained primarily by groundwater discharge from springs and delayed subsurface drainage. Baseflow represents the minimum streamflow you'd see during dry periods (think late summer in a temperate climate). It's controlled by geology, aquifer properties, and long-term precipitation patterns. On the hydrograph, baseflow forms the "floor" beneath the storm response curve.

Types of Flow Contributions

The water making up a hydrograph doesn't all arrive the same way. Three distinct flow pathways contribute to the total discharge, each on a different timescale.

• Direct runoff (overland flow) — water that flows across the land surface and reaches the channel quickly. This is the dominant contributor to the rising limb and peak. It increases sharply when rainfall intensity exceeds the soil's infiltration capacity, or when impervious surfaces (roads, rooftops) prevent infiltration altogether. Antecedent moisture conditions matter too: saturated soils can't absorb more water, so more rain becomes direct runoff.
• Interflow (throughflow) — water that infiltrates the soil surface but then moves laterally through upper soil layers before reaching the stream. It's slower than overland flow but faster than deep groundwater. Interflow contributes to the hydrograph after the direct runoff peak but before the curve settles back to baseflow. Soil permeability, slope angle, and root systems all influence how much and how fast interflow occurs.
• Baseflow (groundwater contribution) — water that percolates deep enough to recharge the groundwater table and then slowly seeps into the stream. This is the slowest pathway and sustains flow during dry spells. In areas with karst geology (limestone with solution channels), baseflow behavior can be unusual because groundwater moves through conduits rather than slowly through pore spaces.

Types of Hydrographs

Different hydrograph types capture streamflow behavior at different timescales and for different purposes.

Storm Hydrographs

A storm hydrograph shows the streamflow response to a single rainfall event. It's the type you'll work with most often in this unit. The curve typically rises rapidly to a peak, then recedes more gradually over hours to days. Its shape is controlled by:

• Rainfall intensity and duration
• Antecedent moisture conditions (dry soils absorb more, producing a flatter hydrograph; wet soils produce a sharper one)
• Basin characteristics like area, slope, soil type, and land cover

Annual Hydrographs

An annual hydrograph plots streamflow variation over an entire year (often a water year, which in the U.S. runs October 1 to September 30). Instead of a single peak, you'll see seasonal patterns: spring snowmelt peaks, summer low flows, or monsoon-driven surges depending on the climate. These hydrographs are useful for water supply planning and understanding long-term flow regimes.

Flood Hydrographs

A flood hydrograph specifically captures high-flow events where discharge exceeds bankfull discharge (the flow at which water begins spilling onto the floodplain). These look similar to storm hydrographs but with much higher peaks. Engineers use flood hydrographs to design infrastructure around specific recurrence intervals, such as the 100-year flood (a flood with a 1% chance of occurring in any given year). Floodplain storage can significantly attenuate the peak and stretch out the recession.

Interpreting Runoff Events

Three timing metrics help you characterize how fast a basin responds:

1. Time to peak — duration from the start of the rising limb to peak discharge (typically measured in hours)
2. Lag time — duration between the center of mass of the rainfall and the peak discharge (hours). This is the most common measure of basin response time.
3. Time of concentration ($T_c$) — the time it takes water to travel from the most hydraulically distant point in the basin to the outlet (minutes to hours). Once $T_c$ has elapsed after rainfall begins, the entire basin is contributing to outlet discharge.

Three magnitude metrics quantify how much water moved:

• Peak discharge — maximum streamflow during the event, measured in $m^3/s$. This is the critical number for flood design.
• Runoff volume — total volume of water discharged during the event, calculated as the area under the hydrograph curve (in $m^3$).
• Runoff depth — the runoff volume divided by the basin area, expressed in $mm$. This lets you compare runoff across basins of different sizes, since it normalizes volume to a uniform depth of water spread over the entire catchment.