A.2.1 Hydrograph Characteristics Notes

Analyzing River Responses to Rainfall Events

1. Imagine standing by a river’s edge after a heavy rainstorm.
2. The water begins to rise, flowing faster and stronger until it surges past with tremendous force.
3. What causes this dramatic response? Why doesn’t the river react immediately to rainfall, and why does the flow eventually recede?
4. These questions lead us to hydrographs, tools that help us visualize and analyze how rivers respond to rainfall events.
5. By studying hydrographs, you can uncover the processes shaping river discharge and assess factors that influence flood risk.

Key Components of a Hydrograph

A hydrograph typically has two axes:

• The x-axis represents time (hours or days).
• The y-axis represents discharge, measured in cubic meters per second (m³/s).

The graph itself consists of:

1. Base Flow: The normal, sustained flow of the river, fed by groundwater.
2. Rising Limb: The steep increase in discharge after rainfall begins.
3. Peak Discharge: The highest point on the graph, indicating the maximum flow.
4. Lag Time: The delay between the peak rainfall and peak discharge.
5. Falling Limb: The gradual decrease in discharge as water drains away.

Understanding Key Hydrograph Terms

To analyze hydrographs effectively, it’s essential to understand three key terms that describe a river’s response to rainfall:

1. Base Flow: The portion of river discharge supplied by groundwater seeping into the channel, sustaining the river during dry periods.
2. Lag Time: The delay between peak rainfall and peak discharge (the highest flow rate in the river). This reflects the time it takes for water to travel from where it falls to the river channel.
3. Peak Discharge: The maximum volume of water flowing in the river at a given time after rainfall.

These terms are visualized on a hydrograph, a graph showing river discharge over time during and after a storm or rainfall event. The shape of the hydrograph reveals how quickly a river responds to rainfall and highlights the factors influencing that response.

Lag Time: The Key to Flood Risk

1. Lag time is a crucial factor in determining flood risk.
2. A shorter lag time means water reaches the river channel more quickly, increasing the likelihood of flooding.
3. Conversely, a longer lag time allows more water to infiltrate the ground or flow slowly through the landscape, reducing flood risk.
4. Several factors influence lag time:

• Basin Shape and Size: Smaller, circular basins tend to have shorter lag times because water converges quickly. Larger, elongated basins have longer lag times as water takes more time to reach the river.
• Slope Gradient: Steeper slopes encourage faster runoff, leading to shorter lag times. Flatter areas promote infiltration and slower water movement, increasing lag time.
• Drainage Density: Basins with a higher density of streams and channels transport water more efficiently, reducing lag time.
• Land Use: Urban areas with impermeable surfaces, such as roads and buildings, reduce infiltration and increase runoff, shortening lag time. Rural areas with vegetation and permeable soils tend to have longer lag times.

Peak Discharge: The Flood Peak

1. Peak discharge represents the highest volume of water flowing in the river after a rainfall event.
2. It is influenced by the characteristics of the rainfall and the drainage basin:

• Rainfall Intensity and Duration: Heavy, prolonged rainfall results in higher peak discharge as more water enters the river system.
• Soil and Rock Type: Impermeable soils (e.g., clay) and rocks (e.g., granite) limit infiltration, increasing surface runoff and peak discharge. Permeable soils (e.g., sandy soils) and rocks (e.g., limestone) allow more infiltration, reducing peak discharge.
• Vegetation Cover: Dense vegetation intercepts rainfall, slows runoff, and promotes infiltration, reducing peak discharge. Deforestation or sparse vegetation has the opposite effect.

Base Flow: The River’s Lifeline

1. Base flow is the steady contribution of groundwater to the river, maintaining its flow during dry periods.
2. While it is typically unaffected by short-term rainfall, base flow plays a critical role in the overall hydrograph.
3. Areas with permeable rocks and high groundwater storage (e.g., chalk or sandstone regions) have higher base flows, while areas with impermeable rocks have lower base flows.

Influences on Hydrographs: Geology and Seasonality

1. Now that you understand the components of a hydrograph, let’s explore two key factors that shape river responses: geology and seasonality.

Geology: The Foundation of River Behavior

1. The type of rock and soil in a drainage basin significantly affects infiltration, runoff, and the shape of the hydrograph.

• Permeable Rocks: Rocks like sandstone and limestone allow water to infiltrate, increasing groundwater storage and base flow. Hydrographs in these areas typically have gentler rising and recessional limbs, lower peak discharge, and longer lag times.
• Impermeable Rocks: Rocks like granite and clay prevent infiltration, leading to rapid surface runoff. Hydrographs in these areas have steep rising limbs, high peak discharge, and shorter lag times.

Seasonality: Timing Matters

1. Seasonal variations in climate also play a significant role in shaping hydrographs:

• Winter and Snowmelt: In colder climates, snow accumulates during winter and melts rapidly in spring, causing sudden increases in river discharge. This results in steep rising limbs and high peak discharge.
• Dry Seasons: In arid regions or during dry seasons, base flow dominates the hydrograph as rainfall is minimal. Hydrographs in these periods are flatter and show lower discharge levels.
• Monsoon Climates: Regions with monsoon climates experience intense seasonal rainfall, leading to sharp peaks in discharge and shorter lag times.

Water Balance Diagrams: Tracking Inputs, Outputs, and Storage

1. While hydrographs focus on rivers, water balance diagrams provide a broader view of the hydrological cycle.
2. They show how water moves into, out of, and is stored within a system, such as a drainage basin.

The Water Balance Equation

1. The water balance equation is the foundation of these diagrams:

$$ P = Q + E + \Delta S $$

Where:

• $P$ = Precipitation (input)
• $Q$ = Runoff or river discharge (output)
• $E$ = Evapotranspiration (output)
• $\Delta S$ = Change in storage (e.g., groundwater, soil moisture)

Components of a Water Balance Diagram

1. A water balance diagram typically includes:

• Inputs: Precipitation, snowmelt, or water inflow.
• Outputs: Evaporation, transpiration, and runoff.
• Storage: Water held in soil, aquifers, or reservoirs.

Interpreting Water Balance Diagrams

1. These diagrams help identify periods of water surplus or deficit:

• Surplus: When precipitation exceeds evapotranspiration and storage is full, leading to runoff.
• Deficit: When evapotranspiration exceeds precipitation, reducing water availability.

Applications: Managing Flood Risk

1. Understanding river responses to rainfall is essential for effective flood risk management.
2. Engineers and planners use hydrographs and water balance diagrams to design flood defenses, predict flood events, and mitigate damage. For example:

• Urban Planning: In urban areas, shorter lag times and higher peak discharges necessitate stormwater systems and flood defenses like levees and detention basins.
• Reforestation: Planting trees in deforested areas increases interception, reduces runoff, and lengthens lag times, lowering flood risk.
• Reservoir Construction: Reservoirs store excess rainfall during storms, reducing peak discharge and protecting downstream communities.

Reflection and Broader Implications

1. Hydrographs (as well as water balance diagrams) offer valuable insights into how rivers respond to rainfall, helping us manage flood risks and protect communities.
2. However, they also raise broader questions: How do human activities like urbanization and deforestation amplify flood risks?