Geography (9696) | Discharge relationships within drainage basins

🌊 Discharge Relationships within Drainage Basins: Study Notes

Hello Geographers! Welcome to one of the most fundamental (and flood-related!) topics in physical geography: Discharge Relationships.

In this chapter, we explore how water moves through a drainage basin and, crucially, how quickly a river's flow responds to rainfall. This is the science behind understanding flood risk, managing water resources, and predicting environmental hazards. Don't worry if it seems tricky at first; we will break down the graphs and the jargon piece by piece!

1. Understanding Discharge and Hydrographs

1.1 What is Discharge?

Discharge (Q) is simply the volume of water flowing past a specific point in a river channel per unit of time.
It is usually measured in cubic metres per second (m³/s) or cumecs.

To calculate discharge, we use this basic relationship:
\( Q = A \times V \)
Where:

• \( Q \) = Discharge (m³/s)
• \( A \) = Cross-sectional area of the river channel (m²)
• \( V \) = Velocity of the water (m/s)

1.2 What is a Hydrograph?

A hydrograph is a graph that shows how the discharge of a river changes over time, usually in response to a rainfall event (a storm hydrograph) or over a whole year (an annual hydrograph).

1.3 Components of a Storm Hydrograph

The storm hydrograph is essential for understanding short-term flood behaviour. It plots both rainfall (precipitation) and river discharge over time.

Step-by-step breakdown of the components:

1. Baseflow: This is the normal, steady, background level of river flow, fed by groundwater (subsurface flows). It’s the flow that keeps the river running even when it hasn't rained recently.
2. Rising Limb: The part of the graph where river discharge is increasing. This is due to increasing inputs of water from surface runoff and rapid throughflow reaching the channel.
3. Peak Rainfall: The highest intensity point of the storm event (often shown on a separate bar chart or marked axis above the main graph).
4. Peak Discharge: The highest level of discharge reached by the river in response to the storm. This is the point of maximum flow.
5. Lag Time: The time difference between the Peak Rainfall and the Peak Discharge.
   • Crucial Point: The shorter the lag time, the faster the flood risk develops. A short lag time suggests water is getting to the river quickly, usually via surface runoff.
6. Falling Limb (or Recession Limb): The part of the graph where the discharge decreases as the storm water input fades.
7. Storm Flow: The discharge volume that is above the baseflow. This is the water resulting directly from the storm event itself (a mix of surface runoff, throughflow, and infiltrated water that quickly returned to the channel).

1.4 Annual Hydrographs

While a storm hydrograph shows hours or days, an annual hydrograph shows discharge patterns over an entire year (e.g., 12 months).

This type of hydrograph helps us understand the seasonal variations in discharge, often clearly showing:

• High flow periods (e.g., during monsoon seasons or spring snowmelt).
• Low flow periods (e.g., during summer droughts or winter freezes).
• The link between annual temperature cycles (affecting evapotranspiration) and river flow.

2. Influences on Hydrographs: What Makes Rivers React Differently?

The shape and timing of a hydrograph—specifically the height of the peak and the length of the lag time—depend entirely on two main sets of factors: Climate and Drainage Basin Characteristics.

2.1 Climate Influences

Climate factors determine how much water enters the system and how much is immediately lost.

a) Precipitation Type and Intensity

• Type: Snow and Ice hold water as a store. Discharge remains low until a thaw (melting) event releases a large volume of water quickly, causing a sharp, delayed peak. Rain creates an immediate response.
• Intensity: High intensity (heavy rain) means water arrives faster than the soil can absorb it (infiltration capacity is exceeded). This leads to increased overland flow (surface runoff), a short lag time, and a high peak discharge.

b) Temperature, Evaporation, Transpiration, and Evapotranspiration

• High Temperature: Increases rates of Evaporation (water turning to vapour from the ground/surface) and Transpiration (water released by plants).
• Evapotranspiration (ET): The combined loss of water through evaporation and transpiration. High ET (common in hot summers) reduces the amount of water available for runoff, resulting in lower baseflow and discharge.
• Low Temperature: If temperatures are below freezing, water is stored as snow or ice, reducing immediate discharge.

c) Antecedent Moisture

This refers to the wetness of the soil and ground before the storm event begins.

• High Antecedent Moisture (Saturated Soil): If the soil is already wet (like a wet sponge), its pores are full. Infiltration is low, and almost all new rain immediately becomes overland flow. This results in a very short lag time and a very high peak discharge. (High flood risk!)
• Low Antecedent Moisture (Dry Soil): If the soil is dry, it can soak up a lot of water initially. Infiltration is high, leading to a longer lag time and a lower, more gradual peak discharge.

Key Takeaway (Climate): Heavy, long-duration rainfall on already saturated ground is the perfect recipe for a flash flood and a peaked hydrograph.

2.2 Drainage Basin Characteristics (Physical and Human Factors)

These factors describe the physical landscape of the river's catchment area.

a) Size and Shape

• Size: Larger basins collect more water, leading to a higher total discharge volume. However, they generally have a longer lag time because the water takes longer to travel from the edges to the central measuring point.
• Shape:
  • Circular Basins: Water from all parts of the basin reaches the river's measuring point almost simultaneously, resulting in a short lag time and a high, flashy peak.
  • Elongated Basins: Water is spread out, taking different travel times, leading to a longer lag time and a lower, more gentle peak.

b) Drainage Density

Drainage Density is the total length of all river channels (streams and tributaries) divided by the total area of the basin. In simpler terms, it measures how well the basin is "plumbed."

• High Density: Many streams and rivers in the area. Water has a shorter distance to travel over land before entering a channel. This means faster transfer, a shorter lag time, and a flashy hydrograph.
• Low Density: Fewer channels. Water must travel further over land, increasing resistance, leading to a longer lag time.

c) Porosity and Permeability of Soils and Rock Type

These factors control how much water infiltrates and how quickly it moves underground.

• Porosity: The percentage of rock/soil that is empty space (pores) that can hold water. (Like a sponge).
• Permeability: The ability of the rock/soil to allow water to pass through it. (Like a filter).

• Impermeable Rock/Soil (e.g., Clay, Granite): Low infiltration, high surface runoff. Result: Flashy hydrograph (short lag time, high peak).
• Highly Permeable Rock/Soil (e.g., Chalk, Sandstone): High infiltration, high storage. Water enters the groundwater store, resulting in a flat hydrograph (long lag time, low peak).

d) Slopes (Gradient)

• Steep Slopes: Water moves down quickly under gravity, leading to rapid surface runoff and fast throughflow. Result: Short lag time and high peak. (Think of water racing down a steep roof).
• Gentle Slopes/Flat Land: Water moves slowly, allowing more time for infiltration and storage. Result: Longer lag time and a lower, extended peak.

e) Vegetation Type (Land Cover)

Vegetation plays a huge role in slowing down water and increasing infiltration.

• Forests/Dense Vegetation (e.g., Afforestation):
  • They intercept rainfall (water stored on leaves).
  • Roots increase soil porosity, promoting infiltration and throughflow.
  • High rates of transpiration reduce overall basin water content.
  • Result: Low peak, long lag time. (Hydrograph is "subdued" or flattened).
• Bare Soil or Grassland: Low interception and infiltration capacity. Result: Faster runoff, shorter lag time.

f) Land Use (Human Impact)

Human activity often makes the hydrograph "flashier" by reducing infiltration and speeding up flow (This links directly to Syllabus 1.4: Human Impact).

• Urbanisation: Replacing natural soil with impermeable surfaces (concrete, roads, roofs). This drastically reduces infiltration, increases overland flow, and uses artificial drainage systems (drains/sewers) to move water rapidly to the river.
  • Result: The most extreme flashy hydrograph—very short lag time, massive peak.
• Deforestation: Removes the protective vegetation cover, reducing interception and increasing soil erosion and surface runoff.