The concept of river discharge and its measurement

River discharge refers to the volume of water that passes through a river at a given point over a specific time. It is a crucial measure in understanding how rivers behave, especially during different weather conditions.

Key aspects of river discharge

• Definition and units - Discharge is measured as the volume of water flowing per second, expressed in cubic metres per second (m³/s), often abbreviated as cumecs.
• Role of runoff - High levels of runoff, which is water flowing over the land surface, directly contribute to increased river discharge, particularly after rainfall events.

Understanding hydrographs and their components

Hydrographs are graphical representations that illustrate how river discharge changes over time. They are particularly useful for analysing river responses to rainfall, especially during storm events.

Types of hydrographs

• General hydrographs - Show river discharge trends over varying time periods.
• Flood hydrographs - Also known as storm hydrographs, these focus on discharge during and immediately after a storm, typically covering hours or days.

Components of a hydrograph

• Peak discharge - The highest point on the graph, indicating the maximum volume of water flowing through the river at any given time.
• Lag time - The time difference between the peak of rainfall and the peak discharge, showing the delay in water reaching the river.
• Rising limb - The section of the graph where discharge increases as rainwater enters the river system.
• Falling limb - The section where discharge decreases as the influx of water into the river reduces.
• Shallow falling limb - A gradual decline in discharge, suggesting water continues to flow into the river from storage areas long after rainfall ceases.
• Bankfull discharge - The level at which the water reaches the top of the river channel, beyond which flooding occurs.
• Flashy hydrograph characteristics - Seen in basins with rapid runoff and low storage capacity, featuring a short lag time, high peak discharge, and steep, nearly symmetrical rising and falling limbs.

Factors influencing runoff and hydrograph shapes

The shape and characteristics of a hydrograph are influenced by various features of the drainage basin, which determine how quickly water reaches the river and the volume of runoff.

Basin characteristics affecting hydrographs

• Size of drainage basin - Larger basins collect more precipitation, leading to higher peak discharge, while smaller basins often have shorter lag times due to quicker water travel to the river.
• Shape of drainage basin - Circular basins tend to produce flashier hydrographs as water arrives at the measuring point simultaneously, compared to long, narrow basins where water arrival is staggered.
• Ground steepness - Steeper slopes accelerate water flow, reducing lag time and limiting infiltration, resulting in more surface runoff.
• Rock and soil type - Impermeable surfaces, such as clay or rock, prevent water from soaking into the ground, increasing surface runoff and elevating peak discharge.

Physical factors affecting the water cycle in drainage basins

Natural environmental conditions play a significant role in how water moves through a drainage basin, influencing river discharge and the overall water cycle.

Climatic and environmental influences

• Intensity of storms - Heavy, intense storms deliver more precipitation, leading to greater peak discharges compared to light, prolonged rain.
• Temperature effects - Below 0°C, water freezes, reducing flow through the basin. Rising temperatures can cause ice to melt, significantly boosting flow volumes.
• Role of vegetation - Vegetation intercepts rainfall, slowing its journey to river channels, and increases water loss through transpiration and evaporation, which reduces runoff and peak discharge.

Human activities impacting the water cycle and river discharge

Human actions can alter the natural flow of water within a drainage basin, affecting river discharge through changes in land use, farming practices, and water management.

Effects of farming practices

• Ploughing - Breaking up soil through ploughing enhances infiltration, allowing more water to seep into the ground rather than run off.
• Crop cultivation - Crops increase both infiltration and interception compared to bare soil, reducing surface runoff.
• Livestock impact - Grazing animals compact soil, decreasing infiltration and increasing surface runoff.
• Irrigation - This practice can elevate runoff while depleting groundwater or river levels, impacting overall water availability.

Consequences of land use changes

• Deforestation - Removing trees reduces interception, leading to more surface water and runoff. Forest floor materials, which retain water and promote infiltration, are also lost.
• Construction and urbanisation - Building creates impermeable surfaces like concrete, preventing infiltration and heightening flood risk due to increased runoff.

Impact of water abstraction

• Definition and purpose - Water abstraction involves removing water from natural stores such as lakes, rivers, reservoirs, and groundwater to meet human needs.
• Regional variations - Areas with higher population density experience greater abstraction to satisfy demand.
• Seasonal effects - During dry seasons, abstraction intensifies for consumption and irrigation, further reducing water stores and affecting river discharge.