Understanding river discharge and regimes

River discharge refers to the volume of water flowing through a river channel at a specific point, measured in cubic metres per second (cumecs). This measurement is crucial for assessing how much water a river carries at any given time and understanding its capacity to handle rainfall without flooding.

River regimes

A river regime describes the pattern of discharge variations over time, which can be monthly, daily, or seasonal. River regimes often mirror local weather patterns, especially rainfall. For instance, rivers in monsoon regions show high discharge during wet seasons, while others may peak in winter due to seasonal rains.

Examples of variation:

• In a tropical river like the Amazon, discharge might surge between May and September due to heavy monsoon rains, reaching up to 42,000 cumecs at its peak.
• In contrast, a temperate river like the Severn in the UK might peak at around 110 cumecs during winter months like January due to consistent rainfall.

The role of hydrographs in analysing river behaviour

Hydrographs are graphical representations of river discharge over time, providing vital insights into how rivers respond to climatic conditions. They help predict flooding risks and inform water management strategies in populated drainage basins.

Types of hydrographs

Annual hydrographs:

• Display average monthly discharge over a year.
• For example, a hydrograph for a river in a monsoon climate might show low discharge from November to April, with a sharp rise from May, peaking at 48,000 cumecs in July, before declining.

Seasonal hydrographs:

• Focus on discharge over a specific period, often showing daily fluctuations.
• A hydrograph for a British river might reveal jagged peaks between December and February, indicating short bursts of heavy rain, with discharge varying between 20 and 140 cumecs.

Hydrographs allow for the assessment of flood risks in drainage basins, which are often densely populated due to fertile soils. This data supports investments in infrastructure and flood prevention measures.

Components and significance of storm hydrographs

A storm hydrograph specifically tracks a river's discharge following a rainstorm, illustrating how quickly water reaches the channel and the potential for flooding. It breaks down the river's response into distinct components.

Key elements of a storm hydrograph

• Rainfall input - Represented as bars on the graph, showing the timing and intensity of a rainstorm, with a peak indicating the heaviest rainfall.
• Discharge curve - A line showing the river's discharge over time, starting from a base level, rising to a peak, and then falling as the storm passes.
• Base flow - The normal, background discharge of the river before the storm, unaffected by immediate rainfall.
• Storm flow - The additional discharge caused by the rainstorm, shown as the area above the base flow on the graph.
• Lag time - The delay between the peak of rainfall and the peak of discharge. A shorter lag time means water reaches the river faster, leading to a steeper rise in discharge and a higher flood risk.
• Flood threshold - A marked level on the hydrograph above which the river will overflow its banks, indicating potential flooding.

Interpreting storm hydrographs

• Rising limb - The steepness of the curve as discharge increases after rainfall reflects how quickly water enters the river. A steep rise suggests rapid runoff and higher flood risk.
• Falling limb - The decline in discharge after the peak, showing how the river returns to normal levels post-storm.

Storm hydrographs are essential for predicting flood events, allowing authorities to prepare for and mitigate impacts on communities and infrastructure.

Factors influencing river regimes and storm hydrographs

Several environmental and human factors affect how a river's discharge behaves over time and during specific storm events. These factors influence lag times and the likelihood of flooding.

Environmental factors

• Rainfall amount and intensity - Heavy, intense rain leads to rapid overland flow as it cannot infiltrate the ground quickly, shortening lag time and increasing discharge peaks.
• Temperature and precipitation form - Cold temperatures can result in snow, which delays water reaching the river until it melts. Frozen ground prevents infiltration, causing quicker runoff once melting occurs.
• Slope steepness - Steep terrain accelerates surface runoff, reducing lag time as water flows swiftly to the river. Gentle slopes allow more infiltration, delaying water arrival.
• Rock type - Impermeable rocks, like granite, prevent water absorption, speeding up runoff. Permeable rocks, such as limestone, allow infiltration, slowing water delivery to the channel.
• Vegetation and land use - Dense vegetation intercepts rainfall, delaying its journey to the ground and increasing lag time. Bare or urban areas with concrete surfaces speed up runoff due to reduced interception and infiltration.

Human factors

• Dams and reservoirs - These structures store water, reducing downstream discharge during heavy rain and lowering flood risk. Controlled release through sluice gates can manage flow.
• Water abstraction - Increased removal of water for human use, such as irrigation or industry, alters natural river regimes, potentially reducing overall discharge and affecting hydrograph patterns.