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Precipitation is a vital part of the water cycle and plays a crucial role in shaping river environments. It's the main source of water for rivers and directly impacts their flow, characteristics and surrounding ecosystems.
Key Definitions:
Rain: Liquid water drops that fall when water vapour condenses in the atmosphere.
Snow: Ice crystals formed when water vapour freezes in the atmosphere.
Sleet: A mixture of rain and snow, or partially melted snow.
Hail: Balls or lumps of ice that form in thunderstorm clouds when water droplets freeze in layers.
Rain gauge: A simple instrument that collects and measures rainfall.
Weather radar: Uses radio waves to detect precipitation in the atmosphere.
Satellite imagery: Provides large-scale views of precipitation patterns.
Units: Usually measured in millimetres (mm) or inches.
Precipitation isn't evenly distributed across the Earth. Several factors influence where, when and how much precipitation falls:
Areas near the equator receive more rainfall due to intense heating, causing air to rise and form clouds. The tropics typically receive high precipitation, while subtropical regions (around 30° N and S) often have dry climates.
Mountains force air to rise, cool and release moisture on the windward side (orographic rainfall). The leeward side experiences a rain shadow effect with much less precipitation.
Coastal areas typically receive more precipitation than inland areas. Maritime air masses carry moisture from oceans, while continental air masses are often drier.
The amount, intensity and duration of precipitation directly affect river systems in several ways:
The UK shows clear regional differences in precipitation. Western areas, particularly Wales, northwest England and western Scotland, receive significantly more rainfall (over 3,000mm annually in some upland areas) than eastern regions (less than 700mm in parts of East Anglia). This is due to prevailing westerly winds bringing moisture from the Atlantic Ocean, which falls as orographic rainfall when air rises over western hills and mountains.
These precipitation patterns directly influence the UK's river systems, with western catchments typically having higher discharge and flashier regimes than eastern rivers.
Before precipitation reaches the ground and enters river systems, it may be intercepted by vegetation and other surfaces. This process, known as interception, plays a crucial role in the water cycle and river environments.
Key Definitions:
When rain falls on vegetation:
1. Some is caught and held by leaves and branches (interception storage)
2. Some drips through gaps in the canopy (throughfall)
3. Some runs down stems and trunks (stemflow)
4. Intercepted water may evaporate back to the atmosphere
Reduces runoff: Less water reaches the ground immediately
Delays water entry: Slows the movement of water into river systems
Reduces flood risk: Helps prevent sudden surges in river discharge
Increases evaporation: Returns water to the atmosphere
The amount of precipitation intercepted varies widely depending on several factors:
Different types of vegetation intercept varying amounts of precipitation:
- Coniferous forests: 25-45% interception
- Deciduous forests: 10-25% interception
- Grasslands: 5-15% interception
- Crops: Varies by type and growth stage
- Light rain: Higher percentage intercepted
- Heavy rain: Lower percentage intercepted as canopy becomes saturated
- Duration: Longer events lead to lower overall interception rates
- Type: Snow may be intercepted differently than rain
- Deciduous trees intercept more in summer when in leaf
- Winter interception rates are lower in deciduous forests
- Coniferous forests maintain relatively consistent interception year-round
- Crop interception varies with growth stage
Human activities significantly affect interception processes and, consequently, river environments:
Pickering has historically suffered from flooding, with major events in 1999, 2000, 2002 and 2007. Instead of traditional flood defences, a natural flood management approach was implemented, including:
- Planting 29 hectares of woodland upstream to increase interception
- Creating 167 small dams made of logs and branches to slow water flow
- Building a small bund to store floodwater temporarily
This "Slowing the Flow" project has successfully reduced flood risk by increasing interception and slowing water movement through the catchment. During heavy rainfall in December 2015, when nearby areas flooded, Pickering remained dry, demonstrating the effectiveness of using natural processes like interception to manage river environments.
Interception has a measurable impact on river discharge patterns, which can be seen in river hydrographs:
Understanding the relationship between precipitation, interception and river systems is crucial for managing river environments sustainably and reducing flood risks in a changing climate.