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examBoard: Cambridge
examType: IGCSE
lessonTitle: Transpiration, Evaporation, Evapotranspiration
Rivers are dynamic systems that constantly change due to various processes. One of the most important aspects of river environments is how water moves not just through the channel, but also between the land, vegetation and atmosphere. This movement significantly impacts river discharge and the overall water cycle.
Key Definitions:
Evaporation, transpiration and evapotranspiration are crucial components of the water cycle that directly affect river environments. Understanding these processes helps explain why rivers behave differently across seasons and locations.
Evaporation occurs when water is heated and changes from liquid to gas. In river environments, evaporation happens from:
The rate of evaporation depends on:
Transpiration is like plant sweating! Plants draw water from the soil through their roots and release it as water vapour through tiny pores called stomata in their leaves.
The rate of transpiration depends on:
Evapotranspiration (ET) combines both evaporation and transpiration. It's a critical measurement for understanding how much water is returning to the atmosphere from a given area. This affects how much water remains available to feed into river systems.
The maximum possible water loss through ET if there's unlimited water available. It's what could happen under ideal conditions.
PET is highest when:
The real amount of water that evaporates and transpires in an area. This is usually lower than PET because water availability is limited.
AET depends on:
Evapotranspiration directly affects how much water reaches rivers. When ET rates are high:
This creates seasonal patterns in river flow, especially in areas with distinct wet and dry seasons.
High temperatures and more daylight hours increase evapotranspiration rates. This often leads to lower river levels and reduced discharge, even if rainfall remains consistent.
Lower temperatures and reduced daylight hours decrease evapotranspiration. More water remains in the system, potentially leading to higher river discharge relative to rainfall.
Tropical regions experience high year-round ET rates, while polar regions have very low rates. Temperate regions like the UK have significant seasonal variations in ET.
Scientists use several methods to measure evapotranspiration:
Human activities significantly affect evapotranspiration rates and consequently river environments:
When forests are cleared:
For example, deforestation in the Amazon has been linked to changes in local rainfall patterns and river flow regimes.
Urban development changes ET patterns by:
This often leads to flashier hydrographs with higher peak discharges.
The Three Gorges Dam on the Yangtze River created a massive reservoir covering 1,045 square kilometres. This dramatically increased the water surface area available for evaporation.
Studies estimate that evaporation from the reservoir surface is approximately 1,050 mm per year. This has measurably altered local humidity and rainfall patterns.
The dam also changed the local vegetation patterns, affecting transpiration rates. Areas that were once forested are now underwater, while new vegetation has grown in other areas due to changes in the water table.
These changes to evapotranspiration have contributed to:
This case study demonstrates how large-scale human interventions can significantly alter evapotranspiration processes and, consequently, river environments.
Climate change is altering evapotranspiration patterns globally:
These changes create new challenges for water resource management and river conservation.
Understanding evapotranspiration helps with:
Evaporation, transpiration and evapotranspiration are vital processes in river environments that:
Understanding these processes helps us better manage and protect river environments for the future.
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