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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Evapotranspiration Changes
Evapotranspiration is a crucial but often overlooked part of the water cycle that affects everything from local weather patterns to global climate systems. When we change landscapes through deforestation or urban development, we alter this vital process with far-reaching consequences.
Key Definitions:
In natural ecosystems, plants pull water from the soil through their roots and release it as water vapour through their leaves. This transpiration, combined with direct evaporation from soil and water bodies, creates a cooling effect and contributes moisture to the atmosphere that eventually forms clouds and precipitation.
A single large tree can transpire up to 400 litres of water per day! Forests are responsible for about 10% of the moisture in our atmosphere. In tropical rainforests, evapotranspiration can return up to 90% of rainfall to the atmosphere, creating a self-sustaining cycle of precipitation.
Human activities have dramatically altered natural landscapes, disrupting evapotranspiration patterns across the globe. These changes affect local and regional climate, water availability and ecosystem health.
When forests are cleared, the complex relationship between vegetation and the water cycle is disrupted. Trees are nature's water pumps and moisture regulators - removing them has multiple effects:
Without trees, less water returns to the atmosphere through transpiration, reducing atmospheric moisture and rainfall in the region.
Deforested areas become warmer as they lose the cooling effect of evapotranspiration, creating "heat islands" that can affect local climate.
With fewer plants to absorb water, more rainfall becomes surface runoff, increasing erosion and flooding risks while reducing groundwater recharge.
The Amazon rainforest generates about 50% of its own rainfall through evapotranspiration. Scientists have discovered "flying rivers" - massive air currents of moisture released by the forest that carry more water than the Amazon River itself! Deforestation has reduced this moisture recycling, contributing to more frequent droughts in parts of Brazil. Some areas have seen rainfall decrease by up to 20% following significant forest clearing.
As cities expand, natural vegetation is replaced with impermeable surfaces like concrete and asphalt. This transformation fundamentally changes how water moves through the environment.
Cities can be 2-5°C warmer than surrounding rural areas. This "urban heat island" effect occurs partly because cities lack the cooling benefit of evapotranspiration. Hard surfaces absorb heat and have no way to cool themselves through water evaporation like plants do.
In urban areas, up to 75% of rainfall becomes immediate runoff (compared to 10% in forested areas). This not only increases flood risk but also means less water is available for evapotranspiration, further intensifying the heat island effect.
Changes in evapotranspiration don't just affect the water cycle - they have cascading effects on ecosystems, weather patterns and human communities.
Reduced evapotranspiration can lead to decreased rainfall, especially in continental interiors that rely on recycled moisture. This can trigger or worsen drought conditions.
Evapotranspiration changes can create feedback loops in the climate system. For example, drier conditions lead to plant stress, which reduces transpiration further, creating even drier conditions.
Changes in local humidity and temperature due to altered evapotranspiration can make habitats unsuitable for native species, contributing to biodiversity loss.
Singapore has implemented a "City in a Garden" approach to combat the urban heat island effect. The city has increased green cover from 36% to nearly 50% since the 1980s. Green roofs, vertical gardens and park connectors have been strategically placed throughout the city. These green spaces increase evapotranspiration, helping to cool the city by up to 4°C in planted areas. The approach also reduces flooding by slowing water runoff and improves air quality as plants filter pollutants.
Scientists use various methods to measure evapotranspiration to understand how it's changing and what impacts these changes might have.
Evapotranspiration can be measured using lysimeters (devices that measure water moving through soil), eddy covariance systems (which measure water vapour flux) and remote sensing from satellites. These measurements help scientists track changes over time and understand how human activities are affecting the water cycle.
Computer models combine data on vegetation cover, climate and soil conditions to predict evapotranspiration rates. These models help scientists forecast how changes in land use might affect future water availability and climate conditions.
Understanding the importance of evapotranspiration has led to various strategies to maintain or restore healthy water cycles in human-modified landscapes.
Replanting trees in deforested areas can help restore evapotranspiration rates. Strategic reforestation in watersheds can improve water security by maintaining rainfall patterns.
Green roofs, rain gardens and urban forests increase evapotranspiration in cities, cooling urban areas and reducing flood risk by slowing water runoff.
Agroforestry (combining trees with crops) and conservation agriculture maintain higher evapotranspiration rates than conventional farming while protecting soil moisture.
Evapotranspiration is a perfect example of how human activities can disrupt natural cycles with far-reaching consequences. By understanding these processes, we can develop more sustainable approaches to land management that work with natural systems rather than against them.
Remember these key points:
For your IGCSE exam, be prepared to explain how human activities affect evapotranspiration and the consequences of these changes. Make connections between evapotranspiration, the water cycle, climate and biodiversity. Remember to use specific examples like the Amazon rainforest or urban heat islands to support your answers.
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