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4.2: The water + Carbon cycle in contrasting locations: The Amazon…
4.2: The water + Carbon cycle in contrasting locations: The Amazon Rainforest case study
Carbon cycle
Stores
Trees (biosphere)
Large forest trees store 180 tonnes of carbon above ground and 40 tonnes in their roots.
60% of carbon in the rainforest is stored in biomass above the ground, such as tree stems, branches, and leaves. The remainder is below ground.
Soil
Soil stores average between 90 and 200 tonnes.
The amazon is a major global store of carbon, absorbing 2.4 billion tonnes a year.
The rainforest is a carbon sink: it absorbs 2.4 billion tonnes of carbon a year and released 1.7 billion through decomposition.
Geology
Most of the amazon rainforest is made up of ancient igneous metamorphic rocks. Carbonates are largely absent from the mineral composition of these rocks.
However, in the western parts of the basin, close to the Andes, there are outcrops of limestone. These store carbon in the slow carbon cycle and are signifcant regional carbon stores.
Flows
Decomposition
Much more speedy due to warm, humid conditions which quickly releases CO2.
1.7 billion tonnes of carbon are released a year through decomposition.
Overall rapid exchange between atmosphere, biosphere and soil.
Photosynthesis
Rates are higher than usual
What it looks like:
Leaf litter and other dead organic matter accumulates temporarily at the soil surface and rainforest soils.
High temperatures and humid conditions promote rapid decomposition of organic litter by bacteria, fungi, and other soil organisms.
Decomposition releases nutrients for immediate ake - up by tree roots systems, and emits CO2 which is returned into the atmosphere.
Background context:
An area that occupies 6 million km squared
The majority of the rainforest is in Brazil, but it also spans into 6 other neighbouring countries.
Water cycle in the Rainforest
Climatic features that affect the water cycle
High average annual temperatures (between 25-30 degrees C)
Low seasonal variation in temperature
High average annual rainfall (above 2000mm) with no dry season.
High average temperatures are due to intense insolation (from the sun) each year: but cloud coverage ensures temperatures do not reach the extremes of desert climates.
How flows and stores are different / affected
Evapotranspiration
Trees don't lose their leaves so there is a high level of interception (20 percent of rain intercepted by trees is evaporated)
High levels of evaporation due to high temperatures, dense vegetation and abundant moisture.
Evapo transpiration returns half of all rainfall back to the atmosphere
Due to dense vegetation, most evaporation occurs from intercepted moisture from leaf surfaces.
Moisture lost in the soil is derived from the soil via tree roots.
Runoff
Rapid runoff due to high and intense rainfall.
River discharge may peak in one or two months of the year due to seasonal distribution.
Atmosphere (store)
High temperatures allow the atmosphere to hold large amounts of moisture.
Humidity is high (contributes to high level of precipitation)
Soil/groundwater (store)
Abundant rainfall and deep permeable soil leads to significant water storage in soils and aquifers.
Precipitation
High average annual rainfall
Rainfall is usually intense and convectional (convectional rainfall is when air is heated from the ground and then cooled which causes rainfall)
'Recycling' system:
50-60 percent of precipitation in the amazon is recycled by evapotranspiration.
Water losses from the Amazon Basin are a result of river flow and atmospheric vapour moving to other regions.
Interception by forest trees is high (around 10 percent)
Vegetation (store)
Abundant vegetation absorbs and stores water from the soil and releases it through transpiration.
Physical factors affecting the stores and flows of water
Geology
Impermeable rocks
Large parts of the amazon form an ancient shield volcano that is formed of crystalline, impermeable rocks.
These have minimal storage capacity and cause rapid runoff.
Permeable rocks
Porous rocks such as limestone and sandstone in areas store rainwater and slow runoff.
Relief
Most relief is compromised of extensive lowlands.
Steep relief
Temperature
Human factors affecting stores and flows of Water
Deforestation
Figures:
Deforestation averaged 17,500 km a year between 1970 and 2013.
Since 1970 almost one fifth of the rainforest has been degraded or deforested.
Annual rates decreased initially since 2009 but have risen in recent years.
This has also contributed to devestating floods occuring on the maderia river, the largest tributary on the Amazon, where 60 people died.
Stores:
In the upper madeira drainage basin, deforestation has affected all stores of water. Deforestation has reduced water storage in trees, soils (which have been eroded) and permebale rocks (due to more rapid runoff) and in the atmosphere.
Trees:
Lower level of interception
Lower level of moisture absorbed from the soil
Lower level of water released into the atmosphere through transpiration.
Processes:
Runoff:
Speed of runoff has increased in areas that have been deforested.
For example, in the upper basin of the Madeira River, the main drier of the floods was deforestation in Bolivia and Peru. This was caused by 30,000km 2red of forest being cleared for subsistence farming and ranching.
The result of this was a massive reduction in water storage in trees and accelerated runoff.
Converting rainforest to grassland increases runoff by a factor of 27, and half of all rain falling on grassland goes directly to rivers which causes floods.
Evapotranspiration
Fewer trees: lower levels of evapotranspiration, less water rising and condensing into the atmosphere, less precipitation.
Condensation:
Lower level of humidity
Precipitation: Projections in future deforestation of Amazonia predict a 20 percent decline in regional rainfall as the rainforest dries out and as forest trees are replaced by grassland.
It isn't only regional areas that experience a reduction in rainfall: disruption of the regional water cycle caused by deforestation causes a loss of precipitation in areas hundreds of km away from deforested sites.