How do water and carbon cycles operate in the Amazon Rainforest?

Location and Context

The Amazon Rainforest is the largest tropical rainforest on Earth, covering around 6.1 million km² across eight countries in South America, with around 60% located in Brazil. The region has a hot, humid equatorial climate, with average temperatures of 26–28°C and annual rainfall typically between 2,000 and 3,000 mm. The Amazon River Basin accounts for around 15–20% of global river discharge, making it the most important drainage basin in the tropics.

Water and Carbon Cycles in the Tropical Rainforest

Water Cycle: Key Stores and Flows

• Canopy interception: Around 10–20% of rainfall is temporarily held on leaves and branches.
• Soil moisture: Deep tropical soils store large volumes of water, with high infiltration in undisturbed forest.
• Groundwater and baseflow: Sustain year-round river discharge.
• Evapotranspiration: Around 1,000–1,200 mm per year, returning up to 60% of annual rainfall to the atmosphere.
• Moisture recycling: About 25–50% of rainfall within the basin originates from evapotranspiration.
• Run-off: Generally low in intact forest but increases rapidly when land is cleared or compacted.

Carbon Cycle: Key Stores and Fluxes

• Above-ground biomass: Typically 150–200 tonnes of carbon per hectare
• Below-ground roots: Around 30–50 tonnes of carbon per hectare.
• Soil organic carbon: Around 80–120 tonnes of carbon per hectare.
• Net primary productivity (NPP): Approximately 6–9 tonnes of carbon per hectare per year.
• Decomposition: Rapid due to warmth and humidity; slower in flooded forests where organic matter can build up.

From an Individual Tree to the Whole Rainforest

A single large tree can transpire 300–1,000 litres of water per day, driving convectional uplift and cloud formation. Deep roots access moisture during dry periods and maintain transpiration, influencing local and regional rainfall patterns. Each tree stores several tonnes of carbon, and collectively the rainforest acts as a major global carbon sink.

Physical Factors Affecting the Water Cycle

• Temperature: High year-round temperatures sustain strong evapotranspiration and convectional rainfall.
• Rock permeability and porosity: The Amazon Basin sits mainly on weathered sedimentary rocks and lateritic soils with high porosity, encouraging infiltration. In contrast, crystalline rocks have low permeability, leading to more surface run-off.
• Relief: The low-lying Amazon plains promote flood storage and delayed river response, while the Andean headwaters are steep, producing rapid run-off and high sediment loads.

Physical Factors Affecting the Carbon Cycle

• Temperature: Warm conditions enhance both photosynthesis and respiration, with small climatic shifts able to change the forest from a sink to a source.
• Vegetation: Evergreen, multi-layered canopies capture large amounts of carbon through photosynthesis.
• Soil organic matter: Decomposes rapidly in warm, oxygen-rich conditions; slower decomposition in waterlogged areas allows carbon accumulation.
• Mineral composition of rocks: Highly weathered oxisols contain iron and aluminium oxides that limit nutrients, leading to more carbon stored in vegetation than in soil.

Drainage Basin Example: The Rio Madeira, Brazil

The Rio Madeira is one of the largest tributaries of the Amazon and an area that has experienced widespread deforestation and agricultural expansion.

Natural Conditions

Before large-scale clearance, forest cover promoted high infiltration, low overland flow, and stable river discharge throughout the year.

Human and Natural Changes

• Deforestation: Reduces canopy interception and evapotranspiration, lowering local rainfall and increasing surface run-off and sediment loads.
• Farming: Cattle grazing and mechanised crops compact soils, lowering infiltration and increasing flash floods.
• Hydropower dams: Modify flow regimes, trap sediment, and alter seasonal flooding.
• Climate variability: During El Niño years, reduced rainfall and higher temperatures increase the risk of wildfires and droughts.

Impacts of Human Activity on Carbon Flows, Soils and Nutrients

• Deforestation and burning: Release large quantities of CO₂, CH₄ and N₂O. Soil organic carbon can decline by 10–40% within a decade of clearance.
• Loss of nutrients: Burning and erosion remove phosphorus, potassium and calcium from the soil.
• Degradation: Selective logging and edge effects reduce canopy density, increase tree mortality and make the forest more vulnerable to drought.
• Agriculture: Pasture and cropland store far less carbon, have shallower root systems and lower evapotranspiration.

Strategies for Managing the Amazon Rainforest

Forest Protection and Restoration

• Protected areas and Indigenous reserves preserve canopy cover, maintain interception and evapotranspiration, and protect biodiversity.
• Riparian buffer zones stabilise riverbanks, improve infiltration and store carbon in regrowing vegetation.
• Forest restoration and afforestation through assisted natural regeneration increase carbon storage and improve local water cycles.

Sustainable Agriculture

• Agroforestry: Combines trees with crops such as cocoa and Brazil nuts, maintaining canopy cover and storing over 100 tonnes of carbon per hectare.
• Silvopastoral systems: Integrate trees into grazing land, improving soil porosity and water retention while storing additional biomass carbon.
• Improved pasture management: Rotational grazing and ground cover crops help rebuild soil structure and organic carbon.
• No-till farming: Reduces soil disturbance, retains moisture, and limits carbon loss.

Fire and Land-use Governance

• Controlled burning and firebreaks reduce the risk of high-intensity wildfires.
• Zero-deforestation policies (e.g. the Soy Moratorium and Forest Code) reduce conversion of primary forest to agriculture.
• Together, these strategies restore interception and evapotranspiration, increase infiltration and baseflow, and enhance biomass and soil carbon storage.

Summary

The Amazon Rainforest plays a crucial role in the global water and carbon cycles. At the scale of an individual tree, high transpiration and carbon uptake influence rainfall and climate regulation.

At the basin scale, the forest’s structure and extent determine how much water infiltrates, evaporates, or flows to the ocean, and how much carbon is stored or released.

Human activity—particularly deforestation and farming—has disrupted these cycles, while sustainable management and forest restoration can help re-establish balance and strengthen the Amazon’s role as a global carbon sink.