Amazon River Basin Subsoil Moisture Levels

Overview

Subsoil moisture in the Amazon River basin is a critical component of the region's hydrological cycle, influencing river discharge, vegetation health, and the overall climate system. The Amazon's subsoil moisture levels are shaped by seasonal rainfall patterns, evapotranspiration, groundwater interactions, and episodic climate events such as El Niño.

Distribution and Seasonal Variability

• Soil moisture in the Amazon basin exhibits significant spatial and temporal variability. The first 1.5 meters of soil depth are particularly important for plant water availability and hydrological processes^[1].
• During normal conditions, soil moisture is generally higher in the northwestern and central Amazon, where rainfall is most abundant, and lower in the eastern and southern regions, which are more prone to seasonal droughts and longer dry spells^[1][2].
• Seasonal variations are pronounced, with soil moisture typically peaking during the wet season and declining during the dry season. Groundwater storage, which buffers the seasonal cycle, represents about 20–35% of the total terrestrial water storage variation in the basin^[3].

Impacts of Drought and Climate Events

• Severe droughts, such as those linked to El Niño events, can cause substantial reductions in subsoil moisture. For example, during the 2015/2016 El Niño, soil moisture in the top 1.5 meters dropped by nearly 30% across 75% of the basin, with the greatest deficits in the northeastern Amazon^[1].
• Recovery from such droughts can be slow, particularly in the northeastern region, with soil moisture anomalies persisting into the following dry season and not returning to average levels until several months later^[1].

Hydrological Balance

• The Amazon basin's water budget is dominated by precipitation, evapotranspiration (ET), surface runoff, and base flow (groundwater contribution). On average, ET accounts for 45–49% of precipitation, while base flow (groundwater-fed river flow) represents 77% of total runoff, underscoring the importance of subsoil and groundwater reserves for sustaining river discharge during dry periods^[2].
• Soil moisture, surface water, and groundwater together make up the basin's terrestrial water storage, with soil moisture accounting for about 27% of this total^[4].

Trends and Threats

• Rising temperatures and changing rainfall patterns, driven by climate change, are expected to exacerbate soil moisture deficits and increase water stress in the Amazon, with potential impacts on forest health and the broader regional climate^[5][6].
• The transition from rainforest to savanna in some parts of the basin is a concern if prolonged soil moisture deficits become more frequent due to climate change or deforestation^[6].

Summary Table: Water Storage Components in the Amazon Basin

Component	Share of Terrestrial Water Storage
Surface Water	49%
Soil Moisture	27%
Groundwater	24%

Source: Frappart et al. (2019); Zanin et al. (2024)^[4][3]

Key Points

• Subsoil moisture in the Amazon is highly dynamic, with strong seasonal and spatial patterns.
• Droughts cause significant and sometimes prolonged reductions in subsoil moisture, especially in the eastern and northeastern basin.
• Soil moisture is crucial for ecosystem health and river flow, and is increasingly threatened by climate change and deforestation^[1][5][6].

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• https://royalsocietypublishing.org/doi/10.1098/rstb.2018.0084     
• https://journals.ametsoc.org/view/journals/hydr/15/6/jhm-d-14-0068_1.xml  
• https://www.sciencedirect.com/science/article/abs/pii/S0309170816305681  
• https://www.sciencedirect.com/science/article/abs/pii/S0168192324001552  
• https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GL111462  
• https://wwf.panda.org/discover/knowledge_hub/where_we_work/amazon/amazon_threats/climate_change_amazon