Research Memo: Amazon rainforests mitigate drought in Rondônia

By Ye Mu and Cassie Sevigny

Map of the water cycle and the moisture sources for Rondônia

The Connections between Water and Rural Production project (CAP) investigates whether and which farmers adapt their production systems when they experience changes in rainfall, what adaptations they make, and whether these adaptations reduce income losses expected when drying occurs. Improved understanding of these responses will inform efforts by government agencies and civil society to help farmers adjust to changes in water availability. This research memo explores the impact of moisture from forests, non-forested areas, and the ocean on important agricultural areas in Rondônia, a state in the Brazilian Amazon.

As the largest tropical rainforest in the world, the Amazon forest has an extremely important impact on the regional and global water cycle and ecological balance. Moisture in the atmosphere over the Amazon mainly comes from air from over the Atlantic Ocean and evapotranspiration from rainforests. Evapotranspiration is the process of plants absorbing water from the soil, using it to grow, and releasing it into the air. Some of this water then condenses and returns to the earth in the form of rainfall. In a recent paper by Ye Mu, Trent. W. Biggs, and Fernando De Sales, our team found that the ocean and forested area each provide just under half of Rondônia’s moisture, and non-forest area only contributes about 1-6%.

In the past two decades, the atmosphere above the Amazon rainforest has become drier, leading to increased water demand, and making ecosystems vulnerable to fires and droughts. Amazonian droughts are generally triggered by ocean conditions: Occurrences of El Niño/La Niña (which depend on above or below average ocean temperatures combined with wind patterns) and the abnormal rise of sea level temperatures in the North Atlantic. In studying droughts in the Amazon river basin in 2005, 2010, and 2015, the team found about a 20% decrease in moisture from the ocean, while forest moisture remained the same. This indicates that forested areas are crucial in supplying moisture to Rondônia during droughts.

However, deforestation in the Amazon River Basin in Brazil has increased, which reduces evapotranspiration and atmospheric moisture at the source of the Amazon. Agricultural land for crops and pasture produces less moisture through evapotranspiration than forests, so changes in land use can worsen the effects of reduced water supply from the ocean. The loss of forests due to fire, logging, drought and potential feedbacks may threaten the forest's water sources, increase the risk of reduced rainfall, and therefore affect agricultural productivity and ecosystem functions. These risks may be higher as the climate changes, potentially outweighing the forest’s ability to provide steady moisture even during droughts.

Ye Mu, Trent. W. Biggs, and Fernando De Sales (2021). Forests mitigate drought in an agricultural region of the Brazilian Amazon. Geophysical Research Letters, 48:5. DOI: https://doi.org/10.1029/2020GL091380