Tropical rainforests provide essential ecosystem services to agricultural areas, including moisture recycling. Tropical deforestation has the potential to disrupt the hydrologic cycle and reduce rainfall. In the Amazon basin, drought frequency has increased in the late 20th and early 21st centuries, but the role of forests, ocean, and non-forested areas in causing or mitigating drought has not been determined. Using a precipitationshed moisture tracking framework, we quantify the contribution of forests, non-forested areas, and ocean sources of evaporation to rainfall in the state of Rondônia in the Brazilian Amazon. Forest sources account for approximately 48% of annual rainfall on average over 1981-2018, and more than half of that forest source (27% of annual rainfall) is from protected areas (PAs). Depending on the land cover dataset used to summarize the data, ocean and non-forested regions contribute 48-53% and 1-6%, respectively. During droughts (2005, 2010), moisture supply decreased significantly from oceans (19-21%) and non-forest (33-44%), while supply from the forest was stable (-4 to +1%) and compensated for the decrease in the ocean and non-forest sources, resulting in a much smaller decrease in rainfall (10-13%). Remote sensing and other land surface models corroborate the relative insensitivity of forest evapotranspiration to droughts. We conclude that 1) rainfall in a key agricultural region of the Amazon is vulnerable to forest loss, as much of the moisture source is the forest with varying levels of protection, and 2) forests mitigate drought, providing an important ecosystem service that could be disrupted with further deforestation. The introduction is the solo work of the author. Chapter 1.2, 1.3 and chapter 2, 3, 4 were written by the author and co-authors, Trent Biggs and Fernando De Sales. Chapter 1.2 – Chapter 4 were published in a journal paper at Geophysical Research Letters (Mu et al., 2021).