Fig. S6.

Annual variability of monthly mean $\delta$D in precipitation according to samples collected at Porto Velho (8.77°S, 63.92°W) and Manaus (3.12°S, 60.02°W) under the Global Network for Isotopes in Precipitation (GNIP) measurement program. Data at Porto Velho were collected sporadically between 1965 and 1981, with the sample sizes ranging from four (July and August) to nine (November). Data at Manaus were collected sporadically between 1965 and 1990, with sample sizes ranging from 10 (September) to 17 (December). Effectively zero fractionation occurs during rainforest transpiration (34). $\delta$D in transpired water vapor should therefore approximately match $\delta$D in soil water; lacking direct observations, we estimate this as the precipitation-weighted mean $\delta$D in rainfall. The precipitation-weighted mean $\delta$D based on these data are approximately $-38‰$ at Porto Velho and $-26‰$ at Manaus. Previous studies suggest that this should represent a lower bound on $\delta$D in soil water, because the isotopic content of bound water accessed by tree roots in ecosystems with clearly delineated dry and wet seasons is weighted toward that of rainfall during the dry and transition seasons, when rain rates are lower and runoff fractions are smaller (70). GNIP data can be accessed at www.iaea.org/water.