The water in streams and rivers started as rain and snow falling on the land surface. Yet a raindrop does not find its way immediately into a river, instead taking a journey that may include passing through vegetation, soil, and potentially rock underneath the soil, before emerging back in streams. This process of water transit controls how much freshwater is available, and when – including whether rivers continue to flow during periods of drought, or conversely if water makes its way too quickly to rivers during storms, causing flooding. At the same time, the time that water takes passing through soils and rocks controls the chemical reactions that take place between water and rock. These chemical reactions release nutrients that sustain ecosystems, and they pull carbon dioxide from the atmosphere into dissolved form in water. Many studies have investigated water transit times (the time it takes for water that fell as rain to emerge as streamflow) and their consequences around the world. Yet the tropics are under-represented in such studies, and the Amazon Basin in particular is poorly characterized – especially considering its major role as the world’s largest river system.

This award supported a multi-year investigation of water transit times, and associated chemical and ecological processes, across the transition from the Andes mountains to the Amazon floodplain in the Rio Madre de Dios River watershed of southeastern Peru. The study focused on eight small watersheds, each with areas close to or less than one square kilometer, spanning from steep high mountain terrain in the Andes to low-relief floodplain terraces. This transition defines the headwater of the Amazon River system and allows for testing of hypotheses about how mountain topography controls the storage and release of water, and related geochemical processes.

Approximately one thousand rain and stream water samples were collected and analyzed for the isotope composition of O and H in water, which was used to estimate water transit times using a modified version of established data analysis approaches. Results revealed that the lowland streams carry a low proportion of young water (less than a few months old), except during storms when they rapidly fill with recent rainfall. Additional chemical analyses of nutrient elements in stream waters showed that this storm-driven streamflow may be important in driving the loss of key nutrient elements from the forest, potentially linking forest health with the climate-associated storm frequency.

In the mountainous catchments in this study, water transit times were highly variable, highlighting the multiple factors that control the pathways of water in steep landscapes. Perhaps counter-intuitively, the steepest watersheds were not associated with the highest proportion of young water in streamflow. However, even where young water fractions in streamflow were low, the results from this study suggest that plants in this tropical environment are typically utilizing relatively recent rainfall as their water source.

This project supported the research of five graduate and four undergraduate students at USC, including a lead PhD student who produced many of the core findings from the research. A new field course in tropical hydrology and biogeochemistry was developed and taught twice in Peru, once online, and once in Colorado.

Last Modified: 07/08/2022
Modified by: A Joshua West