CAREER: How streamflow ages vary downstream along river courses

The time that rain takes to move through a landscape and enter a river regulates contaminant transport and the movement of nutrients along streams. This time is influenced by the contribution of younger versus older water coming from direct rainfall events and groundwater. Estimating the proportion of these waters along a river system and how these contributions vary through time and space is key to predicting water quality and evaluating ecosystem health.

This project evaluates how quickly rain moves into rivers by measuring isotope compositions of rain, groundwater and river water. Specifically, the research aims at quantifying how the proportion of young versus old streamflow varies from headwaters to lowlands by collecting water samples and measuring a broad range of hydrologic tracers and common ions in these samples.

Outcomes of the project will advance knowledge on interactions between surface water and groundwater, the type of water that contributes to streamflow and the evolution of solute concentrations as water moves downstream in a river system. Through training of postdoctoral researchers and graduate and undergraduate students the project will contribute to the development of a strong workforce in geosciences.

The project will also build collaboration with the Nature Conservancy to enhance research translation for planners and decision makers.

Despite the importance of understanding the time water takes to flush through a watershed, there is limited understanding of landscape-scale controls on these transit times. Furthermore, the relative contributions of young and old water to streamflow in steep headwaters versus flatter lowlands remains poorly understood. Quantifying watershed transit times is vital to quantifying biogeochemical cycles and predicting contaminant fate and transport.

This project aims at determining the contribution to streamflow from young water derived from recent rain events versus old water derived prior to rain events to better understand how runoff generation and streamflow sources differ among headwaters versus lowlands. It further aims at quantifying the relationships between young versus old streamflow and solute concentrations and their (co)evolution downstream.

The work will conduct intensive monitoring and sampling along a single stream network in a southern and coastal California Watershed. Samples of precipitation, river water and groundwater will be collected at high spatiotemporal resolutions and analyzed for various tracers, including 18O/16O, 2H/1H, Na+, Cl- and 35S. Hydrograph separation techniques will be applied to assess the contributions of young versus old water to streamflow and quantify the relationship between these contributions and solute concentrations.

The study will fill knowledge gaps on groundwater/surface-water interactions in disturbed lands and Mediterranean climates across topographic conditions. Results obtained will be relevant to other Mediterranean ecoregions around the globe. The project will support training of a Postdoctoral researcher, a PhD student and undergraduate students and will provide opportunities to work closely with scientists from the Nature Conservancy to translate scientific results into better water policies and plans.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.