Estimating Watershed Residence Times

Project Title: Estimating watershed residence times in artificially-drained landscapes and relation to nutrient concentrations

Principal Investigator(s): Dr. Lisa R. Welp, Assistant Professor, Purdue University, Department of Earth, Atmospheric, and Planetary Sciences

Dates: March 1, 2018 – February 28, 2019

Total Federal Funds:  Total Non-Federal Funds:

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Dr. Gary Lamberti

Nutrient runoff from agricultural lands leads to Harmful Algae Blooms and eutrophication in freshwater ecosystems including the Great Lakes and the Gulf of Mexico. Best Management Practices (BMPs) implemented over the last few decades aim to reduce nutrient transport to streams and rivers. Evaluations of their effectiveness have found mixed results in reducing nutrient concentrations. This could indicate that BMPs are ineffective in certain areas, or simply that the residence time of water and nutrients in the watersheds are long and the effect of BMPs won’t be seen for decades. Watershed discharge is a combination of recent precipitation, soil water on the order of a year old, and decades-to-centuries old ground water, and the proportions vary with hydrology and land management. We aim to investigate the variability in residence times of local watersheds using stable isotope tracers and radon measurements and examine the relationships with nutrient concentration variability. This work will leverage 4 years of existing water stable isotope data and 8 years of nutrient concentrations from citizen scientist collections of streams during Wabash Sampling Blitz organized by the non-profit Wabash River Enhancement Corporation (WREC). Sampling occurs in the spring and fall under varying weather conditions. Stable water isotope time series have been used extensively for hydrograph partitioning and residence time calculations, but interpreting twice-yearly sampling of highly variable stream waters presents a challenge. We hypothesize that isotope variability in individual watersheds is correlated with residence times resulting in a spectrum of nutrient dynamics within the same land classification. Additionally, we will monitor a subset of watersheds for high-resolution variability (~biweekly) to verify results from the ‘snapshot’ Blitz method. This study will not directly test BMP effectiveness, but will provide a new context to examine the role of water age on nutrient dynamics.