Water Resources Research Act Program

Details for Project ID 2009AR217B

Longitudinal evolution of nutrients in a mixed-use watershed under storm and non-storm flow regimes

Institute: Arkansas
Year Established: 2009 Start Date: 2009-03-01 End Date: 2010-02-28
Total Federal Funds: $20,380 Total Non-Federal Funds: $40,764

Principal Investigators: J. Romeis, Kristofor Brye, Andrew Sharpley, Jerral Skinner

Project Summary: The Eucha-Spavinaw and Illinois River watersheds in Arkansas and Oklahoma are phosphorus (P)-surplus watersheds where eutrophication-based water quality impacts have led to litigation to reduce non-point and point source P loading through P-based nutrient management protocols. While P management approaches are deservedly the focus of much environmental research, the most effective approaches to mitigate eutrophication impacts are those that integrate P and nitrogen (N) by accounting for each nutrient’s spatial distribution of source areas, preferred hydrologic pathways, biogeochemical processes (upland, near-stream, and in-stream), and sinks. Because P and N can differ in such respects, they should hypothetically exhibit unique ratios, concentration-flow relationships, and loading patterns in streams draining a watershed. These characteristics should vary temporally and longitudinally as a function of flow-regime, P and N loading, near-stream vs. in-stream processes, geology, soils, hydrologic connectivity, and other factors. Few studies have tested these hypotheses in small watersheds using direct analysis of in-stream and near-stream nutrient processes to account for observed changes in water quality. Research Branch is a first-order stream (184 hectares) in the upper Illinois River basin of Northwest Arkansas and an ideal site to test the above hypotheses in a small, mixed-use watershed. The stream drains the Watershed Research and Education Center in Fayetteville, Arkansas. Current studies of P and N concentrations in Research Branch suggest P and N exhibit unique longitudinal patterns, but available data is insufficient to determine controlling variables and to thus, be able to define sources, pathways, and effective best management practices (BMPs). The proposed research comprises two studies to examine longitudinal nutrient variability on an event and reach-by-reach basis. Study #1 will be performed during non-storm conditions using direct measurements to differentiate between groundwater and in-stream nutrient loading (or losses). Study #2 will be performed during stormflow conditions to characterize reach-by-reach nutrient-flow relationships and nutrient loads. Hydrologic data collection will include continuous streamflow measurement at five locations, groundwater level measurement at wells installed along five transects perpendicular to the stream, and rainfall. Water quality data collection for Study #1 will utilize mixed-frequency streamflow and groundwater sampling, short-term in-stream nutrient injection studies, and sediment and soil nutrient chemistry. Different statistical analyses will be used to relate longitudinal variability in stream water quality to sediment, soil, and groundwater nutrient chemistry as a function of land use and other factors. Mass-balance calculations will be used to differentiate between in-stream and near-stream nutrient loading in each reach. Water quality data collection for Study #2 will utilize different stormflow sampling methods to characterize storm event loading in all reaches including point source loads from agricultural ditches and urban runoff. Mass-balance methods will be used to estimate non-point storm loads to each reach. Longitudinal evolution in nutrient loads, hysteresis effects, nutrient ratios, and other water quality characteristics will be evaluated and related to land use, soil and sediment nutrient chemistry, and other results of Study #1. Findings from Study #1 and #2 will provide a foundation for future hydrologic and nutrient transfer studies in this and other regional watersheds, including studies aimed at quantifying effects of BMPs for mitigating nonpoint source inputs to streams.