Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2008
Publication Date: 12/11/2008
Citation: Green, M.B. and D. Wang. Watershed flow paths and stream water nitrogen-to-phosphorus ratios under simulated precipitation regimes. Water Resources Res. W12414:1-13. 2008. Interpretive Summary: Watersheds and their complex network of flow paths regulate the movement of water and nutrients to aquatic ecosystems. Overland and subsurface (soil and ground water) flow paths transport nitrogen (N) and phosphorus (P) with differing levels of efficiency; P transport occurs via surface flow paths almost exclusively and N transport can utilize both overland and subsurface flow paths. Stream water N:P ratios influence ecosystem community composition, nutrient limitation of primary production, and trophic dynamics. The Soil and Water Assessment Tool (SWAT) was used to build a model for a small pristine watershed, calibrated and validated with historical and measured field data. Responses of stream water N:P ratio was related to a wide range of precipitation regimes and flow pathways using a relatively simple sensitivity analysis. Results from the study indicate that watershed flow paths influence stream water N:P ratios, and that groundwater connectivity with the stream, defined as the sum of the active watershed flow paths, is a fundamental condition that helps explain the variability of N:P ratios. The finding has important implications for the watershed transport of other elements. Such information allows more informed management of aquatic ecosystems in a changing environment.
Technical Abstract: Stream water nitrogen to phosphorus (N:P) ratios influence algal community composition and nutrient limitation in lotic ecosystems, showing trends of N limited algae in arid climates and P limited algae in humid climates. Yet, little is known about mechanisms that cause spatial and temporal variation of N:P ratios. This study evaluates the relationship between precipitation regime (annual mean precipitation and its frequency), watershed flow pathways, and stream water total N to total P (TN:TP) ratios, using a model based on a central Minnesota watershed. The purpose of the modeling study was to examine the hydrologic control of TN:TP ratios through simple sensitivity analysis of a watershed model. We built a model that represented a realistic combination of hydrology and biogeochemistry, followed by 161 simulations under a wide range of precipitation frequency and intensity scenarios. Precipitation regime controlled total runoff and the subsurface hydrologic connectivity, which had implications for TN and TP concentrations and TN:TP ratios. Results supported the hypothesis that watershed hydrology controls stream water TN:TP ratios, providing further evidence for a hydrologic mechanism driving the difference of stream water N:P ra tios between streams in arid and humid climates.