Flow path influence on an N:P ratio in two headwater streams: A paired watershed study

Abstract

Hydrologic flow paths were assessed and stream water quality was monitored in two adjacent watersheds to improve the understanding of how flow paths impact stream water nitrate nitrogen to total phosphorus ratios (NO3:TP). We hypothesized that a watershed with mostly subsurface flow paths will have high stream water NO3:TP ratios relative to a watershed with a greater tendency to have overland flow paths. Flow paths were assessed with end member mixing analysis (EMMA), stream water temperature, and stream discharge flashiness. The EMMA suggested that groundwater comprised a larger percentage of stream discharge in the west stream compared to the east stream. The analysis of stream water temperature and stream flashiness corroborated the results from the EMMA. Nitrate and TP concentrations were similar in both streams during low discharge. However, during storm flows nitrate decreased and TP increased in the east stream, while nitrate and TP concentrations remained similar during all discharge rates in the west stream. Thus, NO3:TP ratios in the east stream dramatically decreased during peak storm flows, while NO3:TP ratios remained relatively stable during all discharge rates in the west stream. Exploring hydrological processes helped explain the temporal variation within and spatial variation between the two streams, with regard to NO3:TP ratios. Further attention is necessary to understand whether hydrologic flow paths act as a control of other stream water N:P ratios and quantify flow path control relative to in‐stream processes.