Concentration-discharge relationships derived from a larger regional dataset as a tool for watershed management

Abstract

Concentration-discharge (C-Q) relationships have been widely used to assess the hydrochemical processes that control solute fluxes from streams. Here, using a large regional dataset we assessed long-term C-Q relationships for total phosphorus (TP), soluble reactive phosphorus (SRP), total Kjeldahl nitrogen (TKN), and nitrate (NO₃ ) for 63 streams in Ontario, Canada, to better understand seasonal regional behavior of nutrients. We used C-Q plots, Kruskal-Wallis tests, and breakpoint analysis to characterize overall regional nutrient C-Q relationships and assess seasonal effects, anthropogenic impacts, and differences between "rising" and "falling" hydrograph limbs to gain an understanding of the dominant processes controlling overall C-Q relationships. We found that all nutrient concentrations were higher on average in catchments with greater levels of anthropogenic disturbance (agricultural and urban land use). TP, SRP, and TKN showed similar C-Q dynamics, with nearly flat or gently sloping C-Q relationships up to a discharge threshold after which C-Q slopes substantially increased during the rising limb. These thresholds were seasonally variable, with summer and winter thresholds occurring at lower flows compared with autumn and greater variability during snowmelt. These patterns suggest that seasonal strategies to reduce high flows, such as creating riparian wetlands or reservoirs, in conjunction with reducing related nutrient transport during high flows would be the most effective way to mitigate elevated in-stream concentrations and event export. Elevated rising limb concentrations suggest that nutrients accumulate in upland parts of the catchment during drier periods and that these are released during rain events. NO₃ C-Q patterns tended to be different from the other nutrients and were further complicated by anthropogenic land use, with greater reductions on the falling limb in more disturbed catchments during certain seasons. There were few significant NO₃ hydrograph limb differences, indicating that there was likely to be no dominant hysteretic pattern across our study region due to variability in hysteresis from catchment to catchment. This suggests that this nutrient may be difficult to successfully manage at the regional scale.

Keywords: concentration-discharge; nitrogen; nonlinear; phosphorus; regional; threshold; watershed management.

Fig. 1

Example of hydrologic seasonal boundary definitions, with (a) winter as the low‐flow period during the cold season, (b) snowmelt as the point where discharge increases rapidly over time following winter, (c) summer as the point when discharge begins to drop less dramatically following snowmelt, and (d) the period of increased flows between summer and winter. Data are from gauging station 02BA003, which is mainly forested (95%) with an area of 4,222 km².

Fig. 2

Conceptual diagram of differentiation between rising and falling hydrograph limbs used in this study. Discharge peaks were considered to be part of the rising limb, while discharge lows were considered part of the falling limb. Non‐events, in which mean daily discharge was equivalent to baseflow discharge (not shown) were not included in analysis.

Fig. 3

Ontario, Canada catchments used in regional concentration‐discharge analysis.

Fig. 4

Regional concentration‐discharge relationships for TP by hydrograph limb during each season. Concentration is represented by site‐specific z‐scored nutrient concentration and discharge is represented by site‐specific discharge exceedances. Each point is the mean of all data across all sites within the respective discharge bin. Z‐scores indicate how far above (positive values) or below (negative values) the concentration falls from the mean. Asterisks denote significant differences at a corrected alpha level of 0.05 (black) and 0.1 (gray) for individual discharge exceedance bins (Kruskal‐Wallis tests). Error bars represent standard errors. Points lacking error bars represent single measurements rather than averages.

Fig. 5

Regional concentration‐discharge relationships for SRP by hydrograph limb during each season. See Fig. 4 caption for details on axis units. Asterisks denote significant differences at a corrected alpha level of 0.05 (black) and 0.1 (gray) for individual discharge exceedance bins (Kruskal‐Wallis tests). Error bars represent standard errors. Points lacking error bars represent single measurements rather than averages.

Fig. 6

Regional concentration‐discharge relationships for TKN by hydrograph limb during each season. See Fig. 4 caption for details on the axis units. Asterisks denote significant differences at a corrected alpha level of 0.05 (black) and 0.1 (gray) for individual discharge exceedance bins (Kruskal‐Wallis tests). Error bars represent standard errors. Points lacking error bars represent single measurements rather than averages.

Fig. 7

Regional NO₃ C‐Q relationships for snowmelt (top row) and summer (bottom row) by hydrograph limb in catchments with low or high anthropogenic disturbance (agricultural plus urban land use). Concentration is represented by site‐specific z‐scored nutrient concentration and discharge is represented by site‐specific discharge exceedances, with the mean if all data across all sites taken within a discharge bin. Z‐scores indicate how far above (positive values) or below (negative values) the mean the concentration measurement is. Asterisks denote significant Kruskal‐Wallis tests at a corrected alpha level of 0.05 (black) and 0.1 (gray) for individual discharge exceedance bins. Error bars represent standard errors. Points lacking error bars represent single measurements rather than averages.

Fig. 8

Regional NO₃ C‐Q relationships for autumn and winter by hydrograph limb. Error bars represent standard errors. Points lacking error bars represent single measurements rather than averages. Concentration is represented by site‐specific z‐scored nutrient concentration and discharge is represented by site‐specific discharge exceedances, with the mean if all data across all sites taken within a discharge bin. Z‐scores indicate how far above (positive values) or below (negative values) the mean is the concentration measurement.