River Nutrient Loads and Catchment Size

Abstract

We have used a total of 496 sample sites to calibrate a simple regression model for calculating dissolved inorganic nutrient fluxes via runoff to the ocean. The regression uses the logarithms of runoff and human population as the independent variables and estimates the logarithms of dissolved inorganic nitrogen and phosphorus loading with R 2 values near 0.8. This predictive capability is about the same as has been derived for total nutrient loading with processbased models requiring more detailed information on independent variables. We conclude that population and runoff are robust proxies for the more detailed application, landscape modification, and in-stream processing estimated by more process-based models.The regression model has then been applied to a demonstration data set of 1353 river catchments draining to the sea from the North American continent south of the Canadian border. The geographic extents of these basins were extracted from a 1-km digital elevation model for North America, and both runoff and population were estimated for each basin. Most of the basins (72% of the total) are smaller than 10 3 km 2 , and both runoff and population density are higher and more variable among small basins than among larger ones.While total load to the ocean can probably be adequately estimated from large systems only, analysis of the geographic distribution of nutrient loading requires consideration of the small basins, which can exhibit significant hydrologic and demographic heterogeneity between systems over their range even within the same geographic region. High-resolution regional and local analysis is necessary for environmental assessment and management.

Future Nutrient Load Scenarios for the Baltic Sea Due to Climate and Lifestyle Changes

“…As previously shown in Smith et al (2003) and Smith et al (2005), population density and river discharge can, on a larger scale and to a first order, be seen as robust proxies for the flux of total nitrogen (TN) and total phosphorus (TP) from hydrologic catchments. Thus we have chosen to use this simple regression approach to assess the magnitude and trends associated with potential changes in riverine nutrient fluxes from major Baltic Sea sub-basins under future climate and population change scenarios.…”

“…Annual riverine nutrient loads from sub-basins within the Baltic Sea drainage basin (BSDB) were modeled in this study using the regression relationships presented in Smith et al (2005). Based on that work, annual riverine nutrient loads (L) were represented as a simple function of a region's annual discharge (Q) [m 3 ] and human population (X) [heads]:…”

“…In this current study we have calibrated the regression coefficients (k, a, and b in Eq. 1) for the relationships from Smith et al (2005) to sub-basins of the BSDB to model annual loads of TN [tons] and TP [tons].…”

“…The year 2000 was chosen since Eq. 1 was created using data from around the year 2000 (Smith et al 2005). This allows us to scale a person in a country with less animal protein consumption as less than one PE while a person in a country with higher consumption as more than one PE relative to the BSDB average.…”

“…The population-discharge proxies for nutrient loads from Smith et al (2005) (Eq. 1) were successfully calibrated on the available TN load (Table 1) and TP load (Table 2) 93 across all sub-basins for validation on TN loads and TP loads, respectively.…”