Prioritizing river basins for nutrient studies

Abstract

Increases in fluxes of nitrogen (N) and phosphorus (P) in the environment have led to negative impacts affecting drinking water, eutrophication, harmful algal blooms, climate change, and biodiversity loss. Because of the importance, scale, and complexity of these issues, it may be useful to consider methods for prioritizing nutrient research in representative drainage basins within a regional or national context. Two systematic, quantitative approaches were developed to (1) identify basins that geospatial data suggest are most impacted by nutrients and (2) identify basins that have the most variability in factors affecting nutrient sources and transport in order to prioritize basins for studies that seek to understand the key drivers of nutrient impacts. The "impact" approach relied on geospatial variables representing surface-water and groundwater nutrient concentrations, sources of N and P, and potential impacts on receptors (i.e., ecosystems and human health). The "variability" approach relied on geospatial variables representing surface-water nutrient concentrations, factors affecting sources and transport of nutrients, model accuracy, and potential receptor impacts. One hundred and sixty-three drainage basins throughout the contiguous United States were ranked nationally and within 18 hydrologic regions. Nationally, the top-ranked basins from the impact approach were concentrated in the Midwest, while those from the variability approach were dispersed across the nation. Regionally, the top-ranked basin selected by the two approaches differed in 15 of the 18 regions, with top-ranked basins selected by the variability approach having lower minimum concentrations and larger ranges in concentrations than top-ranked basins selected by the impact approach. The highest ranked basins identified using the variability approach may have advantages for exploring how landscape factors affect surface-water quality and how surface-water quality may affect ecosystems. In contrast, the impact approach prioritized basins in terms of human development and nutrient concentrations in both surface water and groundwater, thereby targeting areas where actions to reduce nutrient concentrations could have the largest effect on improving water availability and reducing ecosystem impacts.

Keywords: Basin selection; Federal research; Hydrology; Monitoring design; Nutrients; Water quality.

Fig. 1

The 163 candidate basins overlain with the 18 hydrologic regions as defined by Van Metre et al. (2020). The numbers in parentheses after the region names are used to identify regions in subsequent figures

Fig. 2

Ranking results for candidate basins throughout the contiguous United States (CONUS) for (A) impact ranking approach and (B) variability ranking approach. Ranking and supporting data are provided in the Supplemental material and in Qi et al. (2023)

Fig. 3

Category scores for the impact approach are shown using violin plots. Category scores range from 0 (least impacted basin) to 1 (most impacted). See text for details. Violin plots depict data distributions using density curves; the width of a curve corresponds to the frequency of data at a point. The overall top-ranked basins in each region for the impact and variability approaches are identified with red and blue dots, respectively

Fig. 4

Category scores for the variability approach are shown using violin plots. Category scores range from 0 (least impacted basin) to 1 (most impacted). See text for details. Violin plots depict data distributions using density curves; the width of a curve corresponds to the frequency of data at a point. The overall top-ranked basins in each region for the impact and variability approaches are identified with red and blue dots, respectively

Fig. 5

Plot of source scores versus water quality scores for the impact ranking approach. Symbol colors are based on the 30-year mean precipitation values for the date range of 1990–2020 (PRISM, 2021). Basin scores for the Southwest Deserts and Atlantic Coast regions are outlined with circles and squares, respectively, to illustrate regions with low and high precipitation rates, respectively

Fig. 6

Top-ranked candidate basins for each hydrologic region throughout the contiguous United States for (A) impact ranking and (B) variability ranking. The names of the top-ranked basins are provided in Table 3. Ranking and supporting data are provided in the Supplemental material and in Qi et al. (2023)

Fig. 7

Distribution of median predicted total N concentration (top), minimum predicted total N concentration (middle), and range in predicted total N concentration for HUC08-sized basins within candidate basins in each region. Distributions are shown using violin plots. Violin plots depict data distributions using density curves; the width of a curve corresponds to the frequency of data at a point. The overall top ranked basins for the impact and variability approaches are identified with red and blue dots, respectively