Assessing interannual variability in nitrogen sourcing and retention through hybrid Bayesian watershed modeling

Abstract

Abstract. Excessive nutrient loading is a major cause of water quality problems worldwide, often leading to harmful algal blooms and hypoxia in lakes and coastal systems. Efficient nutrient management requires that loading sources are accurately quantified. However, loading rates from various urban and rural non-point sources remain highly uncertain especially with respect to climatological variation. Furthermore, loading models calibrated using statistical techniques (i.e., hybrid models) often have limited capacity to differentiate export rates among various source types, given the noisiness and paucity of observational data common to many locations. To address these issues, we leverage data for two North Carolina Piedmont river basins collected over three decades (1982–2017) using a mechanistically parsimonious watershed loading and transport model calibrated within a Bayesian hierarchical framework. We explore temporal drivers of loading by incorporating annual changes in precipitation, land use, livestock, and point sources within the model formulation. Also, different representations of urban development are compared based on how they constrain model uncertainties. Results show that urban lands built before 1980 are the largest source of nutrients, exporting over twice as much nitrogen per hectare than agricultural and post-1980 urban lands. In addition, pre-1980 urban lands are the most hydrologically constant source of nutrients, while agricultural lands show the most variation among high- and low-flow years. Finally, undeveloped lands export an order of magnitude (∼7–13×) less nitrogen than built environments.

Contrasting Annual and Summer Phosphorus Export Using a Hybrid Bayesian Watershed Model

“…This pre/post‐1980 categorization of urban lands outperformed other categorizations, such as pre/post‐2000 and high/low‐density categorizations (not shown), as described in Miller et al. (2021) for TN. Potential reasons for higher phosphorus export from older development include higher impervious cover connectivity (Walsh et al., 2005), fewer stormwater control measures (Howells, 1990; NCDEMLR, 2013), more canopy and leaf litter over impervious surfaces (Janke et al., 2017), and older, leakier sewer infrastructure (Kaushal et al., 2011; Pennino et al., 2016).…”

“…WWTP data were only available for 1994-2017, which limited modeling of watersheds with major WWTPs (i.e., HR1,3,5,NH1,2,3,and FL1,3,10) to these years. Minor WWTP loads were projected backward from 1994, as described in Miller et al (2021).…”

“…(2012) considered multiple approaches to account for temporal variability within a hybrid watershed model, and results suggested that stream retention is inversely related to annual flows. Recent studies have incorporated precipitation‐driven interannual variability directly into the nitrogen export and retention formulations of the hybrid model (Miller et al., 2021; Strickling and Obenour, 2018). In addition, a Bayesian hierarchical framework (used by some of these studies) allowed incorporation of prior knowledge and rigorous uncertainty quantification, which can provide more robust management decision support (Reichert, 2020).…”

“…Han et al, 2021;Wiltsie et al, 2018). We build on the hybrid watershed modeling approach developed by Strickling and Obenour (2018) and Miller et al (2021), which is similar to SPARROW, but (unlike SPARROW) allows us to model interannual variability in loading and retention as a function of precipitation and land use change. Also, while these previous studies focused on total nitrogen (TN), here we explore total phosphorus (TP).…”

Ecosystem‐level effects of re‐oligotrophication and N:P imbalances in rivers and estuaries on a global scale

“…Thus, management measures should be mostly directed to further decrease N loads, which are often related to non-point sources coming from farming activities and to a lesser but still significant extent, urbanization (Miller et al, 2021).…”