Optimal Transport for Assessing Nitrate Source‐Pathway Connectivity. Issue 10 (13th October 2020)

• Record Type: Journal Article
• Authors: Husic, Admin; Fox, James; Mahoney, Tyler; Gerlitz, Morgan; Pollock, Erik; Backus, Jason
• Is Part Of: Water resources research. Volume 56:Issue 10(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-10-13
• Subjects: optimal transport -- connectivity -- pathways -- stable isotopes -- EMMA -- numerical modeling; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2020WR027446
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: Excessive nitrate threatens a wide range of water resources, aquatic habitats, and sensitive infrastructure. Despite this problem, tracing a nutrient from its eventual fate back to its origin remains an elusive challenge due to heterogeneity in how nutrient sources and hydrologic pathways are connected. Typically, this problem is underdetermined (i.e., too many unknowns, not enough equations) and cannot be solved with existing methodologies. The theory of optimal transport allows for the solution of underdetermined systems, and here we construct a novel formulation for its use in water quality modeling. Our objective was to develop an optimal transport modeling framework—coupled to Bayesian source unmixing, loadograph pathway separation, and geospatial connectivity analysis—to apportion nitrate loading from three sources (soil, fertilizer, and manure) across three pathways (quick, intermediate, and slow), resulting in nine possible source‐pathway couplings (soil‐quick, soil‐intermediate, …, manure‐slow). We apply this model to a 30 month elemental (NO3 − ) and isotopic (δ 15 N and δ 18 O) nitrate data set from a karst watershed in Kentucky, USA. Modeling results indicate that—of the nine possible source‐pathway couplings—nearly 60% of nitrate export is facilitated by just three: fertilizer‐quick (16.4%), manure‐intermediate (15.4%), and soil‐slow (27.2%). Further, we reinforce the need to explicitly consider heterogeneity in source‐pathway connectivity as homogeneous assumptions lead to erroneous inferences. The applicability of the model, its input requirements, and transferability to other sites is discussed. Lastly, we simulated two land management scenarios (field buffers and septic repair) and demonstrate how optimal transport can be used to test nutrient reduction strategies. Key Points: Optimal transport modeling allows for the solution of underdetermined source‐pathway transport problems Spatial heterogeneity in the connectivity of sources and pathways shifts the dominant couplings of nitrate transport We provide a new tool for identifying source‐pathway couplings that lead to high nutrient loading to streams