A lagged variable model for characterizing temporally dynamic export of legacy anthropogenic nitrogen from watersheds to rivers

Abstract

Legacy nitrogen (N) sources originating from anthropogenic N inputs (NANI) may be a major cause of increasing riverine N exports in many regions, despite a significant decline in NANI. However, little quantitative knowledge exists concerning the lag effect of NANI on riverine N export. As a result, the N leaching lag effect is not well represented in most current watershed models. This study developed a lagged variable model (LVM) to address temporally dynamic export of watershed NANI to rivers. Employing a Koyck transformation approach used in economic analyses, the LVM expresses the indefinite number of lag terms from previous years' NANI with a lag term that incorporates the previous year's riverine N flux, enabling us to inversely calibrate model parameters from measurable variables using Bayesian statistics. Applying the LVM to the upper Jiaojiang watershed in eastern China for 1980-2010 indicated that~97 % of riverine export of annual NANI occurred in the current year and succeeding 10 years (~11 years lag time) and~72 % of annual riverine N flux was derived from previous years' NANI. Existing NANI over the 1993-2010 period would have required a 22 % reduction to attain the target TN level (1.0 mg N L −1 ), guiding watershed N source controls considering the lag effect. The LVM was developed with parsimony of model structure and parameters (only four parameters in this study); thus, it is easy to develop and apply in other watersheds. The LVM provides a simple and effective tool for quantifying the lag effect of anthropogenic N input on riverine export in support of efficient development and evaluation of watershed N control strategies.

Legacy Nutrient Dynamics at the Watershed Scale: Principles, Modeling, and Implications

“…Another recent attempt is applying Hydrus 1D model with meteorological and soil physical data, and boundary conditions to evaluate catchment lag time for nutrient or solute transport through unsaturated zone in a grassland catchment and an arable catchment ($10 km 2 ) in Ireland (Vero et al, 2017). By adopting simple mass balance and equivalent substitution rules, Chen et al (2014aChen et al ( , 2015a developed two new dynamic models to express the watershed legacy N mass and the indefinite number of lag terms from previous years' NANI by functions of the previous 1 year's riverine N flux. The two models were well calibrated by 31 years recodes in the Yongan watershed eastern China and yield close results concerning the legacy N source contributed riverine N fluxes (i.e., 72%-85% vs 64%-81%) and N leaching lag time lengths (i.e., 12 years vs 11 years).…”

“…The two models were well calibrated by 31 years recodes in the Yongan watershed eastern China and yield close results concerning the legacy N source contributed riverine N fluxes (i.e., 72%-85% vs 64%-81%) and N leaching lag time lengths (i.e., 12 years vs 11 years). These two models also allow partitioning of the complete long-term mass balance for the fate (e.g., transient storage, riverine export, and loss/retention by denitrification, biomass uptake, and wood product export) of annual NANI (Chen et al, 2014a) and to estimate dynamic export of annual NANI by river over years (Chen et al, 2015a). Based on these efforts, Van Meter and Basu (2015) coupled the MODFLOW/MODPATH approach and a simple SON decomposition function to address hydrologic (nitrate in groundwater) and biogeochemical (sorbed organic N within the root zone) N legacies.…”

“…As a result, there is a continuous built-up of legacy nutrients within the watershed Van Meter et al, 2016). Legacy nutrients are the surplus anthropogenic nutrient inputs from previous years that are temporarily stored within the watershed (e.g., soil, vadose zone, groundwater, and sediments) and have the potential to contribute nutrients to the atmosphere, biomass, and waters (Chen et al, 2015a;Sharpley et al, 2013;Van Meter et al, 2016). At the global scale, net P accumulation rates (deducing P loss to waters via runoff ) in cropland soils increased from 1 Tg P year À1 in 1950 to 8 Tg P year À1 in 2000 with cumulative accumulation of about 210 Tg P between 1970 and 2010 Sattari et al, 2012).…”

“…Legacy nutrients are of increasing concerned due to their considerable magnitude and more dispersive spatial distributions compared to current anthropogenic nutrient additions, and they have significant environmental and agronomic implications (Chen et al, 2015a;Meals et al, 2010;Powers et al, 2016;Rowe et al, 2016;Van Meter et al, 2016). Legacy N or P can be released and remobilized, acting as a continuing source to downstream waterbodies for years, decades, or even centuries (Chen et al, 2014b;Haygarth et al, 2014;Meals et al, 2010;Sanford and Pope, 2013;Sharpley et al, 2013;Van Meter et al, 2016).…”

“…Yongan River watershed, eastern China, 1980-2010Chen et al (2015a Processes The outlet concentration is assumed as a function of both the hydrologic and biogeochemical legacy, and the patterns of land-use change:…”