An improved process-based representation of stream solute transport in the soil and water assessment tools

Abstract

Hydrological models have long been used to study the interactions between land, surface and groundwater systems, and to predict and manage water quantity and quality. The soil and water assessment tool (SWAT), a widely used hydrological model, can simulate various ecohydrological processes on land and subsequently route the water quality constituents through surface and subsurface waters. So far, in-stream solute transport algorithms of the SWAT model have only been minimally revised, even though it has been acknowledged that an improvement of in-stream process representation can contribute to better model performance with respect to water quality. In this study, we aim to incorporate a new and improved solute transport model into the SWAT model framework. The new process-based model was developed using in-stream process equations from two well established models-the One-dimensional Transport with Inflow and Storage model and the Enhanced Stream Water Quality Model. The modified SWAT model (Mir-SWAT) was tested for water quality predictions in a study watershed in Germany. Compared to the standard SWAT model, Mir-SWAT improved dissolved oxygen (DO) predictions by removing extreme low values of DO (<6 mg/L) simulated by SWAT. Phosphate concentration peaks were reduced during high flows and a better match of daily predicted and measured values was attained using the Mir-SWAT model (R 2 = 0.17, NSE = −0.65, RSR = 1.29 with SWAT; R 2 = 0.28, NSE = −0.04, RSR = 1.02 with Mir-SWAT). In addition, Mir-SWAT performed better than the SWAT model in terms of Chlorophylla content particularly during winter months, improving the NSE and RSR for monthly average Chl-a by 74 and 42%, respectively. With the new model improvements, we aim to increase confidence in the stream solute transport component of the model, improve the understanding of nutrient dynamics in the stream, and to extend the applicability of SWAT for reach-scale analysis and management. K E Y W O R D S OTIS, QUAL2E, soil and water assessment tool, stream solute transport, SWAT, tracer test, transient storage, water quality model

Representation of hydrological processes in a rural lowland catchment in Northern Germany usingSWATandSWAT+

“…The negative PBIAS indicates a slight underestimation of the measured values in all model applications. In comparison to previous SWAT modelling studies in the Kielstau catchment, which used different input data and time spans, all three model setups performed similarly or better according to goodness‐of‐fit indicators for the validation period (SWAT 3S model by Pfannerstill et al, 2014: NSE = 0.72, PBIAS = −4.4; SWAT 2012 model by Femeena et al, 2020: NSE = 0.78, RSR = 0.46; SWAT 2009 model by Fohrer et al, 2014: NSE = 0.76; SWAT 2005 model by Kiesel et al, 2010: NSE = 0.78; SWAT 2005 model by Schmalz, Tavares, & Fohrer, 2008: NSE = 0.63; SWAT 2005 model by Fohrer et al, 2007: NSE = 0.71). The slightly better performance of the models during the validation period suggests that the parameter set from the drier calibration period (851 mm mean annual precipitation) is similarly or even better suitable for the wetter validation period (963 mm mean annual precipitation).…”

“…In 2010, the Kielstau catchment was designated as a UNESCO demonstration site for ecohydrology (Fohrer & Schmalz, 2012; UNESCO, 2011). The Kielstau catchment has been modelled with different SWAT versions, focusing on the integration of tile‐drained areas (Fohrer et al, 2007), challenges in lowland hydrology (Schmalz, Tavares, & Fohrer, 2008), nitrate loads (Schmalz, Bieger, & Fohrer, 2008), sediment transport (Kiesel et al, 2010), the environmental fate of herbicides (Fohrer et al, 2014), representation of shallow groundwater layers (Pfannerstill et al, 2014), and enhancing SWAT with in‐stream process equations from other models (Femeena et al, 2020).…”

Temporal and Spatial Propagation Characteristics of the Meteorological, Agricultural and Hydrological Drought System in Different Climatic Conditions within the Framework of the Watershed Water Cycle

“…The distributed hydrological model, the Soil and Water Assessment Tool (SWAT), developed by USDA-ARS, simulates hydrological processes by incorporating various basin characteristics [25,26]. It has been widely used in watershed studies and has demonstrated good simulation performance [27][28][29][30]. In this study, the SWAT model is chosen to simulate water cycle processes in the basin.…”

Modeling in-stream biogeochemical processes at catchment scale: Coupling SWAT and RIVE models

“…Moreover, the numerical method employed to solve the QUAL2E differential equations in SWAT can lead to instabilities and solution inconsistencies, particularly in reaches with extended residence times (Woldegiorgis et al, 2017). Femeena et al (2020) noted high daily fluctuations in dissolved oxygen simulations, whereas the ability to simulate dissolved oxygen concentrations was critical for water quality modeling, due to their influence on nitrogen and phosphorus cycles. Regarding phytoplankton, the module represents a total algae biomass, without considering the diverse physiological properties (growth, mortality, nutrients uptake rates) of different algal groups, nor the influence of silica on algal development.…”

“…These mentioned studies focused on one aspect of water quality (algae, organic matter).To our knowledge refinement was made on both phytoplankton and organic carbon simulations, as well as on processes occurring in the benthic zone, in order to comprehensively describe the fate of nitrogen and phosphorus in streams. Further, Femeena et al (2020) integrated a new solute transport model developed based on OTIS (Onedimensional Transport with Inflow and Storage) to account for advection, dispersion, transient storage exchange and calculate at sub-daily scale and smaller segments. However, the computational time was 60 times longer, so they recommended to use the model only for small time period simulations.…”