Evaluation of River Water Quality Simulations at a Daily Time Step – Experience with SWAT in the Axe Catchment, UK

Abstract

The Water Framework Directive (WFD) was established for the protection of surface waters (rivers, lakes, transitional and coastal waters) and ground waters in the European Union. The main environmental objective is to achieve and maintain a good status for all waters by the target date of 2015. Models which are able to address the majority of environmental objectives are proposed within the WFD to inform the management changes required to meet current water policy goals. The use of the Soil and Water Assessment Tool (SWAT) catchment model is widespread throughout the world, especially to support river basin management as required by the WFD. This paper provides a critical evaluation of the use of the model by placing model performance in the Axe catchment, UK, in the context of international performance of the model. Within the constraints of the available data, SWAT represents hydrology, sediment and ortho-phosphorus concentration well for this heterogeneous catchment, but the representation of daily nitrogen concentration dynamics is poor. Temporal aggregation of model outputs from daily to monthly improved the performance metrics for all the river outputs, including nitrate. Wider review of SWAT studies showed widespread reporting of monthly performance metrics within the SWAT studies, despite the model operating at a daily time step. Poor performance for nitrate identified in this current study may be a significant factor in the choice to not report daily results. This demonstrates the importance of ascertaining the reasons for the use of temporal aggregation in modelling studies.

Modelling of discharge, nitrate and phosphate loads from the Reda catchment to the Puck Lagoon using SWAT

“…In the Reda catchment such underestimation has not been noted, though. In contrast, Glavan et al (2011) reported poor goodness-of-fi t for nitrate simulation using SWAT in the Axe catchment in south-western England. Their results were particularly poor for daily concentrations (the model showed excessive variability compared to observations), while they were substantially better for nitrate loads.…”

“…Their results were particularly poor for daily concentrations (the model showed excessive variability compared to observations), while they were substantially better for nitrate loads. Interestingly, Glavan et al (2011) showed excellent model performance for phosphate simulation. This can be partly explained by the fact that point sources constitute almost half of the P-PO 4 loads in this catchment, while in the Reda catchment it is approximately 5%.…”

Strengths, Weaknesses, Opportunities and Threats of Catchment Modelling with Soil and Water Assessment Tool (SWAT) Model

“…There is also a question, whether to consider set guide levels for the rivers that does not represent an economic interest (Lohse, 2008); however rivers are not only economic asset. Careful evaluation of each scenario has to be performed according to its positive and negative issues on the environment, agriculture, social life and economy (Glavan et al, 2011, Volk et al, 2009. Climate change scenarios have a significant impact on the concentration of sediment, since both catchments would decrease by the legislation recommended concentration of 25 mg l -1 (Table 14).…”

Modelling of Surface Water Quality by Catchment Model SWAT

“…There is also the question of whether to consider set guide levels for the rivers that do not represent an economic interest (Lohse, 2008), however rivers are not only economic asset. When recommending possible agri-environmental mitigation measures to deliver water quality improvements, careful evaluation and prioritization of each measure has to be performed according to its positive and negative issues on the environment, agriculture, social life and economy (Bockstaller et al, 2009;Everard, 2004;Glavan et al, 2011).…”