Conceptual modelling for water quality management

Abstract

Water quality models are replete with implicit assumptions. Some of the assumptions may be legacy in nature, having originated from early development of a model and subsequently taken for granted within the scientific community. For instance, Easton et al. (2008) reported that the SWAT model implicitly assumes an infiltration-excess response to rainfall when predicting storm runoff, which is not applicable to humid, well vegetated regions and where saturation-excess is the dominant process. The re-conceptualisation of the SWAT model then led to the development of a new modelling approach SWAT-VSA. Development of an appropriate suite of conceptual models should be one of the first steps in water quality modelling – appropriate in the sense that they capture key operating processes (runoff generation, transport and delivery) and landscape connectivities within catchments. The history of water quality modelling is one of evolution with most of the popular models built on, or adapted from, existing models or experimental studies. As a consequence, the conceptualisations underpinning these models are rarely scrutinised sufficiently. In this paper, we use the eWater Source modelling framework as a case study for critically analysing conceptualisations of constituent generation and transport fluxes. We have selected this product as its water quality modelling component is currently being enhanced and our analysis can usefully inform its future. Our observations, which are pertinent to many water quality models, include: • The current filtration and transport processes are basic representations. Depending on the water quality parameters being modelled, these may not be sufficient to capture the range of processes needing to be quantified in catchment modelling projects. • The base spatial unit for generation is an unlinked functional unit (FU), which is a property-based semidistributed approach to spatial discretisation. This limits the ability to capture important factors such as the distance of FU to streams, drainage/channelisation on FUs and relationships between FUs in the same subcatchment. • The temporal scale for constituent generation is generally daily. Moving to sub-daily scales may be needed to better capture the influence of rainfall intensity, especially in urbanised catchments. • Some aspects of management impacts are not adequately considered, such as lag-time between catchment management actions and system response. It could be argued that many of these conceptualisations are simplifications designed to balance complexity with usability and computational efficiency. Nevertheless, they need to be challenged and alternatives explored if we are to advance the science and practical effectiveness of water quality modelling. In this paper we report our initial investigations and propose some potential improvements in conceptualisation to allow for better representation of the system for water quality management.