Simulating high frequency water quality monitoring data using a catchment runoff attenuation flux tool (CRAFT)

Abstract

High resolution water quality data has recently become widely available from numerous catchment based monitoring schemes. However, the models that can reproduce time series of concentrations or fluxes have not kept pace with the advances in monitoring data. Model performance at predicting phosphorus (P) and sediment concentrations has frequently been poor with models not fit for purpose except for predicting annual losses. Here, the data from the Eden Demonstration Test Catchments (DTC) project have been used to calibrate the Catchment Runoff Attenuation Flux Tool (CRAFT), a new, parsimonious model developed with the aim of modelling both the generation and attenuation of nutrients and sediments in small to medium sized catchments. The CRAFT has the ability to run on an hourly timestep and can calculate the mass of sediments and nutrients transported by three flow pathways representing rapid surface runoff, fast subsurface drainage and slow groundwater flow (baseflow). The attenuation feature of the model is introduced here; this enables surface runoff and contaminants transported via this pathway to be delayed in reaching the catchment outlet. It was used to investigate some hypotheses of nutrient and sediment transport in the Newby Beck Catchment (NBC) Model performance was assessed using a suite of metrics including visual best fit and the Nash-Sutcliffe efficiency. It was found that this approach for water quality models may be the best assessment method as opposed to using a single metric. Furthermore, it was found that, when the aim of the simulations was to reproduce the time series of total P (TP) or total reactive P (TRP) to get the best visual fit, that attenuation was required. The model will be used in the future to explore the impacts on water quality of different mitigation options in the catchment; these will include attenuation of surface runoff.

Identifying Flow Pathways for Phosphorus Transport Using Observed Event Forensics and the CRAFT (Catchment Runoff Attenuation Flux Tool)

“…This case study used data collected from two instrumented sub-catchments investigated by the EdenDTC (Demonstration Test Catchments) project; these are located in the River Eden catchment in Northwest England [6,20,25,26], see Figure 2 below. Figure 2.…”

“…In terms of P (principally total phosphorus (TP) and reactive phosphorus (RP)) the key points from several case studies in agricultural catchments are that firstly the flow pathways for each form of P can be quite different, for dissolved and particulate forms of P [10,15,20,27,28]. The faster flow pathways, typically referred to as "surface runoff", capture the dynamics of P transfer during events but can be relatively short-lived.…”

“…When selecting the model, it was found that one criticism of water quality models is that they contain too many calibrated parameters and possess equifinality [29]. Therefore, the spatially lumped rainfall runoff model-the Catchment Runoff Attenuation Flux Tool (CRAFT) was used to glean additional information on the flow pathways in the catchments [20,21]. The CRAFT is designed to be as parsimonious as possible.…”

“…The model has an appropriate number of flow pathways, the flexibility to swap or remove pathways and works at the spatial and temporal resolution of the observed data, i.e., catchment scale and hourly data. The philosophy behind the development of the CRAFT was to mimic the physical processes of a catchment and its flow pathways using the minimum information requirement (MIR) approach [8,20,33], in this case by utilizing a set of linear storage-discharge relationships to represent outflows from four stores in the CRAFT (Figure 1). Three stores are associated with their flow pathway to the outlet, with the fourth store being an attenuation store that is connected to the surface runoff pathway.…”

“…All RP in the slow groundwater and fast subsurface pathways is assumed to be soluble so will be referred to subsequently as SRP. Particulate P (PP) is also modelled in the surface runoff pathway only, this is typically seen as an enrichment of P from the soil to water [20]. The model structure will be described in more detail below (Section 2.3).…”