Abstract
Identifying key flow pathways is critical in order to understand the transport of Phosphorus (P) from agricultural headwater catchments. High frequency/resolution datasets from two such catchments in Northwest England enabled individual events to be examined to identify the flow (Q) and Total P (TP) and Total Reactive P (TRP) dynamics (forensics). Detailed analysis of multiple flow and water quality parameters is referred to here as the event forensics. Are there more flow pathways than just surface runoff (dominated by overland flow) and baseflow (mainly groundwater) contributing at the outlet of these catchments? If so, hydrograph separation alone will not be sufficient. This forensic analysis gives a classification of four storm event response types. Three classes are based on the balance of old and new water giving enrichment and dilution of TRP pattern in the subsurface flow. A fourth type was observed where a plume of nutrient is lost to the channel when there is no observed flow. Modelling is also essential when used in combination with the event forensics as this additional tool can identify distinct flow pathways in a robust form. A case study will apply the Catchment Runoff Attenuation Flux Tool (CRAFT) to two contrasting small headwater catchments in Northwest England, which formed part of the Demonstration Test Catchments (DTC) Programme. The model will use data collected during a series of events observed in the two catchments between the period 2012 and 2014. It has the ability to simulate fast near surface (that can represent flow in the upper soil horizons and field drains) and event subsurface soil flow, plus slower groundwater discharge. The model can capture P enrichment, dilution and the role that displacement of “old” P rich water has during events by mixing these flows. CRAFT captures the dominant flow and P fluxes as seen in the forensic analysis and can create outputs including smart export coefficients (based on flow pathways) that can be conveyed to policy makers to better underpin decision making.
Highlights
Catchment modelling is a significant tool used for investigating water quality issues to provide evidence to inform policy makers and improve catchment management strategies [1]
The monitoring data collected at the Newby Beck Catchment (NBC) outlet at Morland were summarized by previous studies [6,20,25,26] and comprise 15-minute flow, electrical conductivity (EC), turbidity data and rainfall data and 30-minute Total Reactive P (TRP) and Total P (TP) concentrations
A modelling study using the Catchment Runoff Attenuation Flux Tool (CRAFT) with observations from two Eden part of the DTC programme (EdenDTC) catchments showed that fast subsurface flow was the key pathway for P export in most events in the Pow Beck Catchment (PBC), but the near surface runoff pathways dominated for Particulate P (PP) export in both catchments, more so in the NBC
Summary
Catchment modelling is a significant tool used for investigating water quality issues to provide evidence to inform policy makers and improve catchment management strategies [1]. Modelling can add to the knowledge base to better understand catchment dynamics, the identification of flow pathways that can affect water quality by transporting Phosphorus (P) and sediment from agricultural areas to the catchment outlet. Key hydrological and pollutant delivery processes that occur in small, temperate catchments include: critical source areas [2,8,9], variable source areas [10], overland flow [11], near surface flow/interflow [12], soil processes and leachate [11], subsurface stormflow [11], drain flow including land drainage [13], “shallow groundwater flux” / “return flow” [11], and lastly, deep groundwater flux [14] These processes all impact on P losses through either mobilising high concentrations of P and/or transporting P to the catchment outlet (or other observation points). There exists a need to capture the dominant P loss activity in models at many sites and scales to avoid being too site specific
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