Estimates of in-river concentrations (mg/l) and loads (kg/day) of nutrients to rivers in England and Wales from multiple sector sources, modelled with SAGIS (Source Apportionment GIS). The nutrients include nitrate (mg/l N) and ortho-phosphate (mg/l P); the estimate loads are expressed as kilograms per day (kg/day) and the in-river concentrations as milligrams per litre (mg/l). Sources are both diffuse and point. Diffuse sources include livestock farming, arable farming, highways, urban runoff, background (from soils), onsite wastewater treatment systems and atmospheric deposition. Point sources include treated wastewater effluent, combined sewer overflows and storm tanks, industrial discharges and mine water discharges. Concentrations and loads are modelled using the Environment Agency's catchment river model, SIMCAT, at the locations of model features or every 1 km along each river, taking into account all upstream sources and user defined river losses.
SAGIS is a modelling framework was developed through the UK Water Industry Research Programme (UKWIR) project 'Chemical Source Apportionment under the WFD' , with support from the Environment Agency and SEPA. The model is also described in 
 UKWIR (2012) Chemical Source Apportionment under the WFD (12/WW/02/3). Final report for UK Water Industry Research, 1 Queen Annes Gate, London, ISBN: 1 84057 637 5.
 Comber, S.D.; Smith, R.; Daldorph, P.; Gardner, M.J.; Constantino, C.; Ellor, B. (2013) Development of a Chemical Source Apportionment Decision Support Framework for Catchment Management. Environ. Sci. Technol. 47, 9824-9832
Publication date: 2017-02-23
This dataset has been modelled with The Source Apportionment-GIS (SAGIS) using data from the period spanning from 2006 to 2009. Input datasets include: river network outline, water body boundary, hydrology and river monitoring data, rainfall, land use and agricultural data, on-site wastewater treatment system, sewage treatment works, combined sewer overflows and storm tank locations, soil erosion, highway runoff and pollution inventory industrial discharges to river and sewer including extensive sewage treatment works data.
Loads are converted to concentrations in river waters using the SIMulation of CATchments (SIMCAT) water quality model, which is incorporated within SAGIS, so that the contribution to in-stream concentrations from individual sources can be quantified. Diffuse sources of nutrient pollution are incorporated into SAGIS from the Phosphorus and Sediment Yield Characterisation In Catchments (PSYCHIC) model (developed by a consortium of academic and government organisations led by ADAS Water Quality).
SAGIS models are not time based and do not show seasonal variation; river flow can vary according to seasonal changes which would have an impact on results. The model will also always be a simplified version of reality and so must be used as an advisory tool and not as the sole basis of decisions. Data is often subject to change and so the models require regular updates. Livestock and arable inputs have been calculated from the ADAS models PSYCHIC and NEAPN and have been shown to overestimate inputs in some areas. There is also a related issue, of changing crop types and land use.