Water Resource Associates

Pollution is a massive factor causing health problems across the world, with more than one in six people worldwide - 894 million – not having access to safe freshwater. Water pollution is one of the major causes of death for children under 5 years and every 20 seconds a child dies because of poor sanitation and poor water supply. The UN has created the Sustainable Development Goal 6.3 to provide safe drinking water globally, but this is far from being achieved at present. Across the world there is increasing concern about water quality and new methods to plan and manage sustainable clean water systems are required. Across the EU and the UK there are concerns about the wide range of pollutants that affect water resource systems as well as environmental change such as land use change and climate change.

Climate change is becoming a threat, with changes in flooding, hydrological regimes and temperatures and this will affect water resources, river ecology, agriculture, terrestrial ecosystems and land use. For example, nitrogen (N) in lowland and upland freshwater systems can cause eutrophication, leading to rapid aquatic plant growth. Such increases in growth are often viewed as a nuisance as certain plant species may grow at the expense of others and, within freshwaters, the microbial breakdown of the dead plant matter can lower oxygen levels which is detrimental to invertebrate and fish populations. The problems of freshwater eutrophication are usually associated with lowland, intensively farmed areas where fertilisers provide a significant source of N and P and/or urban areas where domestic and industrial effluent is discharged to the receiving watercourse and groundwaters.

Whilst management strategies have been implemented to control N and P in river systems, these have tended to address single issues: either diffuse or point sources, or upland or lowland areas. However, the chemical concentrations and loads in rivers reflect the integration of the catchment sources such as fertiliser inputs, atmospheric deposition and sewage discharges. Superimposed on these anthropogenic inputs are contributions from the vegetation and mineralisation (and subsequent nitrification) of organic chemicals in soils. Furthermore, the combination of the multiple catchment chemical sources has a downstream effect, influencing the options for further water utilization and impacting the water quality of estuarine and marine areas. Thus, given the holistic nature of the pollution an integrated management approach is required.

The INCA suite of models has been developed to support such an integrated catchment approach and there have been over 50 applications of INCA to catchments around the world.

The INCA model has been designed to investigate the fate and distribution of chemicals in the aquatic and terrestrial environment. The model simulates flow and water quality and tracks the flow paths operating in both the land phase and riverine phase. The model is dynamic in that the day-to-day variations in flow and water quality can be investigated following a change in input conditions form point or diffuse sources such as atmospheric deposition, sewage discharges or fertiliser addition. The model can also be used to investigate a change in land use (e.g. moorland to forest or pasture to arable) or a change in climatic conditions. Dilution, natural decay and biochemical transformation processes are included in the model as well as the interactions with plant biomass such as nutrient uptake by vegetation on the land surface or macrophytes in streams.

INCA has been designed to be easy to use and fast, with excellent output graphics. The menu system allows the user to specify the semi-distributed nature of a river basin or catchment, to alter reach lengths, rate coefficients, land use, velocity-flow relationships and to vary input loads.

INCA provides the following outputs:

• Daily time series of flows, water quality concentrations at selected sites along the river;
• Profiles of flow or water quality along the river at selected times;
• Cumulative frequency distributions of flow and water quality at selected sites;
• Tables of statistics for all sites;
• Daily and annual pollution loads for all land uses and all processes.

Original References for Overall Model Philosophy
Wade, A.J., Durand, P., Beaujouan, V., Wessel, W.W., Raat, K.J., Whitehead, P.G., Butterfield, D., Rankinen, K. and Lepisto, A. (2002) A nitrogen model for European catchments: INCA, new model structure and equations. Hydrology and Earth System Sciences, 6, 559-582.

Whitehead, P.G., Wilson, E.J. and Butterfield, D. (1998) A semi-distributed Integrated Nitrogen Model for Multiple source assessment in Catchments (INCA): Part I - Model Structure and Process Equations. Science of the Total Environment, 210/211: 547-558.

Whitehead, P.G., Wilson, E.J., Butterfield, D. and Seed, K. (1998) A Semi-distributed Integrated Nitrogen Model for Multiple source assessment in Catchments (INCA): Part II Application to large River Basins in South Wales and Eastern England. Science of the Total Environment, 210/211: 559-583.

INCA Versions
Over the past 22 years the INCA suite of models has evolved to include the following:

• INCA-PEco, an extension of INCA-P model to include biological oxygen demand (BOD), dissolved oxygen (DO) and algal growth and decay
• INCA-P for flow, total P, dissolved P, particulate P, macrophytes, epiphytes and phytoplankton
• INCA-N for flow, nitrate and ammonia
• INCA-Sed for flow, sediments (including size fractions) and hydraulic parameters such as stream power and shear velocities
• INCA-C for flow and carbon, including DOC, DIC and particulate carbon transport
• INCA-Metals for flow, and metals (including arsenic, lead, cadmium, copper, nickel, chromium, manganese and molybdenum)
• INCA-Mercury for flow, total mercury and methyl mercury INCA- Trit for radioactive pollution events
• INCA DO/BOD for flow, dissolved Oxygen, and Biochemical Oxygen Demand
• INCA Pathogens for flow, pathogens such as total coliforms, e.coli. etc
• INCA Micro-Plastics for flow and microplastics with 6 size fractions
• INCA Organics for a wide range of chemicals such as POPS, PCBs, Endocrine disrupters, emergent contaminants
• INCA Salinity for flow and chloride

Model Applications, Land Use and the Key Issues Investigated
INCA has been applied in a wide range of application, issues and scales. The table below shows a list of some of the applications ranging from catchments of a few hectares up to very large catchments such as the Garonne at 56,000 km².

Download summary of sites, data and policy issues studied in various INCA projects

Download Map of Location of various INCA projects

Case Study 1: INCA and catchment sensitive farming

INCA has been applied to the River Test Catchment to test the likely effects of new catchment sensitive farming strategies in the UK on flow, nitrate, ammonia, total and soluble phosphorus, sediments and ecology (macrophytes and epiphytes). An important feature of the Test catchment is the relatively low slopes and the underlying geology of chalk which gives the catchment a relatively high base flow index of 0.94. This means that the groundwater is a very important component of the flow regime and will dominate the hydrology of the catchment. Land use is dominated by arable agriculture and grassland for animal production. A range of catchment sensitive farming techniques have been investigated using the model.

The effect of the scenarios on phosphorus and sediment concentrations along the River Test are fairly minimal, as shown in the Figure below.

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Case Study 2: Modelling mercury in Loch Nagar

A new version of INCA has been developed to assess the build up of mercury in catchments from atmospheric mercury deposition and also from terrestrial sources of mercury. The major issue is the release of methyl mercury into upland streams and lakes and the high concentrations of mercury in fish in northern European lakes.

This is a serious problem in Sweden where many lakes have high mercury levels in fish and these are so high that the fish are toxic to humans if eaten.

The new version of INCA has been developed to simulate the build up of mercury on soils and the leaching of mercury into rivers and lakes.

The model has been applied to the Gardsjon catchment in Sweden and Loch Nagar in Scotland.

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Case Study 3: Point and diffuse pollution in urban catchments

The impacts of diffuse and point source pollution climate change on water quality in the Tame and Anker Rivers (sub catchments of the River Trent) has been assessed using the INCA suite of Models to simulate flow, phosphorus, and ecology.



The model has been used to assess the effects of different treatment methods for phosphorus at the STWs. The simulation below shows for the each reach boundary along the rivers and information on the means, plus and minus the standard deviations and the 95 percentile levels.

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Case Study 4: A modelling study undertaken as part of an EA project to assess the likely impacts of climate change on Water Quality across the UK.

A range of climate change (UKCIP) scenarios have been used to generate future precipitation, evaporation and temperature time series at a range of catchments across the UK. These time series have then been used to drive the INCA suite of flow, water quality and ecological models to simulate flow, nitrate, ammonia, total and soluble reactive phosphorus, sediments, macrophytes and epiphytes in the Rivers Tamar, Lugg, Tame, Kennet, Tweed and Lambourn. A wide range of responses have been obtained with impacts varying depending on river character, catchment location, flow regime, type of scenario and the time into the future.



The results from the project are complex as might be expected but consistent patterns are obtained and it is possible to make some statements about the likely outcomes:

• In the lowland southern River Lambourn declining concentrations of Soluble Reactive Phosphorus (SRP) were predicted for winter months and increasing concentrations during summer and autumn months, caused by lower flows and hence reduced dilution, as shown in the Figure below. These changes in concentrations, together with changes in temperature and flow affects the ecology as well, with changes in macrophyte growth in summer months.
• Sediments in the Lambourn are predicted to increase throughout the year but are particularly high in autumn after dry summers. The build up of sediments over dry periods followed by increased autumn flows seems the main mechanism here.
• Results from the urbanized midlands River Tame show similar increased SRP levels in summer but higher increases in winter due to diffuse urban runoff. In the western and rural River Lugg (a tributary of the River Wye) the SRP is predicted to decrease in winter but increase in summer months.
• Nitrate levels in the northern River Tweed show increased winter levels in upland headwaters as organic nitrogen is released and decreased levels in summer months due to drought and increased denitrification in the river. However, the lower Tweed shows increased nitrates in winter but the highest increases are in summer. This change is due to change in land use to agriculture and point source discharges in the lower reaches of the Tweed.
• Nitrate levels in the south-western River Tamar has a similar response to the Tweed with higher nitrate in winter and lower summer nitrates in the upper reaches. In the lower reaches of the river, nitrates increase both in summer and in winter. The rate of change increases over time and also with the severity of the emission scenario for all the river systems.

Full details of the project are given in the EA report Potential Impacts of Climate Change on River Water Quality by Whitehead et al, 2008, Science Report - SC070043/SR1

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Case Study 5: Inca modelling of metals downstream of mines



Mining in the Transylvania region of Romania is a major issue now that Romania has joined the EU. Old mines going back to pre-Roman times have been polluting the rivers for over 2000 years. Moreover there is a desire to reopen and develop the mines to both clean up the polluting discharges and extract metals to boost the local economy.

We have undertaken an extensive study of the Aries and Mures River systems in Transylvania in Northern Romania where an old mine is having a major impact of river water quality.



A new version of INCA has been created to simulated the fate and distribution of mine waste from old mines and the model has been applied to the Aries- Mures River System.

The INCA-Mine model has been used to assess the environmental improvements expected from a clean up operation at the mine as well as the environmental impacts of reopening this old mine.

Whitehead, P. G., Mimouni, Z., Butterfield, D., Bussi, G., Hossain, M. A., Peters, R., Shawal, S., Holdship, P., Rampley, C.P.N., Jin, L. and Ager, D. 2021. "A New Multibranch Model for Metals in River Systems: Impacts and Control of Tannery Wastes in Bangladesh" Sustainability 13, no. 6: 3556.

Whitehead P.G., Bussi G., Hughes J.M.R., Castro-Castellon A.T., Norling M.D., Jeffers E.S., Rampley C.P.N., Read D.S. and Horton A.A. Modelling Microplastics in the River Thames: Sources, Sinks and Policy Implications. Water. 2021; 13(6):861. https://doi.org/10.3390/w13060861.

Crossman, J., Bussi, G., Whitehead, P.G., Butterfield, D., Lannergård, E. and Futter, M.N. (2021) A New, Catchment-Scale Integrated Water Quality Model of Phosphorus, Dissolved Oxygen, Biochemical Oxygen Demand and Phytoplankton: INCA-Phosphorus Ecology (PEco). Water 2021, 13, 723. https://doi.org/10.3390/w13050723.

Whitehead, P.G., Wade, A.J. and Butterfield, D. (2009) Potential impacts of climate change on water quality and ecology in six UK rivers. Hydrology Research, 40(2-3): 113-122.

Whitehead, P.G., Butterfield, D. and Wade, A.J. (2009) Simulating metals and mine discharges in river basins using a new integrated catchment model for metals: Pollution impacts and restoration strategies in the Aries-Mures river system in Transylvania, Romania. Hydrology Research, 40(2-3): 323-345.

Futter, M.N., Skeffington, R.A.,Whitehead, P.G. and Moldan, F. (2009c) Modelling stream and soil water nitrate dynamics during experimentally increased nitrogen deposition in a coniferous forest catchment at Gardsjon, Sweden. Hydrology Research, 40(2-3): 187-197.

Whitehead, P.G. Heathwaite, A.L., Flynn, N.J., Quinn, P.F., Hewett, C. and Wade, A. (2007) Evaluating the Risk of Non-point Source Pollution from Sewage Sludge: Integrated Modelling of Nutrient Losses at Field and Catchment Scales. Hydrology and Earth Systems Science, 11(1): 601-613.

Futter, M.N., Butterfield D., Cosby, B.J., Dillon, P.J., Wade, A.J. and Whitehead, P.G. 2007 Modeling the mechanisms that control in-stream dissolved organic carbon dynamics in upland and forested catchments. Water Resources Research. 43 (2).

Ranzini, M., Forti, C., Whitehead, P.G., Arcova, F., Cicco, V. and Wade, A.J. (2007) Integrated Nitrogen Catchment Model (INCA) applied to a Tropical Catchment in the Atlantic Forest, Sao Paulo, Brazil. Hydrology and Earth Systems Science, 11(1): 614-622.

Whitehead, P.G., Wilby, R.L., Butterfield, D. and Wade, A.J. (2006) Impacts of Climate Change on Nitrogen in Lowland Chalk Streams: Adaptation Strategies to Minimise Impacts. Science of the Total Environment, 365: 260-273.

Whitehead, P.G., Wilby, R.L., Butterfield, D. and Wade, A.J. (2006) Impacts of Climate Change on Nitrogen in Lowland Chalk Streams: Adaptation Strategies to Minimise Impacts. Science of the Total Environment, 365, 260-273.

Wilby, R.L., Whitehead, P.G., Wade, A.J., Butterfield, D., Davis, R.J. and Watts, G. (2006) Integrated Modelling of climate change impacts on water resources and quality in a lowland catchment: River Kennet, UK. Journal of Hydrology, 330, Issues 1-2 , 204-220.

Wade A.J., Neal, C.,Whitehead, P.G., and Flynn, N. (2005) Modelling Nitrogen Fluxes from the land surface to the coastal zone in European systems: the perspective of the INCA project. Journal of Hydrology, 304: 413-429.

Whitehead, P.G., Hill, T., and Neal, C.N. (2004) Impacts of forestry on Nitrogen in upland and lowland catchments: a comparison of the River Severn at Plynlimon in Mid Wales and the Bedford Ouse in South-East England using the INCA model. Hydrology and Earth System Science, 8(3): 533-544.

Whitehead, P.G., Lapworth, D.J., Skeffington, R.A. and Wade, A. (2002) Excess nitrogen leaching and C/N decline in the Tillingbourne catchment, southern England: INCA process modelling for current and historic time series. Hydrology and Earth System Science, 6(3): 455-466.

Whitehead, P.G., Johnes, P.J. and Butterfield, D. (2002) Steady state and dynamic modeling of nitrogen in the River Kennet: impacts of land use change since the 1930s. Science of the Total Environment, 282-283: 417-435.

Wade, A. J., Whitehead, P. G. and Butterfield, D. (2002) The Integrated Catchments model of Phosphorus dynamics (INCA-P), a new approach for multiple source assessment in heterogeneous river systems: model structure and equations. Hydrology and Earth Systems Sciences, 6, 583-606.

Wade, A. J., Hornberger, G. M., Whitehead, P. G., Jarvie, H. P. and Flynn, F., (2002) On modelling the mechanisms that control in-stream phosphorus, macrophyte and epiphyte dynamics: an assessment of a new model using General Sensitivity Analysis. Water Resources. Res., 37, 2777-2792.

Wade, A.J., Durand, P., Beaujouan, V., Wessel, W.W., Raat, K.J., Whitehead, P.G., Butterfield, D., Rankinen, K. and Lepisto, A. (2002) A nitrogen model for European catchments: INCA, new model structure and equations. Hydrology and Earth System Sciences, 6, 559-582.

Wade, A.J., Whitehead, P.G., Hornberger, G.M., Snook, D. 2002. On modelling the flow controls on macrophytes and epiphyte dynamics in a lowland permeable catchment: the River Kennet, southern England. Science of the Total Environment, 282-283, 395-417.

Whitehead, P.G., Wilson, E.J. and Butterfield, D. (1998) A semi-distributed Integrated Nitrogen Model for Multiple source assessment in Catchments (INCA): Part I - Model Structure and Process Equations. Science of the Total Environment, 210/211: 547-558.

Whitehead, P.G., Wilson, E.J., Butterfield, D. and Seed, K. (1998) A Semi-distributed Integrated Nitrogen Model for Multiple source assessment in Catchments (INCA): Part II Application to large River Basins in South Wales and Eastern England. Science of the Total Environment, 210/211: 559-583.