The INCA Models Used in Aquascope

1. Introduction

Versions of the INCA model have been developed for a wide range of water quality parameters and hydrological processes. These models simulate how pollutants, nutrients, sediments, pathogens, carbon, metals, and other materials move through landscapes and rivers, allowing Aquascope to understand and forecast environmental change at a daily temporal resolution. All INCA models represent both terrestrial (land-based) and in-stream (river-based) processes, including inputs from agriculture, wastewater, land management, and atmospheric deposition; transformations such as nitrification, mineralisation, sorption, algal uptake, and carbon cycling; transport of dissolved and particulate materials; and loss pathways including gaseous emissions or burial in sediments. This process-based structure makes INCA uniquely suited for modelling complex ecosystems such as wetlands, peatlands, forests, grasslands, and mixed-use agricultural catchments.

2. The INCA Family of Models

2.1 INCA-N: Nitrogen

INCA-N simulates the fate and transport of nitrate (NO₃⁻) and ammonium (NH₄⁺), capturing soil nitrogen transformations, groundwater transport, river mixing and assimilation, and seasonal agricultural inputs. This model is essential for understanding eutrophication, nitrogen saturation, agricultural leaching, and climate impacts on water quality.

Fig. 3 Nitrogen processes and flow paths (INCA-N)

2.2 INCA-P & INCA-PEco: Phosphorus and Ecology

INCA-P simulates the transport and cycling of dissolved phosphorus (TDP, SRP), particulate phosphorus (PP), organic phosphorus, and soil and sediment interactions (adsorption–desorption, Langmuir isotherm processes). The ecological extension INCA-PEco adds phytoplankton dynamics, macrophytes and epiphytes, DO/BOD behaviour, and streambed and sediment interactions.

Fig. 5 INCA-P nutrient flows and process controls

2.3 INCA-C: Carbon

INCA-C focuses on dissolved organic carbon (DOC) mobilisation and cycling, particularly relevant for peatlands, forested catchments, acidification recovery, and climate-sensitive DOC export.

2.4 INCA-Sed: Sediment

A physically based sediment transport model that simulates soil erosion, sediment mobilisation, deposition, and resuspension. Now embedded within the latest INCA-P versions.

2.5 INCA-Contaminants

Simulates a broad suite of anthropogenic pollutants including persistent organic pollutants (POPs), microplastics, metaldehyde, and PFAS.

2.6 INCA-Path: Pathogens

Models E. coli and other pathogens, assessing agricultural and wastewater influences.

2.7 INCA-Metals & INCA-Hg

Models heavy metal transport, mercury and methylmercury processes.

2.8 PERSiST Hydrological Model

A flexible eco-hydrological rainfall–runoff model used to generate soil moisture deficit, hydrologically effective rainfall, groundwater delays, and flow pathways. PERSiST is often used as the hydrological engine underlying INCA modules.

3. How INCA Works: Model Structure

INCA is a daily, dynamic, mass-balance, semi-distributed catchment model. It is structured into Level 1: River catchment; Level 2: Sub-catchments (1 to 6 land-use types each); and Level 3: Land-phase cell model (soil and groundwater processes).

Fig. 2 Landscape and in-stream integration (INCA model structure)

3.1 GIS-Based Catchment Representation

Sub-catchments defined by digital elevation models, up to six land-use classes per sub-catchment, and combined land and river processes scaled through multi-reach routing.

3.2 Land-Phase Hydrology

INCA simulates soil water movement, groundwater contributions, direct runoff, and subsurface and slow-flow hydrological processes. Driven by Soil Moisture Deficit, Effective Rainfall, Temperature, and Precipitation.

3.3 Land-Phase Biogeochemistry

Processes include Nitrogen: mineralisation, nitrification, denitrification, plant uptake; Phosphorus: adsorption, desorption, erosion, sediment interactions; Carbon: DOC production and leaching; and Sediments: mobilisation and transport.

3.4 In-Stream River Processes

INCA uses a multi-reach river network to simulate dilution and mixing, algal uptake and respiration, DO/BOD dynamics, sorption–desorption exchanges, and settling, burial, resuspension.

Fig. 4 Instream reach structure and nitrogen processes (INCA-N)

4. Data Requirements

INCA can operate in data-poor or data-rich environments. The model requires spatial data (land use classifications, soil and geology, sub-catchment boundaries), meteorological data (precipitation, temperature, soil moisture deficit, hydrologically effective rainfall), and management data (fertiliser amounts and timing, livestock densities, wastewater discharges, growing season parameters).

5. Model Outputs

INCA generates daily flows and concentrations (N, P, DOC, sediments, pathogens, metals), annual nutrient fluxes, land-use-specific loads, reach-by-reach river profiles, cumulative probability distributions, and full mass-balance closure for audit verification. These outputs feed directly into Aquascope's analytics, scenario comparisons, and certification-ready MRV.

6. Global Applications of INCA

INCA has been applied to catchments worldwide, including Europe: Thames, Wye, Cleddau, Tweed, Garonne; Africa: Volta, Awash; Asia: Ganges, Brahmaputra, Mekong, Nepalese headwaters; Oceania: Murrumbidgee, Waikato; North America: Lake Simcoe; and Urban systems: Dhaka River Basin. Applications include eutrophication mitigation, climate change planning, wetland and peatland recovery, pollution control, and heavy metal and contaminant modelling.

7. How Aquascope Enhances INCA

References

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• Whitehead, P.G., Wilson, E.J., Butterfield, D. and Seed, K. (1998b). A semi-distributed nitrogen model for multiple source assessments in catchments (INCA): Part II – application to large river basins in South Wales and eastern England. Science of the Total Environment, 210–211, 559–584.
• 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 System Science Journal, 11(1), 601–613.
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• Neal, C. (Ed.). (2002). Assessing nitrogen dynamics in catchments across Europe within an INCA modelling framework. Hydrology and Earth System Sciences, 6, 297–616.
• Wade, A.J., Whitehead, P.G. and Butterfield, D. (2002b). The Integrated Catchments model of Phosphorus dynamics (INCA-P). HESS, 6(3), 583–506.
• Whitehead, P.G., Johnes, P.J. and Butterfield, D. (2002). Steady state and dynamic modelling of nitrogen in the River Kennet: impacts of land use change since the 1930s. Science of the Total Environment, 282–283, 417–435.