Peatland DOC model selection

Various peatland DOC models have been established over the past few decades. These may be useful to predict what might happen under future climate and atmospheric deposition scenarios to DOC concentrations and fluxes in peat-fed water supply catchments where potable water must be treated to remove the DOC. To determine a suitable model for the above this study considered: (1) model availability; (2) the model’s ability to capture DOC driver factors including rainfall, temperature, and acid deposition; (3) input data availability. Table 4.1 shows a summary of widely-used DOC models which have been employed in peatlands. Only MADOC (Rowe et al., 2014) and Integrated Catchments Model for Carbon (INCA-C) (Futter et al., 2007; Futter and de Wit, 2008) are physically-based models which consider the variables of temperature, rainfall and atmospheric deposition (i.e. sulphate). However, MADOC is more suitable for use over small scales (approximately 100 km2_{) than for large catchments. Since this project is focussed on large}

scale research, using MADOC in this project would require huge amounts of detailed input data, most of which are currently unavailable. Therefore, INCA- C was deemed the most suitable available model to examine the impact of future climate change and atmospheric deposition on DOC release in peatland water supply catchments. The following section provides a brief introduction to the INCA-C model.

The required input data for INCA-C includes daily time series of precipitation, soil moisture deficit (SMD; the difference between the current depth of water and the water holding capacity), hydrologically effective rainfall (HER; the fraction of precipitation which contributes to runoff), temperature (in °C), and precipitation (in mm) for the available dates within the simulation period. HER is the depth of precipitation or snowmelt, net of evaporation that can enter the upper soil horizon while SMD is an estimate of the difference between the amount of water in the soil and the amount of water it can hold. HER and SMD can be derived from a separate hydrological model - Precipitation, Evapotranspiration and Runoff Simulator for Solute Transport (PERSiST) (Futter et al., 2014). As input data, PERSiST requires daily time series of air temperature and precipitation.

As well as time-series data some values used in the parameterisation of the INCA-C model are fixed and site-specific. For example, size of the catchment (ha), length and width of stream reach (m), latitude of the site (important for estimating insolation) and proportion of land-cover type (e.g. bog, moorland, forest, grassland, arable, urban) in the catchment.

Table 4.1 A summary of widely-used DOC models which have successfully

been employed in peatland studies.

Model types Example Note Rainfall Temperature Acid

deposition

Statistical models

Creed et al. (2008)

Relating DOC concentrations in stream water to watershed hydrology,

catchment characterises, or climate

Yes No No Monteith et al. (2015) No Yes Yes Grayson et al. (2012) Yes Yes No Soil moisture and temperature models Birkenes model (Grieve, 1991) Modified Birkenes model (Boyer et al., 2000)

Physically-based. Net DOC production and loss is essentially regulated by soil

temperature, and transport is regulated by soil moisture content, snowmelt, run-off and soil percolation

Yes Yes No Hydrology- biogeochemi- stry models Soil carbon submodule of CENTURY Model (Parton et al., 1988)

Physically-based. The model requiring input information on climate (temperature and precipitation), soil properties (soil texture, soil pH, bulk density, field capacity, wilting point, initial organic and mineral soil C, N, P,

and S), and plant chemistry characteristics (e.g. lignin content,

nutrient content)

Yes Yes No

Dynamic DOC model (Michalzik

et al., 2003)

Physically-based. Combines soil carbon production and loss functions for multiple soil layers and includes a simple hydrological model to simulate soil moisture and runoff processes

Yes Yes No

MADOC (Rowe et al., 2014)

Physically-based. Integrating existing models of vegetation growth and soil organic matter turnover, acid-base dynamics, and organic matter mobility

Yes Yes Yes

INCA-C (Futter and de Wit, 2008; Futter et al., 2007)

Physically-based. Simulating soil carbon stocks and DOC in an arbitrary

number of user-specified land cover types

Model types Example Note Rainfall Temperature Acid deposition ECOSSE (Smith et al., 2010a; Smith et al., 2010b) Physically-based. Comprehensively relating stream DOC driven by daily weather and litterfall, variations in

catchment cover types and soil conditions (upper and lower layers on

uplands and wetlands) and hydrological flow paths

Yes Yes No

Durham Carbon Model (Worrall and Burt, 2005)

Semi-physically based. Formulating DOC production and storage processes in the upper soil layers of a

peat bog as affected by soil temperature and water-table fluctuations with monthly resolution in the context of climate change and land

management

Yes Yes No

An advantage of INCA-C is that this model can simulate effects of hydrological, climate- and atmospheric deposition-related variables on not only daily stream DOC concentration and fluxes, but also different types of overland flow dynamics (which may be important for DOC concentrations and fluxes) in an arbitrary number of user-specified land-cover types at large catchment scale and regional scales.