ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-5537-2012 DO<sub>3</sub>SE modelling of soil moisture to determine ozone flux to forest trees Büker P. 1 Morrissey T. 1 Briolat A. 1 Falk R. 1 Simpson D. 2 3 Tuovinen J.-P. 4 Alonso R. 5 Barth S. 6 Baumgarten M. 7 Grulke N. 8 Karlsson P. E. 9 King J. 10 11 Lagergren F. 12 Matyssek R. 7 Nunn A. 7 Ogaya R. 13 Peñuelas J. 13 Rhea L. 10 Schaub M. 14 Uddling J. 6 Werner W. 15 Emberson L. D. 1 Stockholm Environment Institute at York, Environment Department, University of York, York, UK EMEP MSC-W, Norwegian Meteorological Institute, Oslo, Norway Department of Earth & Space Sciences, Chalmers University of Technology, Gothenburg, Sweden Finnish Meteorological Institute, Helsinki, Finland Ecotoxicology of Air Pollution, CIEMAT, Madrid, Spain Department of Plant and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden Department of Ecology and Ecosystem Management, Life Science Center Weihenstephan, Technische Universität München, Freising, Germany Western Wildlands Environmental Threats Assessment Center, USDA Forest Service, Pacific Northwest Research Station, Prineville, Oregon, USA IVL, Swedish Environmental Research Institute, Gothenburg, Sweden Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, USA Department of Biology, University of Antwerp, Wilrijk, Belgium Department of Physical Geography, Lund University, Lund, Sweden Global Ecology Unit CREAF-CEAB-CSIC, CREAF (Center for Ecological Research and Forestry Applications), Universitat Autònoma de Barcelona, Barcelona, Spain Swiss Federal Research Institute WSL, Birmensdorf, Switzerland Department of Geobotany, University Trier, Trier, Germany 25 06 2012 12 12 5537 5562 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/5537/2012/acp-12-5537-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/5537/2012/acp-12-5537-2012.pdf

The DO<sub>3</sub>SE (Deposition of O<sub>3</sub> for Stomatal Exchange) model is an established tool for estimating ozone (O<sub>3</sub>) deposition, stomatal flux and impacts to a variety of vegetation types across Europe. It has been embedded within the EMEP (European Monitoring and Evaluation Programme) photochemical model to provide a policy tool capable of relating the flux-based risk of vegetation damage to O<sub>3</sub> precursor emission scenarios for use in policy formulation. A key limitation of regional flux-based risk assessments has been the assumption that soil water deficits are not limiting O<sub>3</sub> flux due to the unavailability of evaluated methods for modelling soil water deficits and their influence on stomatal conductance (<i>g</i><sub>sto</sub>), and subsequent O<sub>3</sub> flux. <br></br> This paper describes the development and evaluation of a method to estimate soil moisture status and its influence on <i>g</i><sub>sto</sub> for a variety of forest tree species. This DO<sub>3</sub>SE soil moisture module uses the Penman-Monteith energy balance method to drive water cycling through the soil-plant-atmosphere system and empirical data describing <i>g</i><sub>sto</sub> relationships with pre-dawn leaf water status to estimate the biological control of transpiration. We trial four different methods to estimate this biological control of the transpiration stream, which vary from simple methods that relate soil water content or potential directly to <i>g</i><sub>sto</sub>, to more complex methods that incorporate hydraulic resistance and plant capacitance that control water flow through the plant system. <br></br> These methods are evaluated against field data describing a variety of soil water variables, <i>g</i><sub>sto</sub> and transpiration data for Norway spruce (<i>Picea abies</i>), Scots pine (<i>Pinus sylvestris</i>), birch (<i>Betula pendula</i>), aspen (<i>Populus tremuloides</i>), beech (<i>Fagus sylvatica</i>) and holm oak (<i>Quercus ilex</i>) collected from ten sites across Europe and North America. Modelled estimates of these variables show consistency with observed data when applying the simple empirical methods, with the timing and magnitude of soil drying events being captured well across all sites and reductions in transpiration with the onset of drought being predicted with reasonable accuracy. The more complex methods, which incorporate hydraulic resistance and plant capacitance, perform less well, with predicted drying cycles consistently underestimating the rate and magnitude of water loss from the soil. <br></br> A sensitivity analysis showed that model performance was strongly dependent upon the local parameterisation of key model drivers such as the maximum <i>g</i><sub>sto</sub>, soil texture, root depth and leaf area index. The results suggest that the simple modelling methods that relate <i>g</i><sub>sto</sub> directly to soil water content and potential provide adequate estimates of soil moisture and influence on <i>g</i><sub>sto</sub> such that they are suitable to be used to assess the potential risk posed by O<sub>3</sub> to forest trees across Europe.

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