Modelling and observation of hedgerow transpiration effect on water balance components at the hillslope scale in Brittany Z. Thomas, 1,2 * R. Ghazavi, 3,4 P. Merot 1,2 and A. Granier 5 1 Agrocampus Ouest, UMR1069, Sol Agro et hydrosystème Spatialisation, F-35000 Rennes, France 2 INRA, UMR1069, Sol Agro et hydrosystème Spatialisation, F-35000 Rennes, France 3 Faculty of Agriculture and Natural Resources of Darb, Shiraz University, Shiraz, Iran 4 Faculty of Natural Resources, Kashan University, Kashan, Iran 5 UMR INRA-UHP 1137 Ecologie et Ecophysiologie Forestières, F- 54280 Nancy, France Abstract: Vegetation has a major influence on the water and energy balance of the earth’s surface. In the last century, human activities have modified land use, inducing a consequent change in albedo and potential evapotranspiration. Linear vegetation structures (hedgerows, shelterbelts, open woodland, etc) were particularly abundant but have declined considerably over the past several decades. In this context, it is important to quantify their effect on water and energy balance both on a global scale (climate change and weather prediction) and on a local scale (soil column, hillslope and watershed). The main objective of this study was to quantify the effect of hedgerows on the water cycle by evaluating spatial and temporal variations of water balance components of a hillslope crossed by a hedgerow. Water flow simulation was performed using Hydrus-2D to emphasize the importance of transpiration in the water balance and to evaluate water extraction from groundwater. Model validation was performed by comparing simulated and observed soil matrix potentials and groundwater levels. Hedgerow transpiration was calculated from sap flow measurements of four trees. Water balance components calculated with a one-dimensional water balance equation were compared with simulations. Simulation runs with and without tree root uptake underlined the effect of hedgerow transpiration, increasing capillary rise and decreasing drainage. Results demonstrated that the spatial and temporal variability of water balance components was related to the hedgerow presence as well as to the meteorological context. The relations between transpiration, groundwater proximity and soil-water availability determined the way in which water balance components were affected. Increased capillary rise and decreased drainage near hedges were related to the high transpiration of trees identified in this study. Transpiration reached twice the potential evapotranspiration when groundwater level and precipitation amounts were high. Water balance analysis showed that transpiration was a substantial component, representing 40% of total water output. These results may offer support for improving hydrological models by including the effect of land use and land cover on hydrological processes. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS simulated and calculated water balance components; tree sap flow; hedgerow transpiration; water flow simulation; capillary rise; drainage; meteorological context Received 8 March 2011; Accepted 12 January 2012 INTRODUCTION Vegetation significantly influences the water and energy balance of the earth’s surface. In the last century, dramatic land use changes induced a consequent change in vegetation cover, albedo and potential evapotranspiration (PET; Davin et al., 2007). The presence of trees in a landscape modifies energy and water balances by changing solar energy absorption and latent- and sensitive-heat redistribution. Linear vegetation structures (hedgerows, shelterbelts, open woodland, etc) are com- monly encountered all over the world. In the European landscape, hedgerows were particularly abundant but have considerably declined in density over the past several decades (Baudry et al., 2000). Many authors have highlighted the significant effect of hedgerows on hydrology at the watershed scale, such as the buffering of quick flow during high-frequency storm runoff (Merot, 1999; Van Loon and Stroosnijder, 2000; Viaud et al., 2004) and an increase in the annual actual evapotrans- piration (AET) by 5%–30% (Viaud et al., 2005). The increase in AET depends on hedgerow network density, climatic context and water availability, which can be particularly high in a temperate climate (Herbst et al., 2007; Thomas et al., 2008). Hedgerow trees also play a role as a windbreak (Brenner et al., 1995; Tuzet and Wilson, 2007; Campi et al., 2009) and seriously reduce erosion, surface runoff and flood risk (Gyasi-Agyei et al., 2001; Lin et al., 2009; Marshall et al., 2009). The presence of a hedge can decrease wind speed compared with winds in an open zone (Guyot and Seguin, 1978), thus reducing the local evaporating capacity of the air. In a humid temperate climate, however, the evaporating capacity of air contributes relatively little to PET, and the decreased wind speed due to hedgerows reduces PET by only 5% (Guyot and Seguin, 1978; Guyot, 1997). It is well known that via their well-developed root systems, *Correspondence to: Z. Thomas, Agrocampus Ouest, Laboratoire Physique et spatialisation numérique, 65 rue de Saint-Brieuc, 35042, Rennes Cedex, France. E-mail: zthomas@agrocampus-ouest.fr HYDROLOGICAL PROCESSES Hydrol. Process. 26, 4001–4014 (2012) Published online 21 February 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9198 Copyright © 2012 John Wiley & Sons, Ltd.