A Forest Evapotranspiration Paradox Investigated Using Lysimeter Data

Abstract

In spite of the large number of studies on the role of forests in affecting local and global water and energy cycles, conflicting reports on even the sign of the change in evapotranspiration over forest compared with non-forest land cover can be found depending on the type of data used. Whereas studies based on closure of the water balance suggest higher evapotrans-piration over forests, studies based on turbulent exchange and/or energy balance closure suggest generally higher latent heat flux over non-forest sites. In this study, this forest evapotranspiration paradox was investigated using data from four long-term lysimeter stations in western Europe with contrasting land cover conditions. The results were consistent with evapo-transpiration estimates from catchment-scale water balance studies rather than with eddy covariance estimates. They were also found to be largely consistent with a model previously proposed to predict forest cover effects on evapotranspiration. The results of this study suggest that eddy covari-ance data should be treated with care when used to assess long-term average water balance impacts of land use. Abbreviations: ET, evapotranspiration. Forests are known to have a strong impact on hydrological processes and climate (Bonan, 2008; Spracklen et al., 2012; Teuling et al., 2017; Ellison et al., 2017). The evaluation of forest and land cover change effects on the water cycle, however, requires an accurate representation of land cover and vadose zone effects on the terrestrial hydrological cycle in hydrological and land surface models (e.g., Blyth et al., 2010; Williams et al., 2009). In spite of the large number of studies on the role of forests in local and global water and energy cycles (e.g., Sterling et al., 2013), conflicting reports on even the sign of the change in water balance partitioning (in particular of evapotranspiration [ET]) over forest compared with non-forest land cover can be found depending on the type of data used. Thorough knowledge about the impact of forests on evapotranspiration is of key importance to water management because the difference between precipitation and ET is the main control on water availability in streams and aquifers. Traditionally, the estimation of forest impacts on ET has been performed based on a forced closure of the water balance of a catchment or lysimeter (Seneviratne et al., 2010). Based on long-term measurements of precipitation and discharge (or outflow and seepage in the case of a lysimeter), combined with the assumption that storage changes can be neglected over longer time periods (years), ET is usually determined as the difference between precipitation and discharge averaged across multiple years. Based on this principle, Zhang et al. (2001) investigated ET for a large number of catchments across the world, including western Europe. They showed that average precipitation P and fractional forest cover f were the main determinants of catchment-scale water loss through ET rather than the potential ET. This is consistent with a study by Shuttleworth and Calder (1979), who questioned the use of potential ET over aerodynamically rough forest ecosystems. Zhang et al. (2001) proposed the following relationship to describe the dependency of ET on precipitation and forest cover: Core Ideas • There is disagreement in the sign of reported forest impacts on evapotranspiration. • Independent lysimeter data show higher evaporation over forest. • Lysimeter data show a strong effect of tree age and height on average evapotranspiration.