Abstract
Abstract. Accurate measurements of evapotranspiration are required for many meteorological, climatological, ecological, and hydrological research applications and developments. Here we examine and compare two well-established methods to determine evapotranspiration at the site level: lysimeter-based measurements (EL) and eddy covariance (EC) flux measurements (EEC). The analyses are based on parallel measurements carried out with these two methods at the research catchment Rietholzbach in northeastern Switzerland, and cover the time period of June 2009 to December 2015. The measurements are compared on various timescales, and with respect to a 40-year lysimeter-based evapotranspiration time series. Overall, the lysimeter and EC measurements agree well, especially on the annual timescale. On that timescale, the long-term lysimeter measurements also correspond well with catchment water-balance estimates of evapotranspiration. This highlights the representativeness of the site-level lysimeter and EC measurements for the entire catchment despite their comparatively small source areas and the heterogeneous land use and topography within the catchment. Furthermore, we identify that lack of reliable EC measurements using open-path gas analyzers during and following precipitation events (due to limitations of the measurement technique under these conditions) significantly contributes to an underestimation of EEC and to the overall energy balance gap at the site.
Highlights
Evaporation E from land, termed evapotranspiration, is an essential contributor to the water and energy balances on continents
We identify that lack of reliable eddy covariance (EC) measurements using open-path gas analyzers during and following precipitation events significantly contributes to an underestimation of eddy covariance (EC) flux measurements (EEC) and to the overall energy balance gap at the site
The best-established reference measurement remains ground observations, which can be for example either performed with the lysimeter technique commonly used in hydrology (e.g., Maidment, 1992; Rana and Katerji, 2000; Seneviratne et al, 2012), or the eddy covariance (EC) flux measurement technique established in micrometeorology (e.g., Baldocchi et al, 2001; Aubinet et al, 2012)
Summary
Evaporation E from land, termed evapotranspiration, is an essential contributor to the water and energy balances on continents. Approaches to measure or estimate evapotranspiration are diverse and can include ground observations, remote sensingbased estimates, diagnostic techniques, as well as modeling and reanalyses (e.g., Seneviratne et al, 2010; Jiménez et al, 2011; Mueller et al, 2011; Wang and Dickinson, 2012) Despite their relative scarcity, the best-established reference measurement remains ground observations, which can be for example either performed with the lysimeter technique commonly used in hydrology (e.g., Maidment, 1992; Rana and Katerji, 2000; Seneviratne et al, 2012), or the eddy covariance (EC) flux measurement technique established in micrometeorology (e.g., Baldocchi et al, 2001; Aubinet et al, 2012).
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