Simultaneous estimation of precipitation and actual evapotranspiration by lysimeters - Comparison with tipping bucket and eddy covariance

Friday, 19 December 2014
Harrie-Jan Hendricks Franssen1, Sebastian Gebler1, Thomas Puetz2, Hanna Post1, Marius Schmidt1 and Harry Vereecken3, (1)Agrosphere Institute (IBG-3), Forschungszentrum Jülich, Jülich, Germany, (2)Forschungszentrum Jülich, Jülich, Germany, (3)Forschungszentrum Julich GmbH, Julich, Germany
Although precipitation and actual evapotranspiration measurements have a long tradition, accurate estimates of precipitation (P) and actual evapotranspiration (ETa) remain a challenge. Our study compares actual evapotranspiration estimates acquired with the Eddy-Covariance (EC) method and ETa measurements by a set of six redundant weighable lysimeters for a managed grassland site at Rollesbroich (Eifel, Western Germany). The comparison of ETa measured by EC (accounting for energy balance deficit correction) and by lysimeters is hardly reported in literature and gains more insight into the performance of both techniques. The evaluation of ETa estimates by both methods for the year 2012 shows a good agreement with a total difference of ca. 4 %, which is mainly related to variations in grass height at the lysimeters and in the EC footprint. We also used the lysimeter records to estimate precipitation amounts in combination with the AWAT filter algorithm. The estimated precipitation volumes of the lysimeter measurements show significant differences compared to the precipitation data of the Hellman type tipping bucket rain gauge at the test site. For the entire year 2012 the lysimeter measurements exhibit a 16 % higher precipitation amount than the tipping bucket data. With help of an on-site video surveillance system the precipitation data of the lysimeters were investigated in more detail. It was found that the precipitation surplus in lysimeter records in part is related to the detection of rime and dew, which contributes 17 % to the yearly difference between both methods. We concluded that weighable lysimeter data can be used to simultaneously estimate precipitation and actual evapotranspiration in a reliable fashion. Furthermore, lysimeter allow a plausible detection of rime and dew in contrast to standard rain gauges.