Comparative Study of a Long‐Established Large Weighing Lysimeter and a State‐of‐the‐Art Mini‐lysimeter

Abstract

Core Ideas We saw generally high agreement between lysimeters, despite substantial design differences. Large‐lysimeter processing was optimized in light of highly resolved mini‐lysimeter data. Smoothing filter and setting of ET to zero in wet hours was useful for the large lysimeter. Lysimeter precipitation types were identified using an opto‐electric precipitation monitor. Protrusion of the large lysimeter may cause precipitation under‐catch by flow distortion. Weighing lysimeters are a well‐established means of accurately obtaining local‐scale estimates of actual evapotranspiration (ET), precipitation, and seepage within soils. At the Rietholzbach research catchment in northeastern Switzerland, two weighing lysimeters are in operation. One is a recently installed state‐of‐the‐art monolithic mini‐lysimeter with a tension‐controlled lower boundary; the other is a large backfilled free‐drainage lysimeter in operation since 1976. For this study, the mini‐lysimeter measurements were processed using the Adaptive‐Window and Adaptive‐Threshold (AWAT) filter. The resulting water‐balance estimates were then compared with those of the lower resolution large lysimeter, whose processing has remained unchanged since its installation. A number of additional, retrospectively applicable processing steps for the large lysimeter were then tested to mitigate the main sources of error for this instrument. Those found to be most beneficial were the application of a 10‐min moving average to the mass measurements and the setting of ET and condensation to zero in hours with liquid precipitation. In spite of the differences in design, a generally close agreement between the two lysimeters was observed, which was further improved with the optimized large‐lysimeter processing. A comparison of the lysimeter mass increases associated with liquid precipitation further revealed, however, that the large lysimeter experiences a previously unknown under‐catch of about 11.5%, which could also be important for other lysimeter facilities. This under‐catch led to a reduction in large‐lysimeter seepage during the analyzed period, although the ET flux was not found to be affected.