In-situ long-term monitoring of evapotranspiration via weighable lysimeters and stable water isotopes in seepage

Lysimeters are experimental tools for identifying water fluxes in-situ in the soil-plant-atmosphere continuum. Weighable lysimeters are especially interesting for investigating evapotranspiration as they allow quantifying mass changes. Automated water balance estimations in weighable lysimeters are available in high-temporal resolution offering detailed information about diurnal processes. Besides water balance monitoring natural tracers like stable water isotopes are useful tools to investigate ecohydrological fluxes. Due to fractionation during phase transitions stable water isotopes are especially beneficial to identifying evaporative processes. Understanding the processes affecting evapotranspiration and groundwater recharge are essential for sustainable water management.

We investigated evapotranspiration based on water balance calculations and stable water isotopes at a lysimeter site in Koblenz, Germany between 2022 and 2025 to optimize our understanding of soil-plant-atmosphere water fluxes. The study site is located at the Rhine island Niederwerth in Koblenz and consists of eight drainage lysimeters, four of which are weighable with a grassland-area of 1 m² and a depth of 2 m. The corresponding soil monoliths consist of alluvial clay, loess loam, alluvial sand, and clayed pumice sampled within a 20 km radius surrounding Niederwerth. Seepage is collected weekly for evaluating automated measurements and to sample stable water isotopes. The site further includes a water basin for evaporation measurement and a meteorological measuring set up with air temperature, humidity, precipitation amount, solar radiation and others. The site-specific precipitation from 2022 to 2025 was 2885 mm (721 mm/a) while measured evaporation from a free water surface was 2473 mm (618 mm/a) and evapotranspiration 2291 mm (573 mm/a; mean over all four weighable lysimeters). The years of 2022 and 2025 were especially dry with evaporation exceeding precipitation input.

We found that normalized solar radiation measurements showed similar trends compared to normalized evapotranspiration measurements of the lysimeters which may offer the opportunity to investigate evapotranspiration via remote sensing techniques or to optimize model predictions. Preliminary results regarding stable water isotopes indicate a hysteresis cycle in lc-excess mean of seepage for alluvial sand and clayed pumice. Alluvial clay and loess loam showed little to no seepage in autumn and loess loam seepage lc-excess exhibited low variability. The seepage of loess loam with 142 mm/a was the lowest of the four soil types while alluvial sand showed the highest seepage of 194 mm/a. This supports a general understanding of stable water isotopic mixing in soils as in fine grained soils evaporated precipitation is mixing with surrounding water mitigating the effects of evaporation until percolating to the seepage depth of 2 m while highly permeable soils allow for evaporation effects to be monitored also in deeper soil depths.

With our study we will offer further insights into the variability of evapotranspiration based on soil types and aim to investigate evapotranspiration via tracer-aided modeling. Upcoming steps will include the estimation of the young water fraction and modeling evapotranspiration based on water balance and isotopic composition.