Quantifying evapotranspiration budgets of winter rye using a automated gantry crane – effects of soil type, erosion and management and testing gap filling procedures

In light of ongoing global climate change and related increases in extreme hydrological events, it is becoming increasingly important to have a comprehensive knowledge of the ecosystem water cycle to assess ecosystem stability and in agricultural system to ensure sustainable management and food security. Evapotranspiration (ET) plays a crucial role returning up to 90 % of ingoing precipitation back to the atmosphere. In agriculture, further knowledge about plant transpiration (T) and evaporation (E) of different soils could lead to more efficient water use in the future, which will become necessary for agricultural practice in many regions due to climate change related increase in drought events. Here, we wanted to implore impacts of soil types (representing a ful soil erosion gradient) on ecosystem water budgets (ET) and agronomic water use efficiencies (WUEagro).

We conducted a plot experiment with winter rye (September 17, 2020 to June 30, 2021) at the "CarboZALF-D” experimental field which is located in the hilly and dry ground moraine landscape of the Uckermark region in NE Germany. Along an experimental plot (110 m x 16 m) a modern automated gantry crane was built and used for the first time to continuously determine evapotranspiration with two automated chambers. A major advantage of this system is the opportunity to assess management and soil type effects (compared to eddy covariance setups), without corroborating measurement frequency (compared to manual chamber setups).

Three soil types representing the full soil erosion gradient of the hummocky ground moraine landscape (extremely eroded: Calcaric Regosol, strongly eroded: Nudiargic Luvisol, non-eroded: Calcic Luvisol) within each soil type were investigated (randomized block design, 3 replicates per treatment). In addition, we used five different gap-filling methods and compared them in light of their potential to aquire precise water budgets over the entire growth period as well as reproduce short water flux dynamics realistically. The best performance was achieved with methods based on mean-diurnal-variation (MDV) and support vector machine (SVM), including a validation step SVM yielded best predictions of measured ET. Subsequently, we simulated half-hourly ET fluxes and calculated balances of evapotranspiration for the cropping period.

The results show that there are significant differences in evapotranspiration and yield between soil types, resulting in different water use efficiencies (WUEagro). The Calcaric Regosol (extremely eroded) shows a maximum of around 10% lower evapotranspiration and a maximum of around 35% lower water use efficiency (WUE_agro) compared non-eroded soils.  The key period contributing to 50-65 % of overall ET of the entire growth period was from late April until harvest, however differences in the overall ET budget between soil types and manipulation resulted predominantly from small long-term differences between the treatments over the entire growth period.