Capturing SUbsurface PREferential transport processes in agricultural HILLslope soils: SUPREHILL CZO

<p>Agricultural hillslopes present particular challenges for estimating vadose zone dynamics due to a variety of processes, such as surface runoff, vertical flow, erosion, subsurface preferential flow affected by soil structure and layering, non-linear chemical behaviour, evapotranspiration, etc. To investigate these processes and complexity, the SUPREHILL critical zone observatory (CZO) was started in 2020, at vineyard hillslope site in Croatia. The observatory is extensively equipped for the soil-water regime and agrochemical fluxes monitoring, and includes an extensive sensor network, lysimeters (weighing and passive wick), suction probes, surface and subsurface flow and precipitation collection instruments. The main objective of the SUPREHILL observatory is to quantify subsurface lateral and local scale preferential flow processes. Local-scale nonlinear processes in eroded agricultural hillslope sites have large significance on water and solute behaviour within the critical zone and thus need to be researched in depth using combined methods and various approaches. First results from the sensor and lysimeter network, soil-water regime monitoring, isotope analysis, and agrochemical concentrations in 2021 supported the hypothesis of the observatory, that the subsurface flow plays a relevant part in the hillslope soil-water dynamics. In the wick lysimeter network, although the highest cumulative outflow values were found at the hilltop, the highest individual measurements were found at the footslope. During high-intensity rainfall events, there were differences in weighing lysimeters, possibly related to subsurface lateral flow. Based on the isotope analysis, wick lysimeters exhibit a greater variation of d-excess values than suction probes. Agrochemical fluxes confirmed the sloping effect on their transport in soil and demonstrated the favourability of Cu transport by subsurface flow. Using the comprehensive database presented herein, future analyses of this hypothesis will be carried out in more detail using model-based analyses. &#160;</p>