Identification and quantification of arable hillslope transport processes e.g., preferential flow using soil column study

Hillslope soils are especially challenging due to larger possibilities of soil erosion which can lead to soil heterogeneity and result in nonlinear water flow and solute transport. Thus, to determine water flow and possibly quantify preferential flow we conducted a soil column study within the first Croatian critical zone observatory SUPREHILL. The research was conducted in controlled conditions on undisturbed soil columns (25 cm depth and 16 cm in diameter) taken from the hilltop, middle and hillbottom positions. Since soil hydraulic properties are impacting water flow and thus the solute transport, undisturbed soil cores (250 cm3) were taken from the hilltop, middle and hillbottom in three repetitions and the soil hydraulic properties were estimated using HYPROP and WP4C. After application of 150 mL tracer (potassium bromide, KBr) at concentration of 1g L‒1 KBr, columns are irrigated. Soil moisture is measured with soil moisture sensor on 7 cm depth and tensiometer on 17 cm depth set inside each soil column. At the end of the column, the leachate is collected to determine Br‒ in water samples which helped us to determine dispersivity coefficient and water flow behaviour. Furthermore, to identify preferential flow we applied Brilliant blue dye tracer on the top of the soil columns, afterwards the columns are photographed. Quantification of preferential flow was conducted in ImageJ software using image analyses procedure. First results are showing the fastest water flow in the soil column taken from the middle part of the hillslope which is in line with hydraulic conductivity values obtained using HYPROP and WP4C. In the future, we plan to expand the research by applying KBr and Brilliant blue in the field, as well as by taking undisturbed soil samples for CT scanning and numerical simulations which will give us more specified information about preferential flow and nonlinear dynamics.