Subsurface preferential flow occurrence and relevance in agricultural hillslopes: experimental evidence

Preferential flow (PF) has long been discussed as potential cause for unintended contamination of ground and surface waters with agrochemicals. In agricultural soil landscapes, especially along hillslopes, the mostly vertically-directed preferential flow (VPF) of infiltrating water in unsaturated topsoil horizons fosters the formation of water saturated pore regions at less permeable subsoil horizons that can trigger laterally-oriented preferential flow (LPF) along subsurface preferred flow paths. The occurrence of LPF processes depends on complex interrelations between soil properties and subsurface structures, climatic conditions, crop development, and agro-management, among other factors. Field observations in hillslope agricultural soil landscapes to quantify the relevance of LPF are rare. Here we present studies on LSF processes at two contrasting sites. One is the CarboZALF-D, located in northeastern Germany in hummocky arable soil landscape (Luvisol and Regosol soil types). The second (SUPREHILL) is an agricultural vineyard hillslope with Stagnosol soils located in central Croatia. Both sites show erosion and tillage effects in the soils along slopes. An extensive network of soil moisture sensors, suction cups, and lysimeters are installed at both sites. Relevant soil physical, hydraulic, and chemical properties have been determined for running simulation models. The SUPREHILL site has been equipped also with self-constructed subsurface runoff collection system, while at CarboZALF-D site, LPF was captured by a field tracer experiment; and in the laboratory, LSF along a soil horizon boundary was studied on undisturbed soil monoliths. Different subsurface flow processes were identified and captured at the two sites, for SUPREHILL shortly below the topsoil along the lower permeable Btg horizon and for CarboZALF-D at buried topsoil under colluvium and along coarser-textured bands within compact glacial till C-horizon. The collected experimental results revealed the qualitative importance of LPF and transport in the subsurface; the presented experimental data will be used for the model-based quantitative analysis of the LPF related processes.