Using GPR surveys and infiltration experiments for assessing soil physical quality of an agricultural soil

Time-lapse ground penetrating radar (GPR) surveys in conjunction with automated single-ring infiltration experiments can be used for non-invasive monitoring of the spatial distribution of infiltrated water and for generating 3D representations of the wetted zone. In this study we developed and tested a protocol to quantify and visualize water distribution fluxes under unsaturated and saturated conditions into layered soils. We carried out a gridded GPR survey on a 0.3-m thick sandy clay loam layer underlain by a restrictive limestone layer at the Ottava experimental station of the University of Sassari (Sardinia, IT). We firstly established a survey grid (1 m × 1 m), consisting of six horizontal and six vertical parallel survey lines with 0.2 m intervals between them. The field survey then consisted of six steps, including i) a first GPR survey, ii) a tension infiltration experiment conducted within the grid and aimed at activating only the soil matrix, iii) a second GPR survey aimed at highlighting the amplitude fluctuations between repeated GPR radargrams of the first and second surveys, due to the infiltrated water moving within the matrix flow region, iv) a single-ring infiltration experiment of the Beerkan type carried out within the grid on the same infiltration surface using a solution of brilliant blue dye (E133) and aimed to activate the whole pore network, v) a third GPR survey aimed to highlight the amplitude fluctuations between repeated GPR radargrams of the first and third surveys, due to the infiltrated water moving within the whole pore network (both matrix and fast-flow regions), and vi) the excavation of the soil to expose the wetted region. The shapes of the 3D diagrams of the wetted zones facilitated the interpretation of the infiltrometer data, allowing us to resolve water infiltration into the layered system. Finally, we used the infiltrometer data in conjunction with the Beerkan estimation of soil transfer parameter (BEST) method to determine the following capacitive indicators of soil physical quality of the upper soil layer: air capacity AC (m³ m^–3), plant-available water capacity PAWC (m³ m^–3), relative field capacity RFC (–), and soil macroporosity p_MAC (m³ m^–3). Results showed that the investigated soil was characterized by high soil aeration and macroporosity (i.e., AC and p_MAC) along with low values for indicators associated with microporosity (i.e., PAWC and RFC). These findings suggest that the upper soil layer facilitates root proliferation and quickly drains excess water towards the underlying limestone layer, and, on the contrary, has limited ability to store and provide water to plant roots. In addition, the 3D diagram allowed the detection of non-uniform downward water movement through the restrictive limestone layer. The detected difference between the two layers in terms of hydraulic conductivity suggests that surface ponding and overland flow generation occurs via a saturation-excess mechanism. Indeed, percolating water may accumulate above the restrictive limestone layer and form a shallow perched water table that, in case of extreme rainfall events, could rise causing the complete saturation of the soil profile.