Controls of land use and soil structure on water movement: Lessons for pollutant transfer through the unsaturated zone

Abstract

Summary To address the effects of land use and land cover (LULC) on soil structure formation and the significance on preferential flow during infiltration, dye tracer experiments were conducted on five sites differing in LULC, yet displaying similar soil textural characteristics and parent material. Two grassland sites, two farmland sites (tilled and untilled) and one site located in a deciduous forest were investigated. At each site, the same sprinkling experiment was carried out with a Brilliant Blue FCF solution of 4 g L −1 to visualize flow paths. To explore the effects of different rainfall amounts (20, 40 and 60 mm), each 1.2 × 1.5 m experimental plot was subdivided into three smaller subplots, which were irrigated with an intensity of 15 mm h −1 for 80, 160 and 240 min, respectively. During the tracer application, water content changes were continuously measured with 16 time domain reflectometry probes horizontally installed into the profile at different depths. After the experiments vertical and horizontal soil sections were excavated and photographed. The pictures were processed using digital image analysis and the resulting dye patterns analyzed for volume and surface density, maximum infiltration depth and macropore structure. Additionally, flow processes were classified into distinct flow type categories. The tracer experiments revealed that preferential flow processes significantly differed among sites of differing LULC yet similar soil texture. As primary controlling factors soil structure, surface micro-topography, surface cover and topsoil matrix characteristics were identified. The effects of different rainfall application amounts were complex and strongly varied among sites, stressing the strong control LULC exerts on water flow in soils. Overall this suggests that land use effects on soil properties need to be considered in hydrological models to obtain realistic predictions concerning water quality and quantity.