Effect of land use change on the dynamic behaviour of structural properties of an Andisol in southern Chile under saturated and unsaturated hydraulic conditions

Abstract

Soil structure dependent properties are normally dynamic due to different processes occurring in the soil. However, in classic hydraulic modelling these interactions are generally ignored and may result in higher uncertainty of predicted effects. Such investigations are especially relevant in volcanic ash soils in southern Chile because of their intrinsic properties and the climatic conditions of the region. Therefore, the objective of this study was to describe the effect of the land use change of a Typic Hapludand on some soil structure dependent properties and their variability depending on hydraulic conditions using field, laboratory data and modelling. Two sites on a volcanic ash soil were selected: a native forest (NF) and a prairie used for permanent pasture for over 50 years (P50). In order to register the natural wetting and drying cycles rainfall, soil moisture and temperature at both sites were determined. With the in situ water content data and shrinkage parameters derived from shrinkage curves, we calculated the temporal changes in moisture and void ratios. To validate our calculations, the soil water and shrinkage curves were determined repeatedly in undisturbed soil samples collected during the rainy season in July and October. Finally, the effect of land use change on temporal variations in the saturated hydraulic conductivity was measured at different times during 96 h of continuous water flow. The land use change from NF to P50 exposed the soil to greater mechanical and hydraulic stresses and, consequently, to soil deformation. When considering the soil shrinkage by drying, we assessed that the bimodal character of the water retention curve changed only in macro pore range, when the land use changed from NF to P50 affecting the saturated hydraulic conductivity. The void ratio presented a dynamic behaviour depending on the intensity of natural wetting and drying cycles reflecting also the consequences of these cycles in the water retention and shrinkage curves. The saturated hydraulic conductivity decreased during water infiltration as a function of land use due to a particle release, transport and re-sedimentation which probably affects the pore continuity. Finally, we concluded that the structural properties of the studied Andisol are dynamic, presenting spatial and seasonal variability depending on the land use and soil water content. Therefore, in order to understand processes in soil it is necessary to develop and use hydraulic models that considered this behaviour.