Spatial Variability of Physical Parameters and Processes in Two Field Soils

Abstract

Natural field systems exhibit a large degree of soil heterogeneity which affects the movement of water and solutes and thus leads to highly varying observations of water content and solute concentration. To investigate this problem comprehensive field investigation programs were carried out at two field sites in Denmark representing two different soil types, a coarse sand and a sandy loam, respectively. The field investigations included collection of soil samples for analysis of textural composition, retention, and hydraulic conductivity, measurements of water content and suction, and measurements of radioactive tracer concentration, all carried out at a number of positions within the two field sites. Models for one-dimensional vertical unsaturated flow and solute transport were applied to the two field sites, and the simulation results were compared to field measurements of water content, suction and solute concentration. This paper describes results from model simulations in individual soil profiles, while the variability issues at field scale are described in the two accompanying papers. The modelling approach was based on numerical solutions to Richards' equation for water flow and the convection-dispersion equation (CDE) for solute transport. The model results from the coarse sand field site compared relatively well to measurements of water content, suction, and concentration except for the upper soil layer (∼ 10 cm depth) where the measured water contents appeared to be somewhat uncertain. Due to the neglecting of hysteresis and macropore flow (by-pass) in the model the measured retention curves (drainage based) and the hydraulic conductivity functions at the sandy loam field site had to he modified empirically through the calibration procedure in order to match the measurements.