Spatial Variability of Unsaturated Flow Parameters:Field Study

The field scale spatial variability of soil properties is one of the key factors which have considerable effect on water flow and solute transport. In this paper, an experiment was conducted in a 30 m×30 m field site with sandy loam soil. The capillary tension head was measured with 100 individual mercury tensiometers which were installed at 30 cm deep. Bulk density ρ, initial soil moisture content θ_i and saturated sil moisture content θ_s were measured with 100 soil samples collected from field site, meanwhile the measurements of saturated hydraulic conductivity K_s and soil pore size distribution parameters α in 108 locations were carried out with Guelph Permeameter. Data sets for soil properties and capillary tension head were analyzed with classical statistical and geostatistical methods. The statistical results show that soil parameters ρ,θ_i, θ_s, K_s and soil water capacity C follow approximately normal distributions, while α has a lognormal distribution. According to coefficients of variation of soil properties, it is found that K_s, α and C exhibit large spatial variability, while ρ and θ_s show less variability in the same field. The geostatistical results indicate that K_s and log α are spatially correlated. It is also found that capillary tension head exhibits a moderate spatial variation which is approximately time invariant. The variance of capillary tension head is a quadratic function and increases with its mean values.