Spatial Variability of Soil Parameters of the van Genuchten Model at a Regional Scale

Abstract

Spatial heterogeneity of soil water retention curves (SWRC) plays an important role in modelling the movement of water and solutes in soils. To characterize the spatial variability of SWRC parameters fitted by the van Genuchten (VG) equation, and then identify factors that closely correlated with the VG parameters, we collected undisturbed and disturbed soil samples from the upper 0-5 cm soil layer at 382 sampling sites across the entire Loess Plateau of China ($620 000 km 2 ). We determined the SWRC and six other soil properties as well as nine environmental factors for each site. Results showed that the saturated water content (u s ) and a curve-shape parameter that is related to soil pore size distribution (n) were weakly spatially variable (coefficient of variation (CV) ¼ 15%), and that the residual soil water content (u r ) and a scaling parameter that is related to the inverse of the air entry pressure (a) were moderately (CV ¼ 96%) and strongly variable (CV ¼ 136%), respectively. Semivariograms of ffiffiffiffi u r p and log(a) were best-fitted by an isotropic exponential model, while those of u s and log(n) were fitted by an isotropic spherical model. Fractal analysis indicated that log(a) demonstrated the most short-range variation, while log(n) reflected the importance of long-range variation. Bulk density and the contents of clay, silt, sand and soil organic carbon contributed greatly to the variations in the VG parameters (except a). However, the vegetation, topographic and climatic elements as well as land use also usually explained some of the variation. The VG parameters, at the regional scale, demonstrated different characteristics of spatial variation, which comprehensively reflected the combined effects of processes involving the soil, vegetation, topography and climate as well as effects of land use, which is often determined by human activity.

Recharge and Nitrate Transport Through the Deep Vadose Zone of the Loess Plateau: A Regional‐Scale Model Investigation

“…Using only the PSD data as an input to the ROSETTA , the α parameter is underestimated by an order of magnitude. When including the BD and FC ( θ 330 ) data, the estimated α parameter is comparable to the estimated α values reported by Gates et al () and Wang et al (; Figure ). Note that the other parameters display comparable values.…”

Soil moisture redistribution and its effect on inter-annual active layer temperature and thickness variations in a dry loess terrace in Adventdalen, Svalbard

“…Two additional scenarios, 'max case' and 'min case', represent the high and low ends of the parameter range by combining high alpha with high m, and low alpha with low m, respectively. All of these parameter values are within the range of the variation for silt loam reported in the UNSODA database and in literature (Destouni, 1991;Wang et al, 2015;Watanabe and Wake, 2009).…”

“…2). In all scenarios, residual saturation Sr is set to zero, consistent with commonly adopted practice for silty soils (Destouni, 1991;Painter and Karra, 2014;Wang et al, 2015;Watanabe and Wake, 2009;Weismüller et al, 2011). A reference case is first defined using average values (α=8•10 -4 Pa -1 and m=0.19) obtained from the UNSODA soil hydraulic database (Ghanbarian-Alavijrh et al, 2010) for silt loam soils.…”

Soil moisture redistribution and its effect on inter-annual active layer temperature and thickness variations in a dry loess terrace in Adventdalen, Svalbard

“…Two additional scenarios, "max case" and "min case", represent the high and low ends of the parameter range by combining high α with high m, and low α with low m, respectively. All of these parameter values are within the range of the variation for silt loam reported in the UNSODA database and in literature (Destouni, 1991;Wang et al, 2015;Watanabe and Wake, 2009). Given that each combination of van Genuchten parameters will result in different soil moisture profiles, each simulation test case was set-up to attain unique ice-liquid-gas states.…”

“…2). In all scenarios, residual saturation S r is set to zero, consistent with commonly adopted practice for silty soils (Destouni, 1991;Painter and Karra, 2014;Wang et al, 2015;Watanabe and Wake, 2009;Weismüller et al, 2011). A reference case is first defined using average values ( α = 8×10 −4 Pa −1 and m = 0.19) obtained from the UNSODA soil hydraulic database (Ghanbarian-Alavijrh et al, 2010) for silt loam soils.…”