Comparative analysis of three methods to generate soil hydraulic functions

Abstract

An accurate determination of the soil hydraulic functions is crucial for using soil water simulation models. The purpose of this study is to compare different methods to estimate hydraulic properties and their effects on the simulation of soil water movement. Soil water retention and hydraulic conductivity functions were either measured in the laboratory (core method) and in the field (instantaneous profile method and crust method) or estimated from other soil properties (pedo-transfer functions, PTFs). The data were fitted to the van Genuchten equations and the resulting parameter sets were used as input of the mechanistic WAVE model to simulate soil water flow. Calculated and observed water content values were compared to assess the different methods used for determining the soil hydraulic properties. The hydraulic parameter sets generated from the different approaches were found to be significantly different and to have a strong impact on the soil water flow simulation. Soil water retention data obtained from field experiments and laboratory tests differ significantly near saturation but are similar for lower water content values. On the whole, retention curves given by the Rawls and Vereecken PTFs perform best while the other tested PTFs give poorer results. Unsaturated hydraulic functions given by the Saxton, Cosby and Rawls PTFs are relatively close to the measured data in the silt loam layers while the Vereecken PTF leads to good estimations in the pressure head mid-range values, but poor results in the high and low ranges. In the sandy loam layer characterized by low clay and organic matter content, all the tested PTFs perform very poorly. Globally, when considering prediction of both soil hydraulic functions, the Rawls PTFs were found to be the most adequate. The comparison between observed and simulated soil water content values indicates that the best agreement is achieved with hydraulic properties estimated from field experiments. Results are less satisfactory with the parameter sets obtained from the laboratory tests and very poor with the parameter sets given by the Rawls PTFs.