Comparison of Inverse and Direct Evaporation Methods for Estimating Soil Hydraulic Properties under Different Tillage Practices

Abstract

Tillage and traffic modify soil porosity and pore-size distribution, leading to changes in the unsaturated hydraulic properties of the tilled layer. These modifications are difficult to characterize although they can greatly affect evaporation. This study was conducted to compare field and laboratory methods for estimating the unsaturated hydraulic properties of two soils under different tillage practices. Soils freshly tilled, soils tilled 6 mo earlier that received winter rainfall (340-380 mm), and soils compacted by wheel tracks, were created in loess (Typic Hapludalf) and calcareous (Typic Rendoll) soils to obtain a wide range of soil bulk densities (1.0-1.6 g cm -3 ). The Wind laboratory method (direct evaporation) was compared with an inverse modeling method applied to field measurements of water content and water potential during dry periods in spring. The soil samples were saturated with water from the top (full saturation) or from the bottom (partial saturation) before conducting the laboratory evaporation experiment. The Wind method overestimated water retention, except in the compacted soils, when the soil samples were initially fully saturated with water. On the contrary, there was good agreement between the Wind method and field data for tilled soils with a low bulk density when the samples were only partially saturated from the bottom. In that case, the hydraulic conductivity obtained with the Wind and the inverse modeling methods were similar. The Wind method can be used to estimate the unsaturated hydraulic properties of tilled soils, but care must be taken to saturate the soil cores with water: full saturation (saturation fraction of the total pore space >0.9 m 3 m -3 ) for the cores from compacted soils with a low structural void ratio ( 0.5 m 3 m -3 ).