Tillage and traffic effects on soil hydraulic properties and evaporation

Abstract

Evaporation is a major component of the water loss from the soil whose structure is modified by traffic and tillage. This study was undertaken to analyse, in field conditions, the effect of tillage and traffic on soil structure and evaporation, and to determine the role of the change in hydraulic properties on soil drying using water transfer model. Three structures of the ploughed layer were formed in a loess soil (Luvisol Orthique) and a calcareous soil (Rendzina): a fragmentary structure created by deep soil tillage in autumn or in spring (rotary tiller at 30 cm depth), a compacted ploughed layer created by compaction under wet conditions. The bulk density varied from 1.16 to 1.63 Mg m −3 in the loess soil, from 1.00 to 1.45 Mg m −3 in the calcareous soil. Evaporation was calculated from the change in soil water content and matric water potential profiles measured during the spring season. Soil hydraulic properties were estimated using an inverse modelling method applied to field measurements of water content and water potential or the Wind method. Soil structure greatly affected the drying of the calcareous soil: the evaporation of the compacted plot was about two times that of the tilled plot. The compacted plot dried out homogeneously, with a soil surface which remained wet. Evaporation mainly concerned the first 15 cm of the ploughed layer created by autumn or spring tillage. This effect of soil structure on evaporation was not observed in the loess soil. The unsaturated hydraulic conductivity was higher in the compacted plot than in the tilled plots in the calcareous soil. It was similar in the three plots of the loess soil, because of the formation of relict structural pores by compaction. Experimental and numerical results showed that unsaturated hydraulic conductivity is of major concern in soil drying and that the albedo and surface roughness have minor effects if any. The possible relict structural pores have to be characterised in various soils, as a function of soil sensitivity to compaction, traffic and tillage conditions.