Functional relationships between soil water infiltration and wheeling and rainfall energy

Surface seal formation and subsoil compaction are two important factors of soil structural degradation, both of which reduce infiltration and increase surface runoff. Surface sealing has traditionally been related to rainfall energy; however, there has been very limited work on characterizing subsoil degradation resulting from wheeling energy imposed by agricultural machinery traffic. The objective of this study was to test the hypothesis that wheeling energy, either by itself or in combination with rainfall energy, would be a useful predictor of subsequent soil structural degradation.Field experiments were conducted on a heavy clay soil site in South-east Queensland, Australia which had controlled traffic and conservation tillage treatments for the previous 5 years. Soil was compacted with different axle load and wheel slips, and wheeling energy was calculated from wheel slip and rolling resistance. Simulated rain was applied at different intensities with a portable rainfall simulator. Runoff was measured with a tipping bucket flow meter, and infiltration parameters and the corresponding curve number was determined. The results showed that when soil was protected by residue cover, infiltration and runoff was governed by subsoil compaction. Under this condition, steady state infiltration rate decreased and curve number increased with increasing wheeling energy, but the constant values of steady infiltration rate (14.5 mm h−1) and curve number (93) resulted after wheeling energy exceeded 5 kJ m−2. Under non-wheeled conditions, steady state infiltration rate decreased and curve number increased linearly with increasing cumulative rainfall energy. The limited data set suggests that the degradation of soil hydraulic properties appears to be related to the sum of wheeling and rainfall energy. The data confirms that rainfall energy causes surface soil degradation, while wheeling energy caused subsoil soil degradation. Wheeling energy is relatively easy to measure or estimate, and appears to also be a useful predictor of subsoil soil degradation.