Role of wheel tracks in runoff generation and erosion under vegetable production on a clay loam soil at Pukekohe, New Zealand

The effects of wheel traffic on soil surface hydraulic properties, and cons equent effects on erosion, following planting of vegetable crops in beds ha ve not been widely studied. This paper describes two trials to quantify how wheel tracks influence infiltration and erosion rates, and assesses the va lue of cultivating wheel tracks for reducing erosion. The trials were carri ed out under natural rainfall, on Dystric Nitosols with clay loam texture a nd strong, stable structure. Net rates of erosion from onion (Allium cepa L .) beds with cultivated or uncultivated inter-bed wheel tracks were measure d with erosion pins and repeat topographic surveys of sediment trapped in s ilt fences. Infiltration rates in onion beds, cultivated and uncultivated w heel tracks, and changes in infiltration rates through winter, spring, and summer, were measured using the double-ring, ponded-water method. Differences in erosion rate were only measured in the second trial in which erosion rate from the uncultivated treatment was 21 Mg ha(-1), compared to 1 Mg ha(-1) for the cultivated treatment. Erosion occurred through mobilis ation of soil along the edge and base of the wheel tracks, with no evidence of erosion of the onion beds. Most of the eroded soil comprised soil aggre gates, with 75% between 0.25 and 4 min in diameter, suggesting soil was tra nsported in runoff along the wheel tracks as stable aggregates. Uncultivate d wheel tracks had very low infiltration rates compared to onion beds and c ultivated wheel tracks. The differences in infiltration rates between culti vated and uncultivated wheel tracks were consistent in both trials, with mi nor differences due to rainfall patterns and the implements used to cultiva te wheel tracks. There were clear trends in infiltration rates through time , with rates in the uncultivated wheel tracks increasing during the growing season from 1.4 x 10(-7) to 2.1 x 10(-5) m s(-1) and in onion beds from 1. 1 x 10(-4) to 2.5 x 10(-4) m s(-1), while rates in the cultivated wheel tra cks decreased from 1.7 x 10(-2) to 2.4 x 10(-3) m s(-1). The major increase of infiltration rate in uncultivated wheel tracks occurred after October w hen the soil surface began to dry out, and frequent wetting and drying cycl es caused the compacted surface soil to crack. Most erosion occurred in the winter/early spring period when storm frequency and rainfall intensity was highest, and infiltration rates in the uncultivated wheel tracks lowest. C ultivating wheel tracks is a simple and effective practice to increase infi ltration of rainfall and reduce erosion rates on clay-rich, strongly struct ured soils. (C) 2001 Elsevier Science B.V. All rights reserved.