RILL EROSION ON A STRUCTURALLY DEGRADED SANDY LOAM SURFACE SOIL

This study reports on the degree of rill erosion from a 0.65 ha site with a hardsetting red duplex soil having a structurally degraded surface soil. Rill erosion was measured after a series of relatively low intensity rainfall events (all <1 yr return period) that resulted in above average rainfall over a period of 78 days. The rill erosion was estimated by measuring the volume of rills in the field. The soil characteristics indicated that the soil was a structurally degraded soil with low aggregate stability (2-5% of total soil), very low organic matter levels (0.91% for 0-50 mm) and relatively high bulk density (1.56 g/cm3). The low aggregate stability resulted in the rapid breakdown of soil aggregates and settling of the soil under rainfall causing surface crusting and low infiltration rates (4.08 mm/h) as well as decreasing depressional storage. This resulted in high runoff and rill erosion, the total volume and mass of soil loss being equivalent to 48.3 m3/ha and 78.0 t/ha respectively. This high level of soil loss was considered to result from the combination of the poor structural condition of the soil, the tilled or loosened soil condition at the onset of the rainfall events, the duration and number of the low intensity rainfall events and the low evaporation rates between rainfall events. The results suggest that to reduce soil loss to acceptable levels under the conditions of the study, it is necessary to adopt management practices that increase surface cover, maintain infiltration rates, reduce tillage and increase soil aggregate stability in combination with erosion control earthworks. The study also looked at the prediction of soil loss using the USLE equation, and confirmed that the equation should not be used to predict erosion for individual events or erosion resulting from concentrated flow, rather it should be used to estimate expected long-term soil loss for a particular soil under a given set of management practices.