Soil erosion and tolerable soil loss: Insights into erosion rates for a well-managed grassland catchment

• Soil erosion was determined using an environmental tracer for a grassland catchment.
• Soil depth was also physically measured.
• While erosion was low, it was higher than estimated soil production rates.
• Soil erosion needs to be considered in terms of soil production and tolerable soil loss.

Soil erosion and its spatial and temporal variabilities are rarely placed in the context of soil production and soil depth. This study examines the question of sustainable soil erosion and soil loss in a conservatively managed grassland catchment in South East Australia in what at first appears to be a catchment with a tolerable soil loss. Catchment erosion rates are determined using the environmental tracer 137Cs. A thorough examination of the accuracy and reliability of this method is conducted across a number of spatial scales and years providing confidence in the method and results. Soil depth is measured across the study catchment providing the first bedrock map of a basalt derived soil catchment. Both soil erosion and soil depth are topographically assessed using a high resolution digital elevation model. Results show that soil depth was strongly correlated with elevation and also wetness indices indicating a strong relationship with soil moisture in soil production. Interestingly bedrock topography was decoupled from surface topography. Erosion rates using the 137Cs method and calibrated against independent field data produced a maximum erosion rate of between 0.8 and 2.9 t ha− 1 yr− 1 using two different modelling approaches. Even though the erosion rates are low, given a mean soil depth of 0.44 m for the catchment this suggests that soil is being lost at rates greater than production. This highlights the significance of assessing erosion loss in the context of overall soil depth and production rates and that even in areas with what appears to be low soil loss rates, the loss can be higher than production. The findings provide a rationale to examine soil erosion in the context of whole catchment processes, not simply soil loss in isolation to other hillslope and catchment data.