Soil erosion protection

Cropping regions

Roadside field surveys have been conducted by DEW since 1999 to monitor soil erosion risk in the main dryland cropping Landscape regions of South Australia (Eyre Peninsula, Northern and Yorke, Murraylands and Riverland and Limestone Coast). These survey transects represent an area that covers approximately 8 million hectares of the 9 million hectares of cleared farming land in South Australia.

Geolocated paddock sites along transects are surveyed from the roadside four times a year, in March, May, June and October, corresponding to critical times for ground cover and soil exposure during annual crop growth cycles. This time sequence is used to generate an annual profile of erosion protection based on seasonal changes in crop and pasture growth. The field surveys consist of a simple visual assessment of groundcover, surface looseness and soil and landscape parameters at each site.

Full details of the survey method can be found in the Field Survey Monitoring Manual.

Cropping regions

Since October 2021, the field surveys have been downscaled as DEW has started incorporating satellite data into their monitoring approach. This uses monthly MODIS Fractional Cover satellite data developed by CSIRO which estimates percentage of fractional cover for ‘green’ (photosynthetic) vegetation, ‘dry’ (non-photosynthetic) vegetation and bare soil.

The sum of the green and dry fractions is the estimate of percentage vegetative groundcover which is used as the surrogate for estimating erosion risk. This data is modelled with the ground cover ratings from geolocated sites of previous erosion protection field surveys, to increase its accuracy under local conditions. The MODIS 500 metre pixels are also disaggregated to a 100 metre resolution.

Mapped inherent potential for wind erosion and water erosion across the agricultural zone is also combined with the percentage groundcover data, to estimate actual erosion risk. Modelled erosion risk is produced across the state’s agricultural zone in October, March, May and June each year, corresponding to the field survey months. Using the current October data and groundcover change rates from October to March from previous surveys, erosion risk for March in the following year is also forecast.

While this method does not assess other factors that affect erosion risk such as soil disturbance (looseness), it covers all of the landscape rather than just the transects observed in the field surveys. Smaller scale field surveys are now conducted on an ongoing basis to ground-truth the accuracy of the modelled data.

Cropping and rangeland regions

The MODIS satellite imagery is also used to report the days at risk of soil erosion, as part of South Australia’s Environmental Trend and Condition Report Cards. These report cards summarise current trends in environmental condition in the state, specifically climate, biodiversity, water, land and liveability parameters. These report cards feed into the Environmental Protection Agency’s ‘State of the Environment’ reporting, which is published every five years and summarises environmental management efforts.

MODIS Fractional Cover monthly data is used to monitor relative plant growth and percentage bare soil (i.e. inverse of percentage groundcover) in both the northern rangeland areas of the state, as well as the agricultural zone (cropping areas). The photosynthetic ‘green’ fractional data is used to estimate average plant growth, and the ‘bare soil’ fraction to estimate relative percentage of bare soil.

In the agricultural zone, the ‘year to date’ plant growth average (or bare soil average) in the current year is compared to the average for a representative previous long-term period (2008 – 2017) to show the difference, or ‘anomaly’.

For northern areas of the state, the current 12 month period to date is compared to the long-term average. This analysis helps show how plant growth (or percentage bare soil) is tracking in the current season compared to average, depending on rainfall and other seasonal conditions, and how this is impacting on soil erosion protection, such as during below average rainfall seasons. Using data from previous years, forecast average plant growth and bare soil anomalies for the end of the current year can also be produced for the agricultural zone.