Tracking Grazing Pressure and Climate Interaction - The Role of Landsat Fractional Cover in Time Series Analysis
Over 60% of Queensland's land area is classified as rangeland, areas which are subject to high climatic variability and are primarily devoted to grazing by cattle and other herbivores. Graziers, industry and governments require rangeland land condition data at appropriate spatial and temporal scales for sustainable economic and environmental management, to monitor changes in land condition and fulfil reporting obligations. A large body of work has been undertaken in Australian rangelands studying the relationship between satellite images and field measured vegetation cover leading to several operational monitoring programs. One such program is the Queensland Department of Environment and Resource Management's groundcover monitoring system that routinely produces fractional cover estimates across Queensland and New South Wales.
However, the interaction between climate and management in rangeland environments complicates the interpretation of data on condition and trend. One of the most promising methods to identify cycles and long term trends in rangelands is to examine time sequences of satellite images. Access to the United States Geological Survey (USGS) Landsat image archive affords an unprecedented number of images for use in both research and operational monitoring, permitting the effects of climate and management to be decoupled.
This work reports on the results of a time series analysis based on 15 years of monthly Landsat derived fractional cover over an intensively studied grazing trial in North Queensland. A number of rangeland condition indicators are examined, including vegetation cover and type and their trends and transitions due to climate and management. The results demonstrate a clear link between grazing pressure and the resilience of the grazing resource in the presence of climate variability at the intrinsic temporal and spatial scales at which changes occur in the system. The results highlight the importance of adaptable land management strategies and are being used to inform landholders, regional bodies and policymakers on state scale rangeland condition and trend.
