A delicate balance

Australia’s arid and semi-arid grasslands are home to an economically very important form of extensive livestock farming. The distinctive feature of the climate in these regions is the short rainy season followed by a long dry season and it is the availability of water and vegetation that determines the size of herds. The grasslands are also fragile and overgrazing always brings the risk of land degradation. This is why it is important for the livestock farmers to keep a close eye on the condition of their pastures and to adapt management practices where and when necessary. Is it preferable, for example, to allow the cattle to graze freely over large areas of grassland or to rotate them in smaller confi ned pastures? However, it is not easy to assess the condition of the grasslands. Recent vegetation maps are non-existent, the livestock farms are huge, and the grasslands are in a state of imbalance. The very irregular rainfall also results in wide short-term variations in vegetation cover that can conceal the longer term deterioration of the grasslands, symptoms of which are the appearance of bare earth and a shift from perennial to annual grass varieties. It is therefore important to differentiate changes in the grasslands that are the result of grazing from natural changes in the vegetation. Earth observation can help in this.

A consultancy bureau specialising in the use of geographical information systems in livestock farming and a university pooled their efforts to help Australian livestock farmers to manage their grasslands. Newcastle Waters Station, a farm covering more than 10,000 km2 (1/3 the size of Belgium!) and with 45,000 head of cattle in Australia’s Northern Territory, served as the test area. A method was developed to map the various types of grassland by means of high resolution satellite images. By using archive satellite pictures it was also possible to see what changes there had been to the vegetation over the past 25 years. Finally, by using a vegetation index it was possible to determine grazing gradients around points with a high grazing pressure.