Mapping gullies over large areas requires detailed aerophotos. Google Earth (GE) provides free access to high resolution satellite imagery, but is the quality good enough to map gullies reliably? The aim of this work was to evaluate the accuracy with which gullies in the West Gippsland region could be mapped using GE images. The area comprised the LaTrobe, Thomson and Avon catchments and extended over 11130 km 2 in Victoria, southeastern Australia. GE images available for the West Gippsland area at the time of the study were from three sources: Cnes/Spot Image of 2.5 m ground resolution, Digital Globe of 0.6 m resolution, and GeoEye of 0.5m resolution. Gullies were identified and digitized from GE images, then transferred to GIS. After digitization of a pilot area (192 km 2), on both forested and agricultural land, an initial field survey was conducted in December 2010 to improve interpretation of GE images. The pilot study showed that large canopy cover in plantations and native forest precluded observation of gullies beneath. Gullies under forest canopy could only be recognized on areas felled or burnt before the image was taken. Following the pilot area evaluation, gully mapping in West Gippsland was restricted to agricultural land; forest and plantation areas, covering 64 % of the catchment, were excluded. A second, more extensive field survey was conducted in January 2011 on 39 transects to assess the accuracy (absence/presence) of the gully map on agricultural land. Gullies totaling 2385 km were mapped in agricultural areas across the region. Most gullies (87%) were located along drainage lines and were connected to streams. It was sometimes difficult to separate gully from streambank erosion. Following field observations, streams were defined as drainage lines of third or greater order (Strahler method), whereas incised first and second order drainage lines were classified as gullies. Gully density on agricultural land increased from West to East across the West Gippsland region, varying from 0.59 km/km 2 in the LaTrobe catchment, to 0.65 km/km 2 in the Thomson and 0.86 km/km 2 in the Avon catchment. The field survey showed that 26% of gullies observed were not mapped from GE images, whereas 13% of mapped gullies were not confirmed by the field survey. Mapping errors were correlated to image resolution, with higher errors associated with coarser resolution images. During the survey, 12 representative cross-sections of gullies were measured by recording the maximum depth, width, and taking a perpendicular photograph. Gullies were generally small and inactive, having a median cross section of 2.7 m 2 (1.7-4.1 m 2 interquartile range). By assuming an exponential decay of gully wall retreat in the gully stabilization phase, current gully erosion rate of active gullies was assessed at 0.02 m 2 /y. Together with the revised gully network extent, suspended sediment load originated by gully erosion in the region was estimated at 10.6 kt/y. This estimate is higher than reported in previous research, due in part to ...
