Runoff and erosion from Australia's tropical semi-arid rangelands: influence of ground cover for differing space and time scales

摘要

This paper highlights the relevant issues influencing the amount and arrangement of ground cover in savanna rangelands in Australia, and presents field measurements from hillslope scale flumes, which demonstrate how runoff and sediment loss vary with spatial patterns in ground cover. Hillslopes with relatively high mean cover, but with small patches bare of vegetation, are shown to have between 6 and 9 times more runoff, and up to 60 times more sediment loss than similar hillslopes that do not contain bare patches. The majority of sediment lost from the hillslopes is composed of fine (suspended) rather than coarse (bedload) material, although the absolute sediment loads are comparatively low. These low loads are considered to be the result of lower than average rainfall during the measurement period (2002-2005) and the high and prolonged rates of historical hillslope erosion that have exhausted the erodible material from the A-horizon. The collected data also demonstrate that a large proportion of soil is lost during the initial 'flushing' period of runoff events. The results presented have important implications for the management of savanna grazing systems by highlighting (ⅰ) the significance of bare patches in contributing to runoff and soil loss from hillslopes; (ⅱ) the importance of having medium to high cover patches at the bottom of hillslopes for trapping and storing sediment and therefore reducing its entry into the stream network; and (ⅲ) how maintenance of ground cover during the dry season reduces sediment concentrations in runoff occurring early in the wet season.