Quantifying sediment production in steepland environments

Five published contributions to our understanding of the impacts of erosion processes on sustainable land management are reviewed and discussed. These focus on rapid shallow landsliding and gully erosion which are among the most prevalent forms of environmental degradation in New Zealand's hill country. The over-arching goal of this research has been to quantify the on-site (e.g., soil erosion, land productivity) impacts of these processes. Rather than measure erosion rates over long periods of time, geomorphic techniques such as 'space-for-time substitution' and paired catchment approaches have been employed to overcome the naturally high spatial and temporal variability in erosion processes. Digital elevation models (DEMs) have proven invaluable as a means of measuring rates of gully erosion and of quantifying landform properties at small and large catchment scales. The spatial variability in shallow landslide erosion, recovery of soil on scar surfaces, and long-term evolution of hillslopes is investigated in an area of sandstone hill country in the North Island of New Zealand, to help elucidate the role that vegetation plays in maintaining slope stability. Hillslope evolution is primarily by landsliding on steep slopes and by diffuse creep processes on gentle slopes, punctuated by periods of slope instability related to climatic and vegetative variability. Variation in slope form near to channels, with over-steepened sideslopes and higher benches, implies a history of fluctuating erosion rates, driven by changes in stream base level. Systematic variation in soil depth and slope angle measured at the hillslope scale implies spatial variability in erosion rates and a landscape that is not in morphologic equilibrium. There is about an order of magnitude difference in long-term erosion rates between relatively steep (30deg) and gentle (30deg) hillslopes. Steep slopes are located at the head of 1sup(st)-order drainage basins and are more closely coupled to base level changes in the drainage network and erode at about the rate of tectonic uplift.