Erosion rates for Taiwan mountain basins: New determinations from suspended sediment records and a stochastic model of their temporal variation

We estimate erosion rates from suspended sediment records for 11 basins in the eastern Central Range (ECR) of Taiwan using methods based on mean measured sediment discharge, a rating curve of sediment and water discharge, and a rating curve corrected for periods of limited sediment due to the lack of landslide-supplied sediment. The preferred method for any basin depends on record length and sampling frequency, with higher quality records being analyzed by the latter method. Erosion rate estimates range from 2.2 to 8.3 mm/yr for records with varying sampling frequencies and durations between 8 and 27 yr. This variation in erosion rates does not seem to reflect lithology, tectonic environment, or climate. We interpret the variation in terms of natural stochastic variation in water discharge and sediment supply. To assess the quality of the erosion rate estimates and to better understand the dependence of uncertainty on the duration and frequency of sampling, we construct a stochastic model of sediment supply and transport for the Chihpen River of the ECR. The model stochastically predicts the water discharge and sediment supply from landslides and calculates the transport of suspended sediment through application of a deterministic transport law. We determine that with a 27-yr hydrograph with 780 suspended sediment load measurements for the Chihpen River, assuming an erosion rate of 5.1 mm/yr, there is a 68.3% probability of determining an erosion rate within mm/yr of the actual erosion rate. We provide an estimate of the uncertainty associated with various +/-2.7/4.0 sampling frequencies and record lengths and find that it is difficult to push uncertainties below +/-2 mm/yr.