We report newly obtained data of total suspended sediment concentrations (Cs) and water discharges from the Ma-an Stream in the eastern Taiwan, where intermittent supply-limited conditions with a minimal constant load were invoked for sediment discharge (Hovius et al. 2000. The main evidence for the supply-limited condition is the many zero values of Cs corresponding to low-flow conditions recorded in the hydrological archive of the Water Resources Agency (WRA) of Taiwan. Such conditions form a distinct mode in the rating relationship between loading and discharge, which is discontinuous from the normal mode. However, our data raise serious doubt regarding the sensitivity of the measurements that produced data in support of the minimal constant load. A field sampling from April 4 to May 5, 2003, was conducted in the Ma-an Stream during a very dry period. Our observations showed no zero values, even though the sampling period was one of the driest periods in comparison with historical records. In our study, 11 of 35 daily samples gave Cs values between 10 and 100 mg L -1, whereas only one record from the entire 1980-1994 WRA data set fell in this range, indicating the poor sensitivity of the WRA measurements. Our observations contradict the fact that more than two-thirds of Cs values are null in the WRA records for the rivers at the foothills of the Central Range in eastern Taiwan. The relationships between Cs and discharge and between sediment loading and discharge all demonstrate that our data are consistent with the trend manifested by the nonzero values from the WRA records. Therefore, we conclude that the existing data do not support the proposed discontinuous dual-mode rating relationship for rivers at the foothills of the Central Range in eastern Taiwan.
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