A spatially invariant sediment rating curve and its temporal change following watershed management in the Chinese Loess Plateau

Abstract

Sediment rating curves have long been used as a tool for predicting sediment yield. In this study, the spatial and temporal variations of rating curves were examined in the loess area (~15,000km2) of the Wuding River basin of the Chinese Loess Plateau. Data of mean daily water discharge per unit area, Qd (m3s−1km2), and mean daily sediment discharge per unit area, SDd (kgs−1km2), were compiled for three periods of 1959–1971, 1972–1989, and 2006–2013. Soil conservation measures were rarely implemented for the pre-1971 period but were intensively implemented for the latter periods. The proportional function, i.e. SDd=CdQd, fitted the rating curve well for all periods. The coefficient, Cd, is an approximation of the mean sediment concentration of surface runoff (MSCSR). Contrary to the usual belief, Cd was ca. 730kgm−3 whatever the location for the pre-1971 period; examinations at the instantaneous and the event time scales further demonstrated the spatial invariant MSCSR across six orders of magnitude in drainage area sizes (from 0.006 up to 3893km2). The spatial invariant MSCSR forms as a result of the ready availability of mass-wasted materials in headwaters and holds on as a result of the minimal in-stream erosion and sedimentation along the stream channel. For the post-1971 periods, both Cd and its spatial invariability decreased in a non-monotonous way, which was related to the check-dam (i.e., sediment trapping dams) constructions but not to the implementation of revegetation and terracing measures. As a result, the non-monotonous temporal trends of Cd and its spatial coefficient of variation both captured the intermittency of the check-dam construction well. Temporal changes in MSCSR or the sediment rating curve, which is less disturbed by rainfall, provides important insights into anthropogenic impacts on river systems.