Trend analyses with river sediment rating curves

Sediment rating curves, which are fitted relationships between river discharge ( Q ) and suspended‐sediment concentration ( C ), are commonly used to assess patterns and trends in river water quality. In many of these studies, it is assumed that rating curves have a power‐law form (i.e. C  =  aQ b , where a and b are fitted parameters). Two fundamental questions about the utility of these techniques are assessed in this paper: (i) how well to the parameters, a and b , characterize trends in the data, and (ii) are trends in rating curves diagnostic of changes to river water or sediment discharge? As noted in previous research, the offset parameter, a , is not an independent variable for most rivers but rather strongly dependent on b and Q . Here, it is shown that a is a poor metric for trends in the vertical offset of a rating curve, and a new parameter, â , as determined by the discharge‐normalized power function [ C  =  â ( Q / Q GM ) b ], where Q GM is the geometric mean of the Q ‐values sampled, provides a better characterization of trends. However, these techniques must be applied carefully, because curvature in the relationship between log( Q ) and log( C ), which exists for many rivers, can produce false trends in â and b . Also, it is shown that trends in â and b are not uniquely diagnostic of river water or sediment supply conditions. For example, an increase in â can be caused by an increase in sediment supply, a decrease in water supply or a combination of these conditions. Large changes in water and sediment supplies can occur without any change in the parameters, â and b . Thus, trend analyses using sediment rating curves must include additional assessments of the time‐dependent rates and trends of river water, sediment concentrations and sediment discharge. Published 2014. This article is a U.S. Government work and is in the public domain in the USA. Hydrological Processes published by John Wiley & Sons Ltd.