Trend analyses with river sediment rating curves. Issue 6 (25th April 2014)

• Record Type: Journal Article
• Authors: Warrick, Jonathan A.
• Is Part Of: Hydrological processes. Volume 29:Issue 6(2015:Mar.)
• Page Start: 936
• Page End: 949
• Publication Date: 2014-04-25
• Subjects: Hydrology -- Periodicals; Hydrology -- Research -- Periodicals; Hydrologic models -- Periodicals; Hydrological forecasting -- Periodicals; 631.432
• DOI: 10.1002/hyp.10198
• Languages: English
• ISSNs: 0885-6087

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>Sediment rating curves, which are fitted relationships between river discharge (<italic>Q</italic>) and suspended‐sediment concentration (<italic>C</italic>), 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. <italic>C</italic> = <italic>aQ<sup>b</sup></italic>, where <italic>a</italic> and <italic>b</italic> are fitted parameters). Two fundamental questions about the utility of these techniques are assessed in this paper: (i) how well to the parameters, <italic>a</italic> and <italic>b</italic>, 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, <italic>a</italic>, is not an independent variable for most rivers but rather strongly dependent on <italic>b</italic> and <italic>Q</italic>. Here, it is shown that <italic>a</italic> is a poor metric for trends in the vertical offset of a rating curve, and a new parameter, <italic>â</italic>, as determined by the discharge‐normalized power function [<italic>C</italic> = <italic>â</italic> (<italic>Q</italic>/<italic>Q<sub>GM</sub></italic>)<italic><sup>b</sup></italic>], where <italic>Q<sub>GM</sub></italic> is the geometric mean of the <italic>Q</italic>‐values sampled, provides a better characterization of trends. However, these techniques must be applied carefully, because curvature in the relationship between log(<italic>Q</italic>) and log(<italic>C</italic>), which exists for many rivers, can produce false trends in <italic>â</italic> and <italic>b</italic>. Also, it is shown that trends in <italic>â</italic> and <italic>b</italic> are not uniquely diagnostic of river water or sediment supply conditions. For example, an increase in <italic>â</italic> 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, <italic>â</italic> and <italic>b</italic>. 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. <italic>Hydrological Processes</italic> published by John Wiley & Sons Ltd.</p> </abstract>