Estimating suspended sediment concentrations in areas with limited hydrological data using a mixed-effects model

Sediment rating curves are commonly used to estimate the suspended sediment load in rivers and streams under the assumption of a constant relation between discharge (Q) and suspended sediment concentrations (SSC) over time. However, temporal variation in the sediment supply of a watershed results in shifts in this relation by increasing variability and by introducing nonlinearities in the form of hysteresis or a path-dependent relation. In this study, we used a mixed-effects linear model to estimate an average SSCQ relation for different periods of time within the hydrologic cycle while accounting for seasonality and hysteresis. We tested the performance of the mixed-effects model against the standard rating curve, represented by a generalized least squares regression, by comparing observed and predicted sediment loads for a test case on the Chilliwack River, British Columbia, Canada. In our analyses, the mixed-effects model reflected more accurate patterns of interpolated SSC from Q data than the rating curve, especially for the low-flow summer months when the SSCQ relation is less clear. Akaike information criterion scores were lower for the mixed-effects model than for the standard model, and the mixed-effects model explained nearly twice as much variance as the standard model (52% vs 27%). The improved performance was achieved by accounting for variability in the SSCQ relation within each month and across years for the same month using fixed and random effects, respectively, a characteristic disregarded in the sediment rating curve. Copyright (C) 2012 John Wiley & Sons, Ltd.