In situ and high frequency monitoring of suspended sediment properties using a spectrophotometric sensor

It is well known that sediment properties, including sediment-associated chemical constituents and sediment physical properties, can exhibit significant variations within and between storm runoff events. However, the number of samples included in suspended sediment studies is often limited by time-consuming and expensive laboratory procedures after stream water sampling. This restricts high frequency sampling campaigns to a limited number of events and reduces accuracy when aiming to estimate fluxes and loads of sediment-associated chemical constituents. In this study, we address the potential of a portable ultraviolet-visible spectrophotometer (220-730nm) to estimate suspended sediment properties in a resource efficient way. Several field deployable spectrophotometers are currently available for in-stream measurements of environmental variables at high temporal resolution. These instruments have primarily been developed and used to quantify solute concentrations (e.g. dissolved organic carbon and NO3-N), total concentrations of dissolved and particulate forms (e.g. total organic carbon) and turbidity. Here we argue that light absorbance values can be calibrated to estimate sediment properties. We present light absorbance data collected at 15-min intervals in the Weierbach catchment (NW Luxembourg, 0.45km(2)) from December 2013 to January 2015. In this proof-of-concept study, we performed a local calibration using suspended sediment loss-on-ignition (LOI) measurements as an example of suspended sediment property. We assessed the performance of several regression models that relate light absorbance measurements with the percentage weight LOI. The MM-robust regression method presented the lowest standard error of prediction (0.48%) and was selected for calibration (adjusted r(2)=0.76 between observed and predicted values). The model was then used to predict LOI during a storm runoff event in December 2014. This study demonstrates that spectrophotometers can be used to estimate suspended sediment properties at high temporal resolution and for long-time spans in a simple, non-destructive and affordable manner. Copyright (c) 2016 John Wiley & Sons, Ltd.