Hydroclimatic control on suspended sediment dynamics of a regulated Alpine catchment: a conceptual approach
Authors: Costa Anna , Anghileri Daniela , Molnar Peter 
Published: June 2018

Abstract. We analyse the control of hydroclimatic factors on suspended
sediment concentration (SSC) in Alpine catchments by differentiating among
the potential contributions of erosion and suspended sediment transport
driven by erosive rainfall, defined as liquid precipitation over snow-free
surfaces, ice melt from glacierized areas, and snowmelt on hillslopes. We
account for the potential impact of hydropower by intercepting sediment
fluxes originated in areas diverted to hydropower reservoirs, and by
considering the contribution of hydropower releases to SSC. We obtain the
hydroclimatic variables from daily gridded datasets of precipitation and
temperature, implementing a degree-day model to simulate spatially
distributed snow accumulation and snow–ice melt. We estimate hydropower
releases by a conceptual approach with a unique virtual reservoir regulated
on the basis of a target-volume function, representing normal reservoir
operating conditions throughout a hydrological year. An Iterative Input
Selection algorithm is used to identify the variables with the highest
predictive power for SSC, their explained variance, and characteristic time
lags. On this basis, we develop a hydroclimatic multivariate rating curve
(HMRC) which accounts for the contributions of the most relevant
hydroclimatic input variables mentioned above. We calibrate the HMRC with a
gradient-based nonlinear optimization method and we compare its performance
with a traditional discharge-based rating curve. We apply the approach in the
upper Rhône Basin, a large Swiss Alpine catchment heavily regulated by
hydropower. Our results show that the three hydroclimatic processes –
erosive rainfall, ice melt, and snowmelt – are significant predictors of
mean daily SSC, while hydropower release does not have a significant
explanatory power for SSC. The characteristic time lags of the hydroclimatic
variables correspond to the typical flow concentration times of the basin.
Despite not including discharge, the HMRC performs better than the
traditional rating curve in reproducing SSC seasonality, especially during
validation at the daily scale. While erosive rainfall determines the daily
variability of SSC and extremes, ice melt generates the highest SSC per unit
of runoff and represents the largest contribution to total suspended sediment
yield. Finally, we show that the HMRC is capable of simulating climate-driven
changes in fine sediment dynamics in Alpine catchments. In fact, HMRC can
reproduce the changes in SSC in the past 40 years in the Rhône Basin
connected to air temperature rise, even though the simulated changes are more
gradual than those observed. The approach presented in this paper, based on
the analysis of the hydroclimatic control of suspended sediment
concentration, allows the exploration of climate-driven changes in fine
sediment dynamics in Alpine catchments. The approach can be applied to any
Alpine catchment with a pluvio-glacio-nival hydrological regime and adequate
hydroclimatic datasets.