ASSESSING MODERN RATES OF RIVER SEDIMENT DISCHARGE TO THE OCEAN USING SATELLITE GRAVIMETRY
Résumé
The estimates of the worldwide sediment river discharge to the oceans suffer large uncertainties,
mostly because sediment discharge measurements are scarce in space and time. Yet,
knowing this sediment budget is critical in Earth and environmental Sciences, since it is under
the direct influence of surface processes (such as erosion, transport and sedimentation)
and, in particular, controlled by climate, tectonics and human activities (dam, deforestation).
Since 2002, the GRACE Satellite provides global gravity time series that have proven useful for
quantifying mass transport, including continental water redistribution at the Earth surface (ice
sheets and glaciers melting, groundwater storage variations). Because, sediment accumulation
in the oceanic sinks leads to a mass increase, we propose to use GRACE data to assess these
mass variations, and in turn the sediment discharge to the ocean. However, the order of magnitude
of mass variations due to sediments is only one tenth of the magnitude of continental water.
As a result, unraveling the sedimentological contribution to GRACE signal is a challenge. To
tackle it, we pair the analysis of regularized GRACE solutions at high spatial resolution corrected
from all known contributions (hydrology, ocean, atmosphere) to a particle tracking model
that predict the location of the sediment sinks for 13 rivers with the highest sediments loads
in the world. This particle tracking model combines global reanalysis of oceanic currents with
available data on the grain size and the seasonality of the sediments load at the river mouth
and is run over the same time range as GRACE data. We find encouraging correlation between
GRACE spatial pattern of mass accumulation and the modeled pattern of sedimentation zones
for some rivers, although some discrepancies between modeled and observed mass accumulation
values are observed. The reasons for these misfits are a combination of inaccuracies both
in GRACE solutions and in the particle tracking model: 1) GRACE has a limited spatial resolution
and uncertainties, with residual contributions from off-target effects that are poorly estimated;
2) The particle model is highly dependent on the oceanic currents’ velocity field, which also
suffers from uncertainties and has limited spatial and temporal resolution as well; 3) We use
literature-based estimations of the seasonality of river sediment loads and associated grain
sizes, both of which are notoriously uncertain data. We presently aim at refining our sediment
particle tracking model in order to get better confidence in the spatial distribution of the sediments.
This way, we expect to assess the annual riverine sediment discharge within the uncertainty
of GRACE, independently from uncertain in situ rivers’ sediment loads data.