���񱦵�

Syvitski, James ¹

¹ Environmental Computation and Imaging Group, INSTAAR, University of Colorado at Boulder

The UNH water balance and transport model (WBM), based on the STN-30p network, provides a fundamental structure for analyzing the water and sediment flux distribution by river basin, continent, climatic zone, and receiving ocean or sea. WBM estimates of discharge are constrained by observed hydrographic data from the Global Runoff Data Centre (GRDC) archive, covering 76 Mkm2 (72%) of the world’s actively discharging landmass. WBM discharge estimates derived from modern climatology were used to develop correction factors and then revised to account for human-induced losses of water within basins caused by inter-basin water diversions or irrigation losses to the atmosphere. The resulting composite discharge field is a mix of observed discharge and WBM simulations where observations are not available, at a 0.5° by 0.5° resolution (latitude, longitude) for monthly climatology.

Sediment flux to the coastal zone is conditioned by geomorphic and tectonic influences (basin area and relief), geography (temperature, runoff), geology (lithology, ice cover), and human activities (reservoir trapping, soil erosion). A new model, termed “BQART” in recognition of those factors, accounts for these varied influences. When applied to a database of 488 rivers, the BQART model showed no ensemble over- or under-prediction, had a bias of just 3% across six orders of magnitude in observational values, and accounted for 96% of the between-river variation in the long-term (≈30 years) sediment load or yield of these rivers. The geographical range of the 488 rivers covers 63% of the global land surface and is highly representative of global geology, climate, and socioeconomic conditions. The BQART model makes possible the quantification of the influencing factors (e.g., climate, basin area, ice cover) within individual basins, to better interpret the terrestrial signal in marine sedimentary records.

Combining the BQART model with a load variability (PSI) model allows for time-variations in sediment flux to be estimated for global rivers. The seasonal flux of sediment of global rivers, on a river-by-river basis under modern and pre-human conditions, is estimated. Humans have simultaneously increased the sediment transport by global rivers through soil erosion (by 2.3 ± 0.6 billion metric tons per year), yet reduced the flux of sediment reaching the world’s coasts (by 1.4 ± 0.3 billion metric tons per year) because of retention within reservoirs. African and Asian rivers carry a greatly reduced sediment load; Indonesian rivers deliver much more sediment to coastal areas.