Estimating riverine freshwater flux into the Mediterranean sea


The river discharge is an important component of the in Earth’s water cycles which couples the continents and oceans in the climate system. These fluxes are difficult to estimate due to un-gauged rivers, submarine groundwater discharges, human activities, and measurement errors. One approach is based on the observed river discharge and a simple annual water balance model for ungauged rivers, but it only provides flux estimates with large uncertainties and low temporal frequency which are unsatisfactory for oceanic modelling needs. Another approach is to use off-line land surface models (LSM) which provide daily values but with uncertainties associated to the limitations of the models. In the proposed study, we use data assimilation by merging the river discharge modelled by ORCHIDEE LSM and the observations from Global Runoff Data Center (GRDC) to obtain optimal estimates over the full catchment of the Mediterranean sea (1980-2013). The Mediterranean sea, being a closed basin, is a particularly interesting case for studying the interactions between the continental and oceanic water cycle.

The assimilation, as implemented for this region in ORCHIDEE, uses 338 GRDC stations, and corrects for systematic errors in the model as well as missing processes (dam operations, irrigation, submarine discharges). The atmospheric forcing is at 0.5° degree resolution but the routing scheme within the LSM uses finer scale information to delineate the basins and describe the river graphs. The assimilation improves the discharge estimates by comparing against independent river discharge observations. The assimilation also corrects the discharge bias for the basins without observations assimilated by extrapolating the correction from adjacent basins. This is particularly important for the Mediterranean, which has a great number of small river basins without observed river discharges available.

This assimilation allows to estimate the total fresh water into the Mediterranean at around 558 km3/y, which is higher than the values reported in the literatures (e.g., 300-400 km3/y, depending on the studies and period considered). Comparing these new estimates with the widely used CEFREM data (from CEntre of Education and Research on Mediterranean Environments, Ludwig et al. 2009) shows that the discharge in well observed regions are comparable with the difference being less than 10%. On the other hand, discharge into basins of the Mediterranean dominated by the non-observed rivers (Alboran and Tyrrenian sea for instance) differs by more than 100%. We hypothesize that the large difference with previous estimates is the consequence of the simple water balance assumptions made for un-observed river basins and that probably the estimates proposed here benefit fully from the state of the art land-surface model and the data assimilation technique. The ORCHIDEE LSM takes into account more small rivers than previous studies thanks to the high resolution river network and has a more reliable estimate of the water balance over the continents. Other possible sources of the extra freshwater flows into the Mediterranean sea could be the submarine groundwater discharge and the kastic systems.

Fuxing Wang
salle Darcy (Tours 46-56, 3e étage)
Vendredi, 29 juin, 2018 - 13:00