Assessing the freshwater flux from the continents to the Mediterranean Sea

Fuxing Wang,Jan Polcher

doi:10.1038/s41598-019-44293-1

Abstract

Precipitation minus evaporation over continents is the freshwater flux which can be sustainably exploited by mankind. Over a catchment and longer time periods, this flux is also the amount of water which flows into the ocean. An essential question for semi-arid areas of the world is how well this freshwater flux can be estimated and predicted to evolve under climate change and human water use. Knowing this flux is thus an essential element in regional water resources management. Here we examine this question over the catchment of the Mediterranean Sea. Using a novel data assimilation methodology that incorporates observed discharges of rivers in a land surface model, new estimates of the freshwater flux to the Mediterranean Sea for the period 1980–2013 are proposed. We find that more freshwater (40–60%) flows into the sea than previously estimated. The hypothesis we advance is that previous estimates have underestimated the discharges of the large number of unmonitored coastal basins and neglected submarine ground water flows. The proposed error bars on the estimate indicate that the degrading river gauging station network limits our ability to monitor this branch of the water cycle reliably. Nevertheless, the uncertainty is small enough to allow the identification of regions in which non-climatic decreases in the freshwater flows exist over the period.

Highlights

The Mediterranean region is considered to be one of the most vulnerable regions to climate change as water scarcity is expected to be exacerbated[1,2]
We estimate riverine discharge through the Fusion of ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems)[18,19] land surface model (LSM) and Global Runoff Data Center (GRDC, 56068 Koblenz, Germany) observations over the Mediterranean catchments for the 1980–2013 period
Our result indicates that submarine groundwater discharge (SGD) has an important role in the water cycle of the Mediterranean Sea

Summary

Introduction

The Mediterranean region is considered to be one of the most vulnerable regions to climate change as water scarcity is expected to be exacerbated[1,2]. The renewable water resources are predicted to decrease with climate change as a result of increasing temperature and reduced rainfall[3]. These changes are important for this region with already scarce water resources and increasing demands for water for domestic, industrial, irrigation, and tourism activities[4]. Water discharge from the continents plays an important role for the Mediterranean Sea as it provides a large fraction of the freshwater[6] and most nutrients[7]. Because of the semi-enclosed nature of this sea, these fluxes drive in large part the oceanic circulation patterns and the marine productivity[8]. FOG corrects modelling and forcing errors in the continental moisture convergence and provides adjustments for unmonitored catchments

Methods

Findings

Conclusion