Basin-scale evaporation rate estimation based on isotopic approach: adaptation to the Mediterranean conditions

Being a proxy for water stress monitoring and as a key element of sustainable water management, evaporation helps to assess the vulnerability of Mediterranean ecosystems to droughts. Estimation of annual basin-scale evaporation rate can be done using cost-effective isotope mass balances approaches that exploit the integrative nature of the river isotopic signal.

In Mediterranean regions, the marked climatic seasonality and uneven precipitation distribution complicates the use of isotope mass balances to obtain basin-scale estimation of average evaporation rates. For example, a mass balance approach carried out on the Tavignanu River watershed in Corsica (France), showed unrealistic evaporation rate estimates of 10% for 2017-2018 and 1% for 2018-2019. These results suggest that not only does evaporation alter the seasonal isotopic composition in the river, but that there is a complex variability of the dominant water reservoirs contributing to the streamflow. Therefore, we propose a modified mass balance approach that includes monthly contribution of the different water sources to the river discharge. This allows the discrimination of isotopic variation occurring by evaporation from that originating by mixing processes. By applying this modified approach, we estimated evaporation rates on the Tavignanu River watershed that were in good agreement with results obtained by hydrological modelling: 40% for 2017-2018 and 46% for 2018-2019, respectively.

The proposed approach can be used to determine evaporation rates in river basins in other semi-arid climates where estimating evaporation is of major concern for water management. More generally, these results should encourage investigations of details in water source contributions to river flow prior to conducing isotope mass balance evaporation estimates. Consequences are expected in several research fields where the portioning of evapotranspiration into components is of interest, including hydrology (e.g. water budget estimations), ecology (e.g. carbon budget estimations) and climatology.