**4. Discussion**

By improving the calculation of the freshwater quantity and associated properties reaching the coastal area, managers and scientists would be able to better reproduce the thermal and salinity fronts affecting coastal hydrodynamics and associated ecological processes. The proposed methodology for integrating the water cycle from rainwater runoff to the open ocean with numerical models was demonstrated for western Iberia by simulating the extension of the WIBP during a wet season and an extreme rain event.

The approach taken enables the continuous improvement of the solution through updates/upgrades to any upstream component such as the watershed results or through the use of a better characterisation of the estuarine proxy. The application of this methodology provides more realistic land boundary conditions than river climatology direct discharges, while also having the capacity to describe inter-annual variability between dry and wet years and effects due to extreme rain or drought events. The developed methodology is generic and could be set for any region with open-access data and open-source models.

Enhanced land boundary conditions including water properties such as temperature and salinity can provide a more realistic circulation and thermohaline fields in the coastal area. The included land boundary conditions can grow in complexity from plainly watershed modelling results that are implemented as direct discharges to fully 3D biogeochemical estuarine fluxes (positive and negative) imposed with momentum.

The main advantage of watershed modelling is to complete the hydrometric monitoring networks, providing gapless river-flow data and non-monitored variables while covering areas with monitored data. Additionally, watershed numerical modelling enables the forecasting of river flow and water properties, thereby allowing a more efficient managemen<sup>t</sup> of the modelled systems. However, calibration and validation of many watersheds became challenging when large water reservoirs or intense human managemen<sup>t</sup> modify their natural flow. The use of 5 km horizontal-resolution domains enabled the coverage of a large simulation area, as well as a decrease in the simulation time required, and these domains can adequately represent the channel flow behaviour of larger rivers. However, smaller rivers may not be accurately simulated under these conditions. Generally, better results are obtained from higher resolution grids and single-watershed domains, since calibration is easier to execute.

From an operational point of view, the objective is to generate a land boundary product with the best available information. For this reason, observed data can be combined with modelling results, such as river properties. In any case, the watershed-model approach has shown the ability to produce a more realistic land boundary condition than methods involving the use of river climatologies, but further development is needed for this component of the system.

The estuarine proxy has been a versatile tool that allows researchers to estimate in a simple way estuarine mixing and contributions to the open ocean. The proxy affords the inclusion of estuary time and spatial scales, due to the tides and the rivers' flow combination, in coarse regional ocean models. Its low computational cost would enable the easy construction of two-way coupled systems. Another advantage is that land boundary conditions are independent of the receiving model domain, and thus the same flow/fluxes can be used by several regional ocean models. The estuarine proxy can be regarded as a useful tool when full estuarine models do not exist.

#### **5. Conclusions and Future Approach**

Monitoring networks along the estuarine continuum from river catchment to the open ocean should be encouraged for evaluating the transfer of properties and momentum at the land-ocean interface. While open ocean network is relatively well established, operational estuarine monitoring is far from consolidated. In general, hydrometric networks should be further developed to meet the requirements of coastal region stakeholders. Numerical models can support the design of the monitoring networks, and they can provide a more comprehensive view by spatially and temporally completing the observed data. In addition,

numerical models can provide non-observed variables and forecasts. Model results also permit the calculation of complex indicators such as tidal prims and the area of influence of the estuarine waters, as well as the accurate estimation of estuarine fluxes that would serve as boundary conditions for ocean regional models. The synergies between these two can be observed, and the modelled data pave the way for a more holistic view of the water continuum.

Since salinity remote sensing is starting to take its first steps in coastal areas and since in-situ monitoring has a low frequency, numerical modelling is currently the only tool able to represent and estimate the temporal and spatial scale of the WIBP and other estuarine plumes. Taking into consideration the numerical modelling limitations and assumptions, the salinity modelling results provided by the methodology described here significantly improves salinity fields and helps delimit the region of freshwater influence and salinity fronts.

Future work must include improving the incorporation of human-water managemen<sup>t</sup> in watershed modelling in order to obtain more accurate forecasts. A possible next step in the research is to use artificial intelligence and machine learning techniques. In fact, combining machine learning and physical-based models is becoming popular in the design of predictive systems [29,30]. Including the human component is especially relevant in southern Europe where water retention is a more common practice.

In addition, efforts to identify and make available more river data are crucial for watershed-model calibration and validation. As the results show, to achieve good sea surface salinity fields in regional models, it is not enough to include only the major rivers, but a comprehensive freshwater budget is also required.

**Author Contributions:** Conceptualization, F.C.; methodology, F.C., F.S., L.S., A.R.O., R.F., D.B. and R.N.; validation, F.C. and F.S.; resources, E.O., A.T., M.A. and A.N.; writing—original draft preparation, F.C., F.S., E.O. and A.N.; writing—review and editing, F.C., F.S., E.O. and A.N.; project administration, F.C. and A.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** The present work was performed within the framework of two research projects: LAMBDA project from Copernicus Marine Environment Monitoring Service (CMEMS) Service Evolution 2 (2018–2020) and the iFADO project supported with ERDF funds from the INTERREG Atlantic Area Programme under contract EAPA 165/2016.

**Data Availability Statement:** LAMBDA project products can be found in the LAMBDA Project Data Portal: http://www.cmems-lambda.eu/#data-portal (accessed on 8 April 2022); data from the different ocean model scenarios analysed in the study can be available on request from the corresponding author.

**Acknowledgments:** The authors are grateful to the following experts (and institutions) for their support to this research: Marcos G. Sotillo (Puertos del Estado), Tomasz Dabrowski (MI), Joanna Staneva (HZG), and Marina Tonani (Mercator Ocean international) for their support, evaluation, and contributions to improve the LAMBDA-project products. The authors would like to thank Angelique Melet, Isabel Garcia Hermosa, and Pierre-Yves LeTraon (Mercator-Ocean International) for their continuous advice and evaluation during the LAMBDA project. The authors would like to thank Kieran O'Driscoll for the careful English-language review. The author would like to thank the reviewers for constructive criticism of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
