**4. Conclusions**

This paper integrated two numerical models (PATRICAL and RREA) to assess nitrate concentration in surface and groundwater of the Júcar River Basin District (RBD) and to determine the main drivers of pollution and the effects of nitrate transfers from the aquifer on the nitrate status of the rivers.

It was found that there is a direct linear correlation between the nitrate concentration in the river and aquifer along the main course of the Júcar and Turia rivers. Changes of nitrate concentration in rivers of the Júcar RBD are strongly related to the source of irrigation water, river–aquifer interaction, and the regulation of water flow produced by the dams. The models properly represent the effects of the SW–GW interaction in the nitrate status on the rivers in 87% of the Júcar RBD.

Therefore, this paper proves that PATRICAL and RREA models, after a proper calibration and validation process, allow for assessment of the concentration of nitrates in surfaceand ground-water. This is particularly relevant in in arid and semi-arid areas, such as the Mediterranean basins.

The models may also be used to identify pollution sources, evaluate the efficiency of management strategies to prevent water degradation, and analyze the effects of natural or human-induced changes on the nitrate concentration in the water bodies, among a wide range of applications. Thus, future research could be focused on analysing how climate and land use variations affect nitrate concentration in rivers and aquifers.

**Author Contributions:** Conceptualization, D.Y.D.-G., J.P.-A. and M.Á.P.-M.; methodology, D.Y.D.-G., J.P.-A. and M.Á.P.-M.; software, D.Y.D.-G., J.P.-A. and M.Á.P.-M.; validation, D.Y.D.-G., J.P.-A. and M.Á.P.-M.; formal analysis, D.Y.D.-G., J.P.-A., M.Á.P.-M. and H.T.H.; investigation, D.Y.D.-G., J.P.-A., M.Á.P.-M. and H.T.H.; resources, J.P.-A. and M.Á.P.-M.; data curation, D.Y.D.-G.; writing—original draft preparation, D.Y.D.-G.; writing—review and editing, D.Y.D.-G., J.P.-A., M.Á.P.-M. and H.T.H.; visualization, D.Y.D.-G.; supervision, J.P.-A. and M.Á.P.-M.; project administration, J.P.-A. and M.Á.P.- M.; funding acquisition, J.P.-A. and M.Á.P.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The first author's research is partially funded by a PhD scholarship from the food research stream of the program "Colombia Científica—Pasaporte a la Ciencia", granted by the Colombian Institute for Educational Technical Studies Abroad (Instituto Colombiano de Crédito Educativo y Estudios Técnicos en el Exterior, ICETEX). The authors thank the Spanish Research Agency (AEI) for the financial support to RESPHIRA project (PID2019-106322RB-100)/AEI/10.13039/501100011033.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Water hydrographic network, water demands, nitrate concentration in surface and groundwater, streamflow, groundwater withdrawals and piezometric levels data set area available in Water Information System for the Júcar RBD (SIA Júcar, Available online: aps.chj.es/siajucar/, accessed on 26 March 2021). Precipitation and temperature data set can be found in www.saih.chj.es and www.aemet.es (accessed on 26 March 2021), respectively. Point discharge in surface and groundwater and equivalent population in urban areas can be consulted in the Spanih National census of discharges (MITECO, Available online: www.miteco.gob.es, accessed on 26 March 2021).

**Acknowledgments:** We appreciate the help provided by the Júcar River Basin District Authority (CHJ), who gathered field data.

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