**4. Conclusions**

To minimize water conflicts and prevent drought- and scarcity-related impacts on international river basins, it is essential that both countries have similar planning and managemen<sup>t</sup> approaches. In this regard, Portugal and Spain are not ye<sup>t</sup> in similar stages. Notably, the definition of comparable drought indicators to be applied in common by the two countries in the shared river basins is missing.

In this context, the goal of the present work was the definition of common drought indicators in the shared river basins as a tool for the better coordination of drought and water scarcity managemen<sup>t</sup> in the Iberian Peninsula. For this purpose, examining the Minho and Lima river basins as case study, this work presented the definitions of the indicators in the Portuguese part of the Minho and Lima river basins similar to those defined in the Spanish part of the two basins, namely the Prolonged Drought and Scarcity indicators.

The resulting indicators showed that, in terms of prolonged drought and despite different intensities and durations, there was a general coincidence between the periods of prolonged drought in the Portuguese and Spanish parts of the Lima and Minho river basins between 1980/1981 and 2016/2017, with the major drought events being signaled in the two parts of each basin.

Concerning scarcity situations, both the Portuguese and Spanish parts of the Minho river basins and the Portuguese part of the Lima river basin have low exposure to scarcity situations. The only exception is the Spanish part of the Lima river basin, which has a high level of exposure to this situation.

Based on the results presented, namely due to the apparent adequacy of the indicators obtained for Spanish and Portuguese parts of the Minho and Lima river basins, it can be concluded that the general Spanish methodology presented may be considered to be a valid starting point for the Portuguese part of Minho and Lima river basins and that it has also the potential to be applied in the others Portuguese basins.

The advantage of using these indicators instead of the use of more conventional singular indicators, such as the SPI or other hydrological indicators, corresponds to the fact of diagnosis and indicator results being more clearly comparable between the different basins, despite their proper characteristics. Moreover, the definition of similar indicators has extreme importance for transboundary drought and scarcity managemen<sup>t</sup> since, common definitions enable the common characterization and comparison of droughts and scarcity events in the Portuguese and Spanish parts of the shared river basins. That may allow the common planning, managemen<sup>t</sup> and monitoring of droughts and scarcity situations between the two countries.

In addition to a better integration of both countries with WFD principles and goals, the drought and scarcity common indicators' definitions and managemen<sup>t</sup> shall lead to the improvement of the Albufeira Convention implementation by contributing not only to the envisaged redefinition of the MFR but also to an agreemen<sup>t</sup> on the definition of environmental flows for common stretches of the transboundary rivers.

Nevertheless, it should be emphasized that some work remains to be developed to improve the current system of indicators to be used in common by the two countries. In fact, despite being in a way quite integrated with the WFD (distinguishing prolonged drought situations from scarcity situations), the use of the Spanish system of indicators has some recognized limitations, cautions and reserves that must be considered [34], namely, concerning the PDI. In the Spanish system, the definition of the PDI only considers the intensity and not the duration of the events; the arbitrary definition of the threshold for prolonged drought is equal to an indicator value of 0.30. This definition should be agreed with Portugal, because the measures envisaged to be applied by Spain for prolonged drought—the call in Article 4.(6) of the WFD and reduction of environmental flows—may not only affect Portugal but also in fact themselves constitute the consequences of prolonged

drought events. Therefore, the managemen<sup>t</sup> of prolonged drought situations should be performed in order to achieve a maximum delay on the use of Article 4.(6) and on the reduction of the environmental flows, which ought not to be exacerbated.

**Author Contributions:** Conceptualization, R.M., M.C. and J.M.; methodology development and application, M.C.; result analysis and validation, R.M., M.C. and J.M.; writing—original draft preparation, M.C. and J.M.; writing—review and editing, R.M., M.C. and J.M.; supervision, R.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the RISC\_ML project (reference: 0034\_RISC\_ML\_6\_E), co-funded by the European Regional Development Fund (ERDF) through the Interreg V-A Spain-Portugal Program (POCTEP) 2014–2020.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data used in this study are available in the publicly accessible repositories: SNIRH database (https://snirh.apambiente.pt/, accessed on 20 October 2021) and SAIH Miño Sil (http://saih.chminosil.es/, accessed on 20 October 2021).

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