**1. Introduction**

Drought and water scarcity have always been situations of concern. While drought is a natural phenomenon caused by an abnormal precipitation deficit over a certain region and period of time, water scarcity is a result of human action, referring to an insufficient water availability to satisfy water demands for different socio-economic uses [1]. From an operational perspective (as mostly considered for definition of common indicators), scarcity is considered a temporal problem of lack of water resources. Nonetheless, water scarcity can also be understood as the long-term unsustainable use of water resources, determined by social and political processes [2,3].

Drought and water scarcity events commonly coexist and are interdependent in a same region, making the distinction between them a complex process. Moreover, water scarcity enhances regional vulnerability to drought effects [3,4].

The increasing frequency and intensity of such phenomena, which are expected to worsen from climate change, have been leading to an increasing and urgen<sup>t</sup> need to establish

**Citation:** Maia, R.; Costa, M.; Mendes, J. Improving Transboundary Drought and Scarcity Management in the Iberian Peninsula through the Definition of Common Indicators: The Case of the Minho-Lima River Basin District. *Water* **2022**, *14*, 425. https://doi.org/10.3390/w14030425

Academic Editors: Athanasios Loukas and Luis Garrote

Received: 31 December 2021 Accepted: 27 January 2022 Published: 29 January 2022

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**Copyright:** © 2022 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

specific policies and managemen<sup>t</sup> measures to deal with the associated risks and the mitigation of the related effects [5,6]. These issues become even more relevant in watersheds shared by different countries, since conflicts (defined by opposing national interests and/or policies) over water use tend to be more pronounced, and the harmonization of water policies and bilateral agreements and sustainable water use is required.

At the European Union (EU) level, in order to achieve the environmental objectives established for the protection of water bodies, the Water Framework Directive (WFD) [7] states that, within a river basin shared by different countries, where the use of water may have transboundary effects, water managemen<sup>t</sup> should be coordinated for the whole of the River Basin District (RBD). In this context, the WFD stipulates that Member States (MSs) shall ensure coordination with the aim of producing a single international River Basin Managemen<sup>t</sup> Plan (RBMP). In case that is not possible, the WFD envisages that the MSs develop individual but coordinated RBMPs for each territorial part of the river basin. This directive corresponds to greater country responsibility and interference in water managemen<sup>t</sup> of shared river basins in order to achieve WFD provisions of good environmental status, in line with increasing concern with water quality, namely by southern Member States [8].

One of the purposes of the WFD is to contribute to the mitigation of drought effects. However, the WFD states that in the case of exceptional natural conditions, such as a prolonged drought, the temporary deterioration of water quality will not breach the Directive requirements (Article 4.6). Although "water scarcity" is not directly mentioned in the WFD, its frame is implicit across it, namely by highlighting (Article 11) that MSs shall ensure the establishment of measures to promote efficient and sustainable water use.

To deal with more specific situations, namely for the mitigation of drought and scarcity effects, the WFD highlights the possibility of complementing the RBMPs through special programs and managemen<sup>t</sup> plans (e.g., drought managemen<sup>t</sup> plans—DMPs), which should, whenever possible, also be jointly set by the States involved in the case of international river basins (Article 13.5). Despite not being mandatory, the development of DMPs has been strongly recommended, namely by means of the EU Communications on Drought and Water Scarcity [9–11] and corresponding follow-up reports. In one of those reports [12] the importance and the need of separate indicators to evaluate drought and water scarcity situations is enhanced.

In the Iberian Peninsula (IP), drought is one of the most damaging natural hazards, causing wide-ranging socioeconomic and environmental impacts, which are prospected to continue the recent years' aggravation trends due to climate change. To prevent impacts from drought, close cooperation between Portugal and Spain is required, namely regarding water and drought planning and management, as the two countries share five river basins (Minho/Miño, Lima/Limia, Douro/Duero, Tejo/Tajo and Guadiana) that cover 45% of the Iberian territory (Figure 1). This issue is particularly relevant for Portugal, as 64% of its territory corresponds to shared river basins, with the Portuguese part located downstream, rendering the country extremely vulnerable to the quantity and quality of water flowing from Spain [13].

In 1998, the two countries signed the Convention on Cooperation for Portuguese-Spanish River Basin Protection and Sustainable Use (commonly referred to as the Albufeira Convention), which has been in force since 2000 [14] and was revised in 2008 [15] under the WFD principles. The convention defines the framework for bilateral cooperation to promote and protect the good status and the sustainable use of water resources in the shared river basins, as well as actions to contribute to mitigate the effects of floods, droughts and water scarcity situations.

Under the Albufeira Convention, two institutional bodies were constituted: the Parts Conference (Article 21) and the Commission for Convention Development and Appliance (CADC) (Article 22). The first has mainly a political role and guarantees bilateral cooperation at the highest levels, its representatives being appointed by the Government of each country. The CADC has an operational, deliberative, consultative and supervisory

role, having the responsibility to ensure compliance with the Convention obligations. The CADC is composed of two delegations, one from each country [14].

**Figure 1.** Transboundary river basins in the Iberian Peninsula and respective flow control sections under the Albufeira Convention.

In this context, and in order to secure good water conditions and the current and predictable uses, the Albufeira Convention (AC) defined a minimum flow regime (MFR) at the border sections (and bordering/international river stretches) of each of the Portuguese-Spanish shared river basins (except Lima) (Figure 1). Currently, as stated in the revised version of the convention [15], the MFR consists of minimum volumes of water to be guaranteed at the control (namely bordering) sections: annually and quarterly, for all (4) shared basins; weekly for the two major basins (Douro/Duero and Tejo/Tajo rivers), and; for the southern basin (Guadiana river basin), mean diary flow at the bordering section at the entrance of the Guadiana river in Portugal (Badajoz weir) and at the upstream section (Pomarão) of the estuarine and lower bordering stretch between the two countries. Nevertheless, these flow regimes are not applicable under exceptional drought conditions, those defined by means of cumulative precipitation thresholds and also (only for Guadiana river) on reservoir volumes, based on the weighted values of the referenced monitoring stations in the Spanish part of each basin [14,15].

Moreover, according to Article 19 of the Convention, the parties: (i) should coordinate their actions to control and to prevent drought and scarcity situations and (ii) should define the nature of the exceptions and the establishment of exceptional mechanisms, which can include, among others: (a) the definition of the conditions in which the exceptional measures can be applied and (b) the possible use of indicators that characterize, in an objective way, the drought and scarcity situations. Under the above-referred context and Albufeira Convention principles, the current MFR values and time frame continue to be considered provisional, insufficient and requiring of further revision [16,17].

Spain approved and has implemented drought plans in all the River Basin Districts since 2007 [18]. The 2007 Spanish drought plans were already been revised and approved in 2018 [19]. In the 2018 versions (e.g., [20]), two types of indicators are defined: a Prolonged Drought Indicator (PDI) and a Water Scarcity Indicator (SI).

Portugal approved a national drought plan in 2017, by which two types of drought situations (agrometeorological and hydrological) are identified through the use of simple variables (or standardized indexes), like precipitation, stored dam reservoir volumes, piezometric levels and soil moisture [21]. Specific DMPs by River Basin District are envisaged but still not active.

Considering these national differences, in order to minimize water conflicts and prevent drought-related impacts (e.g., in 1994/95, in the Guadiana river basin, the affluences to Portugal were actually null for six months [22]. In 2019, some Portuguese parts of the Tejo river basin dried out due to Spanish water management, contributing to the water quality degradation in the Portuguese part [23], which led to political contact between the two Government representatives, namely the Environmental Ministers [24]), several efforts should be made by the two countries to adopt coordinated and/or possible joint drought managemen<sup>t</sup> and planning through the establishment of standard and/or common approaches, in the light of the AC and WFD. One of the most important approaches that should be enforced is the definition of common indicators that characterize, in an objective way, the drought and scarcity situations [13,22,25].

In this sense, the Spanish methodology, by using separate indicators for those situations, presents a good basis for the establishment of a common system of indicators for the Iberian Peninsula in order to achieve a better integration with the WFD and AC goals, representing a step forward in the implementation of European Water Policies regarding drought managemen<sup>t</sup> and planning.

The purpose of the presented work was the definition of prolonged drought and scarcity indicators for the Portuguese parts of the shared Minho and Lima river basins, in common with the Spanish parts of the basins, as the basis for the joint international DMP for these basins. In fact, following this work, this DMP is already in the stage of discussion for approval by the Portuguese and Spanish RBD authorities (respectively, APA—Agência Portuguesa do Ambiente and CHMS—Confederación Hidrográfica del Miño-Sil).

The paper describes the procedure used in the definition of the indicators for the Portuguese parts of the basins, as well as its comparison with the corresponding Spanish indicators, for a period from October 1980 to September 2017. The work allowed us to assess the applicability of a common system of indicators for any Portuguese-Spanish transboundary river basin's drought and scarcity management.

This work is intended to be a prototype for the definition of new and similar drought indicators to be applied in common by Portugal and Spain in the shared river basins, and was developed in close collaboration with the Portuguese Minho and Lima RBD and the corresponding Spanish RBD authorities under the scope of the RISC-ML project [26].

### **2. Materials and Methods**
