Abstract

Historically, states have found it useful to regulate their relationship over their shared water resources via treaties. While treaties provide some element of predictability and certainty with regard to the future supplies of water to the riparian states, they also need to incorporate mechanisms that allow flexibility to respond to changes in the quantity of water available for allocation amongst the parties. This requirement is especially relevant when global climate change is causing freshwater resources to shrink. The legal regime of the Guadiana, as an example of a modern legal regime of a transboundary river, includes some provisions that permit the riparian states to accommodate the variability of the Guadiana's streamflow in response to climate change within the water allocation arrangements. The lessons learned from these provisions, which include periodical update of the historical mean precipitation values, updating the flow regime of the rivers and cancelling minimum flow requirements under special circumstances, may contribute to climate-proofing international transboundary agreements within the catchments where climate-change induced water scarcity poses a major threat.

Introduction

Historically, states have found it useful to regulate their relationship over their shared water resources via international treaties. These treaties can provide some element of predictability and certainty with regard to the future access to water for states. However, research shows that most of these agreements lack adequate mechanisms for addressing changing social, economic or climate change conditions (Goldenman, 1990; McCaffrey, 2003; Cooley & Gleick, 2011).

Water allocation agreements are particularly vulnerable to climate change, as most of them include fixed allocation amounts (Wolf, 1998, 2002) that are difficult to amend and are concluded under the assumption that the future's water availability will remain the same as today's (McCaffrey, 2003). With climate change altering the form, intensity and timing of precipitation and runoff, this assumption no longer holds true. It is important, therefore, to move towards designing water allocation agreements that are capable of addressing the issue of hydro-climatic variability, by identifying and improving best existing practices.

Within the European Union, the Convention on Cooperation for the Protection and Sustainable Use of the Waters of Portuguese-Spanish Hydrographic Basins (hereinafter, the ‘Albufeira Convention’) is recognized as one of the rare examples of water allocation treaties to include specific provisions to deal with hydro-climatic variability (Baranyai, 2015). This convention, together with its Amendment Protocol of 2008 (hereinafter, the ‘Amendment’) and the EU Water Framework Directive (WFD), composes the legal framework governing five major basins of Spain and Portugal (the Miño/Minho, Limia/Lima, Douro/Douro, Tajo/Tejo and Guadiana/Guadiana), which are showing a worrisome level of decrease since the first half of the 20th century. Climate change is expected to aggravate the water stress in these basins, by altering precipitation and streamflow patterns and provoking more frequent extreme water events (Estrela et al., 2012).

This paper explores the options available to accommodate climate change-induced variability of water supply in the Guadiana basin under its legal framework, in order to provide suggestions for improving the capacity of international water allocation agreements to adapt to climate change. An assessment of how each of the identified options have been implemented in practice is provided, accompanied, where appropriate, by suggestions for solving challenges and further improvement.

The paper starts by presenting the case-study area, and the impacts of climate change on water availability in the Guadiana basin. It then provides an insight into the legal framework which regulates the use of water from the river. Special attention is paid to the Albufeira Convention and its Amendment, and the EU WFD. The fourth section evaluates the performance of the legal framework of the basin in accommodating the changing water supply and demand, and highlights the lessons learned from such performance. Special focus is placed on the minutes of the meetings of the Commission for Implementation and Development on the Albufeira Convention (CADC) (Minutes of the VI Plenary Session of the CADC, 2006), and interviews conducted by the author and other researchers with the stakeholders involved. The conclusion section provides suggestions for increasing the flexibility of international water allocation agreements under climate change, based on the findings of the case study.

Presenting the case study area: the Guadiana River's geography and climate

Guadiana is one of the most important rivers of the Iberian Peninsula, with 30 major tributaries and a total length of 820 km, of which the last 50 km make the natural border between Portugal and Spain. The river flows westward through south-central Spain and south-eastern Portugal to the Gulf of Cádiz in the Atlantic Ocean (Figure 1). Spain is largely the upstream state. Nevertheless, the Portuguese territory occupies the upper part of the basin in the final reach of the river. The drainage area of the river is 66,960 km2 and is characterized by a hydrological regime of high intra- and inter-annual discharge variation, typical to the Mediterranean region.

Fig. 1.

The Guadiana basin. Source: Creative Commons Attribution Share Alike 3.0, based on an image of http://www.maps-for-free.com/.

Fig. 1.

The Guadiana basin. Source: Creative Commons Attribution Share Alike 3.0, based on an image of http://www.maps-for-free.com/.

The importance of the Guadiana basin for Spain and Portugal lies in the role it plays in supporting the dependent ecosystems and the socio-economic activities in both states. Some of the major wetlands in Spain, including ‘Las Tablas de Daimiel’, depend for their survival on the waters provided from the Guadiana. In Portugal, the Guadiana River is considered the home of the most diverse fish fauna (Cowx, 2008). Agricultural activities, which are the main source of income in the southern parts of Spain and Portugal, are dependent on the Guadiana basin. In Spain, 89% of the whole consumptive use of the waters of the Guadiana is dedicated to irrigation (Martin, 2012). In Portugal, 60% of the water reserved in the 13 dams constructed before Alqueva are dedicated to agricultural uses. The water of the Guadiana at the Alqueva dam is mainly used for irrigation and agricultural diversification. However, the dam also contributes to hydroelectric generation and tourism development.

The water stress experienced in the region surrounding the Guadiana since the beginning of the 20th century is currently threatening the ecological well-being, socio-economic and recreational uses of the river. Considerable modification of the local hydrological regime, via dams, wells and increasing urbanization, is the main force behind such pressure. Meanwhile, the climate change-induced rise in temperature and variations in the amount and intensity of annual and seasonal precipitation in the area have exacerbated the situation, by provoking a significant decrease in the streamflow and increasing the frequency and intensity of extreme water events (Rodrigo & Trigo, 2007).

Studies focusing on the future of the region suggest that the water scarcity problems along the Portuguese-Spanish border will continue to mount as climate change hits the two countries with more intensity. Depending on the model and scenario used, the region may witness between 1 °C and 6 °C increase in temperature by the end of the 21st century (Nogués-Bravo et al., 2007). This trend, together with a predicted decrease of between 25% and 30% in precipitation by 2,100 (Giorgi & Lionello, 2008), will most likely translate into significant reduction of water resources and the soil moisture level (Ayala-Carcedo, 2001).

The Guadiana River is considered to be one of the most vulnerable basins to such implications of climate change. The basin is already under pressure due to overexploitation of aquifers, in particular for agricultural purposes (Bromley et al., 2001). Climate change will result in such pressure by contributing to temperature increase, decreasing the water discharge, and increasing the inter-seasonal variability of streamflow and the frequency of extreme water events. Some studies predict up to 15% reduction of the river's water resources as a result of climate change (Amblar et al., 2016). Such drastic reduction can lead to tension in the region as the competition over the use of water mounts between different sectors in Spain and Portugal. As a result, a rigorous approach to transboundary water allocation that permits accommodating changing water supply and the implications of climate change is urgently needed to manage the Guadiana basin.

The legal framework of Guadiana

The legal regime of the Guadiana River is predominantly defined by three legal instruments: the Albufeira Convention of 1998, its Amendment of 2008 and the WFD. An understanding of these instruments is necessary in order to analyse how they contribute to the flexibility of water management practices within the basin.

The Albufeira Convention

The Albufeira Convention is the cornerstone of the legal framework governing water allocation in the Guadiana basin. The Convention has six sections with a total of 35 articles, two annexes and one additional protocol dealing with a variety of issues, including modes of cooperation (Albufeira Convention, Part II), environmental protection and sustainability (Albufeira Convention, Part III), exceptional circumstances (Albufeira Convention, Part IV) and institutional matters (Albufeira Convention, Part V). The approach of the Convention, clearly modelled on the Convention on the Protection and Use of Transboundary Water Courses and International Lakes, is highly environmentalist. It recognizes a need to balance environmental protection with sustainable use of water, and requires the parties to consider the impacts of their water-related decisions on fauna, flora, climate, soil and air (Albufeira Convention, Preamble).

The specific reference of the Convention to water allocation arrangements is made under Article 16, which requires the states to define the flow regime of each basin, in accordance with the basin's characteristics, including the basin's geography, hydrology and climate, the minimum flow required to guarantee good water conditions, and the historical uses of water. The executive body of the Convention, the CADC, is ultimately responsible for proposing the river's regime flow, which will then have to be approved by the Conference of the Parties (CoP) before implementation. The parties are thereafter obliged to manage their hydrologic infrastructure in such a way that the flow regime is respected. Until such flow regime is established, the Additional Protocol to the Convention defines a preliminary flow regime for each river.

The approach of the Additional Protocol to the Guadiana's flow regime is peculiar. The Protocol divides the river into two parts; one downstream from Azud de Badajoz where the river goes to Portugal from Spain, and the other downstream from the station of Pomarão where the river goes back from Portugal into Spain as a border (Figure 2). An annual and a daily flow regime is designated to the first part of the river from Azud de Badajos. In accordance with the annual regime, the amount of water that is supposed to be passed to Portugal from this station depends on both the amount of precipitation between 1 October and 1 March of each year and the volume of water accumulated by Spain in its dams. The amount of precipitation is compared with a reference value, originally understood as the mean accumulated winter precipitation1 during the period from 1945/46 to 1996/97. However, paragraph 4 of the Annex to the Additional Protocol of the Convention requires the states to update this mean value every 5 years. Such provision permits a certain level of flexibility to respond to changes in water supply. To allow for more flexibility, deviations from the annual flow regime are also permitted during dry years (see Table 1).

Table 1.

The Guadiana's annual flow regime at Azud de Badajoz. Source: Additional Protocol to the Albufeira Convention.

  Accumulated precipitation above the average as at 1 March 
Total amount of water (hm3) accumulated in Spanish damsa >65% <65% 
>4.000 600 hm3 400 hm3 
Between 3,150 and 4,000 500 hm3 300 hm3 
Between 2,650 and 3,150 400 hm3 Exception 
<2,650 Exception Exception 
  Accumulated precipitation above the average as at 1 March 
Total amount of water (hm3) accumulated in Spanish damsa >65% <65% 
>4.000 600 hm3 400 hm3 
Between 3,150 and 4,000 500 hm3 300 hm3 
Between 2,650 and 3,150 400 hm3 Exception 
<2,650 Exception Exception 

aThese dams include: La Serena, Zujar, Cijara, Garcia de Sola, Orellana and Alange.

Fig. 2.

Location of Azud de Badajoz and Pomarão stations on the Guadiana. Source: UNEP/DEWA/GRID-Europe 2007.

Fig. 2.

Location of Azud de Badajoz and Pomarão stations on the Guadiana. Source: UNEP/DEWA/GRID-Europe 2007.

The Additional Protocol provisions for the daily flow from both Azud de Badajoz and Pomarão require the upstream of the respective stations to pass a minimum flow of 2 m3/s into the river, although no annual regime flow is prescribed by the Additional Protocol for the Pomarão station. However, pursuant to article 2 of the Annex to the Additional Protocol, the parties are required to investigate the situation of the river's estuary, and assign an annual regime to this part.

The institutional provisions of the Convention, which deal with the CADC, CoP, their structure and their functions, are extremely relevant to the Convention's approach to water allocation. Articles 22 and 23 on the CADC highlight that the Commission is a technical and secretarial body in its composition and powers. Nevertheless, pursuant to Article 16, it is within the fold of the CADC where parties can define and establish the necessary flow regime of each basin. Meanwhile, the ultimate power to approve the proposed flow regime is held by the CoP.

The WFD of the European Union

The WFD is one of the pillars of the legal regime of the Guadiana River. The Directive results from attempts to harmonize the water regulations within the European Union, starting in the mid-1970s (Bukowski, 2011), and its main goal is to set rules for achieving effective water governance on a European level.

Although adopted after the Albufeira Convention, the traces of the Directive's impacts on the Convention are quite obvious, as its advanced drafts were already known to Spain and Portugal when the convention was prepared. In fact, the objective of the Albufeira Convention (Art. 2) and its Goals (Art. 4) on cooperation for protection of water bodies, dependent aquatic ecosystems and good status of water, seem to reflect the same objectives as those of the WFD which are reflected under its Article 4. Therefore, the provisions of the directive appear to be completely consistent and merely complementary to those of the Albufeira Convention.

The WFD includes no explicit obligations on water allocation arrangements. However, its preamble recognizes the importance of linking quantity to quality issues and requires states to ensure the ‘good status’ of water. This requirement is interpreted as the obligation of the states to implement an ecological flow regime, consistent with the achievement of the environmental objectives in natural surface water bodies (European commission, 2015). In other words, all water-related policies, including water allocation arrangements, must be compatible with the principle of sustainability and guarantee the minimum flow necessary for the well-being of the basin ecosystem. In this sense, the WFD does not offer a mechanism to improve flexibility of water allocation schemes to respond to changes in water supply. However, it requires the member states to fashion their water allocation arrangements in a way that would allow for such flexibility.

Amendment to the Albufeira Convention

The Albufeira Convention was, from the very beginning, recognized as a significant step towards the sustainable management of Portuguese-Spanish shared basins. Nevertheless, following the implementation of the treaty provisions, guaranteeing a minimum streamflow for the downstream state remained problematic.

During the second plenary meeting of the CADC, Portugal started voicing concerns about the reduction of the flow volume in the Tajo and Duero basins. It was under the shadow of these concerns that Portugal proposed that dealing with matters related to the flow regime and extreme water events should be included in the priorities of the CADC. Later on, the failure of Portugal to comply with the flow regime at Ponte de Muge2, concerns of Spain about reduction in water supplies due to the ongoing works in the Alqueva Dam and the drought situation experienced in both states pushed the CADC further towards formulating a new allocation regime for the rivers under Article 16 of the Convention. The Commission's attempts in this respect resulted in the Amendment of the Convention in 2008.

The most important contribution of the Amendment was to establish guaranteed trimestral, weekly and daily minimum flows in the basins3. Prior to the Amendment, the transfer of the minimum flow by the upstream state to the downstream state could in theory be conducted once every year. The Amendment requires the upstream state to proceed with phased water releases, protecting the ecosystem and providing the downstream state with more water security in the face of changing water supplies in dry periods (EFE, 2006; Gooch, 2016). In this sense, the Amendment is clearly influenced by the requirement of WFD for establishing an ecological flow.

In the case of the Guadiana River, the Amendment kept the annual flow regime, guaranteed under the Convention at Azud de Badajoz, untouched. However, it established a trimestral minimum flow, which depends on the water accumulated in the Spanish dams, as well as the amount of precipitation registered at Ciudad Real and Talavera la Real. The reference mean precipitation in this case is calculated based on the mean precipitation observed during the 6 months prior to the first day of the third month of each trimester. More details on the trimestral flow for Azud de Badajoz are reflected in Table 2.

Table 2.

The Guadiana's trimestral flow regime at Azud de Badajoz; source: Amendment of 2008.

(i) Between 1 October and 31 December 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >3,700 63 hm3 42 hm3 
 Between 2,850 and 3,700 53 hm3 32 hm3 
 Between 2,350 and 2,850 42 hm3 Exception 
 <2,350 Exception Exception 
(ii) Between 1 January and 31 March 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >4,000 74 hm3 49 hm3 
 Between 3,150 and 4,000 61 hm3 37 hm3 
 Between 2,650 and 3,150 49 hm3 Exception 
 <2,650 Exception Exception 
(iii) Between 1 April and 30 June 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >3,700 42 hm3 28 hm3 
 Between 2,850 and 3,700 35 hm3 21 hm3 
 Between 2,350 and 2,850 28 hm3 Exception 
 <2,350 Exception Exception 
(iv) Between 1 July and 30 September 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >3,400 32 hm3 21 hm3 
 Between 2,550 and 3,400 26 hm3 16 hm3 
 Between 2,050 and 2,550 21 hm3 Exception 
 <2,050 Exception Exception 
(i) Between 1 October and 31 December 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >3,700 63 hm3 42 hm3 
 Between 2,850 and 3,700 53 hm3 32 hm3 
 Between 2,350 and 2,850 42 hm3 Exception 
 <2,350 Exception Exception 
(ii) Between 1 January and 31 March 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >4,000 74 hm3 49 hm3 
 Between 3,150 and 4,000 61 hm3 37 hm3 
 Between 2,650 and 3,150 49 hm3 Exception 
 <2,650 Exception Exception 
(iii) Between 1 April and 30 June 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >3,700 42 hm3 28 hm3 
 Between 2,850 and 3,700 35 hm3 21 hm3 
 Between 2,350 and 2,850 28 hm3 Exception 
 <2,350 Exception Exception 
(iv) Between 1 July and 30 September 
 Total amount of water (hm3) accumulated in the Spanish dams More than 65% precipitation Less than 65% precipitation 
 >3,400 32 hm3 21 hm3 
 Between 2,550 and 3,400 26 hm3 16 hm3 
 Between 2,050 and 2,550 21 hm3 Exception 
 <2,050 Exception Exception 

There is no annual or trimestral minimum flow regime in place for the station of Pomarão under the Amendment, although Article 8 requires such a flow regime to be established after appropriate studies have been conducted. Meanwhile, the Amendment requires a minimum daily flow of 2 m3/s in both this and the Azud de Badajoz stations.

Another major accomplishment of the Amendment with significant implications for the legal regime of the Guadiana River is the inclusion of a provision that requires the states to consider the impacts of climate change on the basins in their future revisions of the flow regime. Article 8 of the Amendment reads:

For the future revisions of the flow regime, the Parties will consider […] the impact of climate change on the field of hydrological planning.’

By this provision, the Amendment prepares the ground for climate change adaptation.

Provisions for flexibility under water supply change in practice

Within the Guadiana River's legal framework, the following provisions can be identified as possible mechanisms capable of improving the flexibility of water allocation schemes in response to climate change-induced variations in water supply: first, the provision of periodic updates of mean precipitation amounts for the establishment of annual flows; second, the provision to update the flow regime of the rivers; and third, the permission to cancel minimum flow requirements under special circumstances. This section is focused on the implementation of these three mechanisms in practice, and lessons learned for their application in other shared rivers.

The provision of periodic updates of historical mean precipitation values for the establishment of annual flows

The Albufeira Convention includes an obligation for the states to update the historical mean values of precipitation registered in certain rain gauge centres. A comparison between these historical mean values and the actual precipitation registered in the same centres will affect the minimum annual flow required to be passed to the downstream state from a station located on the river.

Within the Guadiana Catchment, the annual stream flow at Azud de Badajoz is affected by the precipitation registered in Talavera la Real and Ciudad Real. The mean accumulated precipitation registered at these two rainfall gauge centres between 1 October and 1 March is calculated, assigning 80% and 20% of sampling weight to the precipitation registered at Talavera la Real and Ciudad Real, respectively. The result is compared to the value of the historical mean accumulated precipitation registered in the same stations. If the result is higher than 65% of the historical mean value, the minimum required streamflow at Azud de Badajoz would fall within a higher bracket; otherwise, Portugal will be entitled to a lower amount of streamflow.

It seems to be clear why updating such historical values is considered as a mechanism capable of increasing the flexibility of water allocation schemes in response to implications of climate change. Precipitation is one of the principal climatic factors that control stream flow. Any changes in the amount of rainfall caused by climate change will inevitably alter the water availability. By requiring states to periodically update the mean annual precipitation series, which would ultimately affect their share of water, the Albufeira Convention attempts to capture the change of water availability in each basin.

The implementation of the provision for updating the historic mean value of precipitation has been carried out smoothly during the lifetime of the Albufeira Convention. The first update took place in 2005, and was reflected in the Hydrological Report of Discharge Regimes for the Hydrological Year of 2005–2006. Accordingly, the value of the historical mean accumulated precipitation during the period between 1 October and 1 March of each year changed from 280.3 mm for the period between 1945/46 to 1996/97 to 280.2 mm for the period between 1945/46 to 2000/01. The preparation works for the second CoP in 2008 paved the way for an update ahead of time. The historical period was changed to between 1945/1946 and 2006/2007 and the mean accumulated precipitation amount was amended to 278.4 mm. The third and the final update so far was proposed by Spain during the XVII plenary session in 2013. Following this proposal the historical series period was amended to between 1945/1946 and 2011/2012, reflecting a mean accumulated precipitation amount of 278.3 mm.

The updating of the historical series has so far not led to a drastic change in the amount of precipitation that impacts Portugal's right to the streamflow from Azud de Badajoz. The difference between the historical mean precipitation in the period from 1945/46 to 1996/97 and 1945/46 to 2011/12 is a mere 2 mm. This is the expected result of the way the update is carried out. So far, the revision of the mean accumulated precipitation has been implemented through a 5-year prolongation of the historical series time window. Changes in precipitation amount during such a 5-year extension are cancelled out by the relative length of the whole historical series, resulting in a minor deviation from the original mean accumulated precipitation value of 280.3 mm. Given the stationarity of the historical data on mean precipitation value, such a ‘cancelling out’ effect seems not to be a major problem in the Guadiana basin. In fact, the minor deviations in the value of the historical mean precipitation might be beneficial to the states in that it offers them a comfortable margin to gradually adapt to the impacts of climate change on their access to water. However, in those catchments where the historical precipitation data shows cyclical patterns of multiple dry followed by wet years, updating the historical mean precipitation data by an extension of the time window might be biased. In those basins, the cancelling out effect might dilute the seasonality of changes in precipitation.

The Albufeira Convention itself does not provide a definite guideline for updating the reference precipitation amount. Thus, it is up to the states to decide the details of such a provision's implementation. If the extension of the time window fails to capture the implications of climate change on the precipitation amount in future, for instance due to the appearance of cyclical patterns, states may take an alternative approach to update the historical mean precipitation value. I addressed this issue during an interview with an expert from the Spanish delegation. The expert admitted that in the future, parties may want to revisit their approach to updating the reference precipitation amount, using a translation4 instead of a prolongation method to better reflect the impacts of climate change. However, the expert pointed out that at this moment a translation of the series is not appropriate. The first years of the original series were quite dry, which means that updating the series by a translation results in a higher mean value for the reference precipitation (see Figure 3). In addition, the original series reflects the period 1990–1995, which was the driest period witnessed in the basin so far, and excluding it from the series would shift the amount of the reference mean precipitation to a higher value. Therefore, while the translation approach will benefit Spain at this point, Portugal is unlikely to accept it, as it will negatively affect its share of the river's flow.

Fig. 3.

Evolution of the Guadiana reference precipitation: comparison of methods. Source: Confederación Hidrográfica del Guadiana.

Fig. 3.

Evolution of the Guadiana reference precipitation: comparison of methods. Source: Confederación Hidrográfica del Guadiana.

The requirement to update the flow regime of the river

The Guadiana's legal framework does not prescribe periodic revision of the river's flow regime as a measure to adapt to climate change. However, the fact that it leaves the doors open to future flow regime revisions provides an opportunity to update water allocation arrangements with a view to climate change adaptation.

The Albufeira Convention requires the parties to update the flow regime in three cases: first, in order to replace the provisional regime prescribed in the Additional Protocol; second, for the international branch downstream of the Pomarão station, after a comprehensive analysis of the environmental situation of the river's estuary is available; and third, before approving any new use from the river in its border sections, or the border sections of its tributaries. Under the provisions of the WFD and the Amendment, such revisions must take into account the ecological well-being of the basin, the impacts of climate change and the principles of sustainability.

The CADC was first asked by Portugal to include the revision of the flow regimes in its priorities in 2001. Despite this petition, the issue remained untouched for a couple of years, as the Commission was busy with procedural matters. During the V Plenary Session of the CADC in 2005, Portugal raised the necessity for the revision of the flow regime once again. A workgroup was then formed to follow up on the matter. In the same year, Spain and Portugal agreed to conduct a final review of the results of the study conducted on the environmental situation of the estuary of the Guadiana in order to prepare the ground for updating the flow regime of the river downstream from Pomarão (Minutes of the V Plenary Meeting of the CADC, 2005).

The first CoP also paid special attention to the issue of revising the flow regime of the rivers covered by the Albufeira Convention, and decided to boost the activities of the workgroup formed with this goal. However, the reshuffling of the workgroups under the CADC, proposed in 2006 (Minutes of the VI Plenary Session of the CADC, 2006), temporarily kept the attention of the Commission away from the matter. The question of the Guadiana's flow regime downstream of Pomarão's station also remained unaddressed. In 2007, the issue was raised as Spain called for the adaptation of the operational conclusion of the studies on the environmental situation of the Guadiana's estuary with the provisions of the WFD (Minutes of the VIII Plenary Session of the CADC, 2007). Following this, it was agreed that the revision of the flow regimes must be in accordance with the implications of the Directive.

The CADC activities during the subsequent year of 2008 proved to have significant impacts on the flow regime of the Guadiana. On 18 February, the Commission finalized the proposal for the new flow regime of the rivers covered by the Albufeira Convention, in preparation for the second CoP. The proposal was approved as an Amendment to the Albufeira Convention the next day by the state parties. The Amendment put in place trimestral flow regimes at Azud de Badajoz station on the Guadiana River, complementary to the annual regime in force. However, it kept the regime of Pomarão untouched.

In 2010, the Spanish delegation renewed its call for the establishment of a flow regime for the Pomarão section. The Commission agreed with the petition and put the workgroup of the flow regime in charge. Following this, an agreement was reached that required Spain to propose an annual, trimestral and daily minimum flow at the beginning of the estuary at Pomarão or a substitute station, by taking into account the amount of water available at Portuguese dams and the mean volume of precipitation (Minutes of the XV Plenary Session of the CADC, 2010).

However, in a later session the CADC distanced itself from such an approach. During a meeting held on 19 December 2012, the Spanish delegation voiced concerns over the difficulties of achieving a regime flow for the part of the Guadiana downstream from Portugal. In response, the Commission tasked the Subgroup for the Guadiana basin to consider the establishment of a flow regime for the river (Minutes of the XVI Plenary Session of the CADC, 2012).

Due to little progress with the issue, Spain renewed its calls for revision of the Guadiana's flow regime downstream from Pomarão during the XVII session of the CADC. Portugal responded positively, however, asking for the regime to be elaborated in harmony with the future uses of water in the region, Portugal's national hydrological plans and the multiple uses of the Alqueva dam. Spain replied that the establishment of a flow regime for the Pomarão station is a treaty obligation which thereby prevails over the national plans. As a result, Spain concluded that Portugal's national hydrological plans and multiple uses of the Alqueva dam shall be restricted to the framework established by the future flow regime. Portugal responded that the multiple uses of the Alqueva dam are aimed at gaining maximum efficiency from the hydrological resources available, in accordance with the conditions put forward by the EU for financing the project. Therefore, it suggested that the flow regime be discussed as part of the hydrological plans for the shared basins between the two states. Eventually, the delegations agreed on the creation of a special workgroup to draft a proposal in this respect, to be sent to the Ministry of Environment of both countries for further discussion (Minutes of the XVII Plenary Session of the CADC, 2013). In the subsequent session of the CADC, Spain suggested more intense work on a proposal to put forward to the next CoP (Minutes of the XVIII Plenary Session of the CADC, 2014). As of today, the negotiations over the flow regime at Pomarão station remain on a technical level (Technical Secretariat of the Albufeira Convention, personal communication, 7 July 2017).

How the negotiations over the Guadiana's flow regime unfolded over the years can be perceived as an indication of how water allocation renegotiation provisions can assist adaptation to water supply variability. The success of Spain and Portugal in securing an allocation regime for the station of Azud de Badajoz based on trimestral amounts was, in fact, a significant step towards making the Guadiana's allocation scheme more flexible to inter-annual water supply variability resulting from climate change. Interviews with a member of the Portuguese delegation in the CADC confirmed that a major concern during the negotiations leading to the Amendment was establishing a ‘normal’ and ‘continuous’ flow for Portugal throughout the year. This matter was especially important since it was feared that the increasing inter-annual variability in precipitation and streamflow could translate into an irregular water supply for Portugal.

Nevertheless, it should not be forgotten that despite the history of a close cooperation between Spain and Portugal, and clear obligation set out by the Albufeira Convention for updating the water allocation scheme over the Guadiana, the renegotiation process proved to be a lengthy one, with no achievement for the part of the river downstream of Pomarão. Even for the Azud de Badajoz station, where the two basin states succeeded in putting a trimestral regime in place, negotiations went on for a couple of years before an agreement could be reached. Of course, the impact of international developments leading to new obligations for the basin states on prolonging this negotiation process shall not be ignored.

For the section of the Guadiana downstream of Pomarão, the renegotiation process has been even more challenging. The most significant factor contributing to the prolongation of the negotiations over this part of the river seems to be the conflict of socio-economic interests of the parties. This conclusion is reinforced in interviews with different stakeholders and experts. Respondents from the Portuguese government believe that Spain's demands with respect to the environmental flow for the part of the Guadiana downstream of Pomarão are far too excessive and are most likely meant to satisfy Spain's agricultural needs (Leal, 2013). Respondents from the government of Spain, however, accuse Portugal of deliberately avoiding discussions on the flow regime due to its unwillingness to agree to a water allocation scheme that may restrict its use from the river. This unwillingness is likely reinforced through the media pressure in Portugal that requires the government to negotiate a higher volume of water with Spain in dry years (Leal, 2013).

The complexity of the studies required to achieve a comprehensive analysis of the environmental situation of the river's estuary and to identify factors related to sustainable use from the river is another factor that prolonged the process of renegotiation of the Guadiana's flow regime from the Pomarão station. Interviews conducted by Leal with an external expert on the Guadiana confirm this conclusion. Some members of the Portugal delegation involved in the negotiations have gone so far as to classify the result of estuary studies as ‘inconclusive’. Furthermore, the position of the Spanish delegation during all negotiations over Pomarão's flow regime shows that there is a difference in position between what Spain and Portugal each consider to be an environmental flow. This experience will likely be repeated in the context of other water agreements with renegotiation provisions. This is especially relevant since climate change is expected to affect the factors contributing to sustainability in a basin, and make requirements to satisfy an environmental flow more challenging.

Such a conclusion, however, does not reject the notion that provisions on updating a river flow regime could be used to increase adaptability of water allocation treaties to climate change. In fact, the success of Spain and Portugal in securing a trimestral flow regime for the Guadiana was in itself an important step towards increasing the climate change adaptability of their water allocation scheme, as impacts of climate change are not equally distributed through the hydrological seasons. Furthermore, the deadlock in negotiating a new flow regime, which happened in the case of Pomarão station, can be avoided for instance by tasking a third party, such as an independent river commission, to design a new allocation scheme compatible with the new climate reality.

The involvement of a technical commission in updating a river's flow regime is also useful for facilitating the studies that are required prior to agreeing on a new allocation scheme. Such a technical commission should monitor the status of the river continuously and provide the parties with the information they require to decide on an allocation scheme compatible with the sustainable use of water.

Cancellation of minimum flow requirements under special circumstances

The legal framework applicable to the Guadiana regime explicitly cancels the obligations of Spain to comply with the minimum flow requirements at Azud de Badajoz under exceptional circumstances. Accordingly, when the amount of precipitation and the water accumulated in the Spanish dams fall below a certain threshold, Spain is under no obligation to comply with the minimum flow requirements set under the Albufeira Convention and its Amendment5. The deviation under such circumstances does not lead to a responsibility to compensate. No exception is applicable to minimum required daily flows. So far, no need to apply for such exceptional circumstances at the Guadiana has occurred during the history of the Albufeira Convention. Therefore, the performance of such a provision in practice may not be evaluated at this time.

However, the provision that relieves one state from complying with a minimum flow obligation has been reflected in other important international water agreements, such as the treaty between Mexico and the United States over the Rio Grande. The agreement, known as the 1944 Water Treaty, gives Mexico a margin of 5 years to deviate from its minimum flow obligations under the situation of an ‘extraordinary drought’. Nevertheless, during the preceding 5-year cycle Mexico has to compensate for the deficiency from its tributaries6. The International Boundary and Water Commission (IBWC), which is the commission responsible for the application of the 1944 Treaty, has also authorized Mexico to repay its water debt through a transfer of its stored water in international reservoirs7.

The approach taken in the 1944 Treaty seems to be a more reasonable one than that used in the Albufeira Convention, as the cancellation of a minimum flow requirement without prescribing a compensation measure would aggravate the harm suffered by the state whose right to the flow has been cancelled. This is especially true as, with climate change, the frequency of extreme water events, including drought, is expected to increase.

Conclusion

Global climate change poses a major challenge to water allocation by altering water supply patterns. For countries whose freshwater resources lie completely within their own territory, adapting to such increasing variability is difficult enough. Such a hurdle becomes more pressing when freshwater resources cross borders and implicate multiple sovereign actors with conflicting interests. When such transboundary resources are governed by a treaty that specifies the water allocation arrangements, an important challenge is to accommodate the changing water supply within the rigidity of such a legal instrument.

The legal framework governing over the Guadiana River contains some provisions that can increase the flexibility of a treaty to respond to possible changes in water supply. These provisions include the requirement for periodic update of the historical mean precipitation values, the requirement to update the flow regime of the rivers, and cancellation of minimum flow requirements under special circumstances.

This paper has analysed the merits of each of these provisions and challenges faced for their implementation in the Guadiana basin, and explained the lessons learned for their application in other catchments.

The periodic update of historical mean precipitation values seems to be an effective way of increasing climate change flexibility of water allocation agreements in the basins that rest their allocation scheme on historical precipitation series. Such revision will partially reflect the impacts of climate change on water supply. Furthermore, states seem to be less reluctant to accept it, as it is based on objective and measurable evidence. However, the method of conducting such updates must be adapted to the special characteristics of the catchment in question.

Keeping the doors open to water allocation renegotiation can increase climate change adaptability of water allocation schemes, as was seen in the case of the trimestral regime for Azud de Badajoz. However, as climate change intensifies, and the conflict of interest of the riparian states regarding their shared basin increases, coming to an agreement over a new water allocation arrangement can be time-consuming and unfeasible. Empowering a joint institution when deciding on the new allocation arrangement is a proposed solution to such a challenge.

Finally, the cancellation of minimum flow requirements under special circumstances may be used as a temporal measure to show flexibility in response to changes in water supply due to climate change. However, its recurrent application will inevitably harm those states whose right to minimum flow has been cancelled, especially if it is not combined with a compensation arrangement for when the special circumstances cease to exist.

1

For the purpose of this paper winter precipitation should be understood as the precipitation falling between 1 October of each year and the beginning of March of the next year.

2

Ponte de Muge is a station located on the Tajo River.

3

The weekly minimum flow regime was only established for Duero and Tajo basins.

4

Meaning changing the beginning period of the historical series.

5

However, it is still obliged to make sure the water is managed so that there is enough water for the use of the population and other social uses, as well as to keep the ecological balance of the river and its estuary intact.

6

Utilization of Waters of the Colorado and Tijuana Rivers and of The Rio Grande (US-Mexico) (3 February 1944) Treaty Series 944.

7

IBWC Minute (2 December 1969) 234.

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