Tagus is the largest river basin in the Iberian Peninsula. Its Spanish section covers an area of 55,781 km2 and hosts a population of 7.2 million inhabitants highly concentrated in Greater Madrid (TRBA, 2013). Average consumptive water use amounts to 2,893 million m3/year (ibid.) with an average natural renewable resource availability of 10,214 million m3/y (1940/41 – 2005/06). Despite significant local scarcity challenges (mainly in the Upper Tagus) and high variability in water resources, severe scarcity is not pervasive and drought vulnerability is still relatively moderate in the watershed.
By contrast, the Segura river basin with a smaller area (19,025 km2) and lower population (1.98 million inhabitants in 2010, increasing in peak seasons up to 2.1 million, depending on tourist inflows) is experiencing a growing demand for water. In 2010, water demand hit 1,760 million m3 per annum (SRBA, 2013), while average renewable rainfall and runoff over the last 40 years is estimated to be only 848 million m3/year (704 million m3/year when considering the period 1980-2006) (ibid.). Water demand composition is mainly driven by the agricultural sector, accounting for around 85 per cent; the remaining part is mainly determined by domestic consumption (10 per cent of the total). Industrial demand for water in the Segura basin is almost negligible (Calatrava and Martínez-Granados, 2012).
Vulnerability to water scarcity and drought has increased due to the joint effect of three driving factors: a large inter and intra-annual variability in the precipitation patterns with implications for water-dependent economic and social development; powerful incentives in the economy leading to increased water use in the short term; and the relative failure to implement public policy responses to water scarcity, with the implications that the overall objectives of water policy have not been met.
Water scarcity in the resource scarce Segura basin is partially compensated by transferring water from relatively more abundant watersheds such as the Tagus basin via water transfer infrastructure that has been in operation since 1978. These water transfers have only been operated to provide a fraction of the planned capacity of 600 million m3 per year. The remaining deficit is mostly covered through the overexploitation of groundwater sources (a buffer stock), sometimes via illegal abstraction, re-used water, and alternative resources from desalination. It has been estimated that, even considering the water transferred from the Tagus basin, the water demand in the Segura basin exceeds the availability of renewable water resources by around 370 million m3/year. This deficit is mainly covered through non-renewable deep groundwater withdrawals.
Several extremely intense drought periods have affected both basins in the past two decades. The persistent water scarcity conditions of the recent past are projected to increase in severity if the climate change driven negative trends of rainfall and runoff continue in the future. Both the fourth and fifth Assessment Reports of the Intergovernmental Panel on Climate Change classified the southern part of Spain among the portions of Europe most likely to be severely impacted by a radical change in precipitation. In addition, the extreme variability of natural supply along with lower ability of water infrastructures and aquifers to store water and stabilize water supply, have severely reduced the robustness and resiliency of the entire water supply system and increased both the likelihood and the severity of droughts (Gómez and Pérez, 2012). By contrast, the economic value generated by the water-dependent agriculture has increased, consistently increasing the demand for the scarce water resources.
Current trends in water use can only be tackled and reversed by putting in place an appropriate set of incentives. Economic incentives will need to be central elements of any water policy alternative that focuses more on the so-called soft options of water management rather than just on hard (physical capital) solutions. Once the potential for additional infrastructural measures has been fully explored, as in the Segura case, available alternatives to match water supply and demand, to reduce water scarcity, to enhance drought resilience and to improve water security must be found. These could be comprised of a combination of new alternatives such as water demand management, an increase in the technical efficiency with which water is applied to any economic use, and/or the development of non-conventional sources such as regenerated or desalinated water.