Reduction of Tagus-Segura Water Transfer volumes – an environmental and socioeconomic impact to be borne in mind

2021 is undoubtedly a key year for the operation of the Tagus-Segura Water Transfer. Firstly, because of the proposal by the Ministry for Ecological Transition and Demographic Challenge to modify its current operating rules, and mainly because of the proposed increase in ecological flows set out in the new Tagus River Basin Management Plan.

The modification of the operating rules of the Tagus-Segura Water Transfer generates uncertainty and rejection on the part of users, given that the intention is to change a rule only six years after its entry into force. Moreover, the intention is to modify it again within a short period of time, once the new Tagus River Basin Management Plan has been approved. From a technical point of view, the new rule is ineffective, as it fails to provide more resources and will lead to a reduction of up to 32% in the volumes that can be transferred in years considered to be very good for irrigation. It is estimated that average annual evaporation losses of water stored in the reservoirs at the headwaters of the Tagus will increase by 13 hm3, a volume which will be cut from the Tagus-Segura Water Transfer.

But the great challenge facing the Tagus-Segura Water Transfer this year will certainly be the change of the legal minimum flows (Act 52/1980) of 6 m3/s in Aranjuez and 10 m3/s in Toledo and Talavera to the minimum ecological flows proposed in the Scheme of Important Issues of the new Tagus River Basin Management Plan. The minimum flows set out in this Plan are 8.52 m3/s in Aranjuez, 17.00 m3/s in Toledo and 17.80 m3/s in Talavera. These values are much higher than those proposed in the studies carried out to characterise the minimum ecological flows in the Aranjuez-Talavera de la Reina section prepared by Professor Luis Garrote of the Universidad Politécnica de Madrid and biologist Dr. Fernando José Mariño. The conclusions of this analysis emphasise that there are no reasons to modify the minimum flow value of 6 m3/s currently applied in the Bolarque-Aranjuez section, given that it is a value much higher than those obtained with the habitat simulation methods, which are the methods established in the guidelines for the drafting of River Basin Management Plans. Moreover, it should be emphasised that in many periods, current legal minimum flows are guaranteed artificially to an excessive extent by the reservoirs at the headwaters of the Tagus, given that they would not be guaranteed naturally. As stated in the 2015-2021 River Basin Management Plan for the Spanish section of the Tagus, the status of the bodies of water that could be affected by the Transfer can generally be considered to be good. Therefore, from an environmental point of view, this potential increase in current minimum flows does not appear to be justified. If this increase were to be implemented, estimates indicate that the average annual volume transferred at source would be reduced from 314 hm3 to approximately 234.5 hm3, 39% of the maximum permitted transfer volume. An average annual volume of just 144.5 hm3 would remain for irrigation, 34% of the maximum envisaged for this use. All of this would have a major environmental and socioeconomic impact in south-eastern Spain.

In this respect, the current Segura River Basin Management Plan highlights the decisive role played by water from the Tagus-Segura Water Transfer in achieving the environmental objectives of this river basin district. It estimates the average annual deficit of this basin at 400 hm³/year, bearing in mind the historical average annual contribution of the Tagus-Segura Water Transfer. In addition, it points out that in order to satisfy the demands of this basin, water transfers are essential, as meeting the demands of the district, including urban demand, is unsustainable with only the resources of the Segura itself. The Plan also points out that reuse of treated wastewater, upgrading of irrigation systems and the use of desalinated resources already feature in the current River Basin Management Plan and, therefore, do not enable the deficit of the river basin district to be reduced beyond the aforementioned volume.

Water from the Tagus-Segura Water Transfer is a basic pillar of irrigation in the Levante area. It is essential in qualitative terms, because of its higher quality. The problems associated with the use of groundwater with high salinity and non-conventional resources (high cost, phytotoxic ions, high emissions) are mitigated and practically disappear through its combined use with water of these characteristics. This results in an increase in agricultural output, a decrease in the risk of leaching and consequently of diffuse pollution, and a reduction in the risk of soil salinisation.

Therefore, desalination cannot be considered a substitute for water from the Tagus-Segura Water Transfer, but rather a complementary resource. In this sense, the combined exploitation of desalinated water with all the available resources, and particularly blending it with water from the Tagus-Segura Water Transfer, resolves all the agronomic and management problems related to the application of desalinated seawater for agricultural irrigation, as demonstrated in the “Report on the background and problems of the application of desalinated seawater for agricultural irrigation” (“Informe Antecedentes y problemática de la aplicación de agua marina desalinizada al riego agrícola”), drawn up by Victoriano Martínez and Bernardo Martín, Professors at the Universidad Politécnica de Cartagena. With respect to climate change, it should be noted that agricultural crops, due to their photosynthetic capacity, act as greenhouse gas sinks. According to research on the carbon balance of the irrigable areas of the Tagus-Segura Water Transfer, carried out by researchers from the Universidad Politécnica de Cartagena, the progressive replacement of water from the Tagus-Segura Water Transfer with desalinated seawater would reduce the capacity of these irrigated areas to act as carbon sinks by 30%.


Mariano Soto García
Coordinator of the Transfer and Sustainability Chair José Manuel Claver Valderas

Published in: Nº77 FuturENVIRO February – March 2021