How long can we continue using water from reservoirs that are almost empty? This is the current situation with La Viñuela and La Concepción in Malaga province

The severe drought on the Costa del Sol and the Axarquia, is expected to hit Malaga province hard in the coming weeks as the weather warms up, means water savings from the reservoir must be sought and extra resources from desalination and acquifers must be seized.

These are times of emergency measures, where pumps are trying to extract water of a reasonable quality from the La Viñuela reservoir, which has just 12.27 cubic hectometres left. But how long can water from a reservoir continue to be used, and down to what level can it stop being used?

Fernando Delgado, professor of hydraulic engineering at the University of Granada, said: "Each reservoir is different. The geology has a great deal of influence. In La Viñuela, the basin is erodible and with much less vegetation and it is more difficult to take more volume than in more rocky beds".

"For example, the Rules reservoir (Granada province) has a large part declared 'dead' because it was thought that the basin was very erosive. There is no single answer. But what does happen, at very low levels, is that there is a lot of sludge content, heavy metals increase, there is much less oxygen and a lot of organic matter," Delgado added.

Water treatment plants

Delgado pointed out that dealing with the water in treatment plants is complicated. "Under normal conditions, drinking water treatment is a physico-chemical process in which, to summarise, it is then passed through sand filters. And then it is chlorinated. But if the water has a lot of organic matter in it, you have to use other treatments," he said.

"The reservoirs, regardless of whether they are made of loose or construction materials (masonry and concrete, mainly) have water intakes for supply and other uses. In the past, the part of the reservoir that could not be used by the intakes was referred to as a dead reservoir. But, in almost all the reservoirs, the bottom outlet has been connected to the outlet for supply and/or other uses," technical sources told SUR, who added that in most cases a pumping station has to be installed to raise the flows to the dam reservoir level.

Dead reservoir

La Viñuela, precisely, is in what is called a 'dead' reservoir status, which has become an outdated concept. "They are not dead at all, they tend to have more sediment and a lower water quality, normally with a greater quantity of iron and manganese ions, but that is what the water purification plants are for," Delgado said. Depending on the reservoir, the last cubic hectometres can be used up, he added.

Oxygen is falling

At very low levels, the quality of the water worsens considerably as the oxygen concentration drops. Often, it is necessary to remove the fish and relocate the fauna. Another authoritative source, also closely linked to water management on the Costa del Sol, pointed out that at around 5% capacity there is already more sludge than water, and before reaching this situation it is advisable to use floating pumps to "catch the little water that can be used". The source believed that the pumping system had been installed late in La Viñuela. More time in advance would have led to better use of the water, they said, while adding that La Concepción had been left virtually empty on two occasions over the past 40 years.

Regarding the treatment of poor quality water, the source added that the decantation processes slow down (despite the use of specific reagents) in the water treatment plants and that more reagents must be added to the process, but he does not consider that it is a crazy increase in costs. At this point, he emphasised that in Spain treatment is mainly with chlorine, given the health regulations in this regard. "In other countries like France, ozone is used a lot. It produces a very good result, but there the supply networks are very short, in addition to having intermediate treatment points. "Ozone disinfects very well but it disperses over a long distance," he concluded.