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La Concepción reservoir near Marbella started the year at a very low level (file image). Josele
Evaporation means reservoirs in the province can lose as much water as the entire population of Malaga city consumes in six months
Drought crisis

Evaporation means reservoirs in the province can lose as much water as the entire population of Malaga city consumes in six months

There are no clear solutions to curb the problem when large bodies of water are stored, but there are some experiments taking place around the world using chemicals

Chus Heredia

Malaga

Monday, 27 May 2024, 15:10

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Malaga province's reservoirs can lose more than 30 cubic hectometres of water through evaporation in average rainfall years, according to data from the Junta de Andalucía's last drought committee.

This is not a fixed amount, but depends on multiple variables such as the amount of rainfall (the greater the volume, the greater the evaporation) and temperatures. The surface of the exposed sheet of water also counts.

As an idea, the maximum volume evaporated can supply Malaga city for more than six months. To put it another way: this is the water that a population of 450,000 inhabitants drinks per year. The situation is worsening, with Malaga closing 2023 as the second warmest and sixth driest year in history, since records started in 1961. This year is projected to be slightly wetter, but also warmer.

Viñuela, up to 11 hm3 per year if it rains, but 2.41 with drought

"Evaporation depends on several factors: mirror surface, temperatures, hours of sunshine and whether or not there is rain. The mirror surface is the surface area at the reservoir level. The greater the capacity of a reservoir, with some exceptions, the greater the mirror surface and, therefore, the greater the evaporation," sources told SUR. Even atmospheric pressure has an influence, as pointed out in one of the previous drought plans for the Mediterranean basins drawn up by the Junta, which put the evaporation of La Viñuela, the reservoir in La Axarquía, at around 11 hm3 per year in a normal year of rainfall. In a dry one like last year only 2.41 hm3 evaporated.

Up to 20 hm3 lost in the Guadalhorce system

Technicians of the Guadalhorce-Limonero system said that with 64 hm3 of rainfall contributed to the reservoirs, equivalent to a normal year without drought, 20 hm3 evaporates. With half the rainfall, evaporation is 15. With a quarter, it drops to 10 and, with drought, around 5 cubic hectometres. For example, in a particularly dry period between October and January last year, the estimated evaporation was 2.05 hm3.

Evaporation is one of the factors in what is called in technical jargon, the law of the reservoir, which is volume supplied minus ecological and maintenance flows (cleaning of drains, bottoms, sludge...) minus consumption and minus evaporation. This gives the final volume of the reservoir. For example, in the whole of the last hydrological year, 10.9 hm3 were released in the system that supplies Malaga to maintain the established ecological flow, and for maintenance and security.

La Concepción records the lowest losses

In the La Concepción reservoir , between Marbella and Istán, the dimensions are smaller. And, therefore, the evaporation is lower. In a year of average rainfall and inflows, technicians estimate it to be around 2.5 hm3.

Scientists are increasingly concerned about evaporation. For example, according to the 2021 drought plan for the Mediterranean basins, "more than 5% of the water that enters the Strait of Gibraltar evaporates due to its closed basin nature".

A worrying trend

Dr. Enrique Salvo, professor of botany and expert in climate change said global warming is happening very fast. And it is not only the contrast between the temperature of the water and that of the air, but also the subsoil is getting warmer. In the 2005 drought plan, he pointed out, the concept of evaporation was already introduced for the classification of severe water shortages.

For Salvo, the area around the Alboran Sea is one of the areas of the planet that is warming the most. "It's not only in cities, but also when we measure outside them. Not to mention an open water surface. It is clear that human consumption has an impact, but the spectacular drops we are seeing are also due to evapotranspiration," he said.

In certain national latitudes, this effect is very noticeable and can produce daily drops of 1cm in the level of reservoirs subjected to extreme temperatures and wind, which is another of the components to be taken into account. In the Guadiana basin, for example, some technicians have put the annual losses at 230 hm3, the reservoir capacity amounts to 9,538 hm3, 15 times more than Malaga province. In the Júcar basin, losses of 0.3 hectometres per day have been recorded on days of maximum heat.

The Penman equation

This phenomenon has specific mathematical formulae. The most widespread is the Penman equation, which includes variables such as net radiation, wind speed at two metres above the surface, air temperature at a height of two metres, vapour pressure deficit, curve slope and ground heat flux.

What is the surface area of the reservoirs in Malaga? The Guadalhorce-Guadalteba system totals 780 hectares; La Viñuela has 546; Conde de Guadalhorce, 546; La Concepción has 214; Casasola, 112; and Limonero, 105.

The reservoirs in Malaga gained 73 cubic hectometres with the rains in March, reaching more than 170 hm3 in storage. However, they have been plummeting for ten days and now stand at 167, barely 27% of their capacity.

Solutions

At the moment, there are no clear solutions to curb evaporation in large bodies of water. There are some experiments taking place worldwide using chemical products. There is even the possibility of partial covers such as the one proposed by Trops and the University of Malaga in their project to build a desalination plant in Axarquia. The plant would be supplied with energy by floating solar panels on La Viñuela reservoir. The panels on platforms would help to reduce the insolation on the water surface and, therefore, evaporation.

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