Until a few months ago, drought was a problem that affected almost the entire country, even to the greatest northern areas that saw an important reduction in rainfall and even the introduction of temporary water savings measures. Now the situation has changed, although the threat is still in force in some areas of the country, especially along the Mediterranean coast.
Odiel, red and stones, the exception. However, if we look at one of the maps that show us the Situation of Basin Reservoirs Hydrographic, perhaps otherwise what caught our attention. Surrounded by the great hydrographic basins of Guadiana and Guadalquivir, the Odiel basin, red and stones seems to show us a humid anomaly in the south of the Peninsula.
The swamps of the basin are, according to the Last data availableat 94.3% of its capacity. This makes it the second largest hydrographic basin, behind the Internal Basins of the Basque Country (95.2%) and ahead of basins such as Galicia coast (87.6%) and the Eastern Cantabrian (83.6%).
A figure that contrasts in a striking way with the state of the reservoirs in the hydrographic basins of its surroundings: 48.4% in the Pantans of the Guadiana, 40.5% in those of the Guadalquivir, and 30.5% in Guadalete-Barbate. What happens in this little basin for?
Statistical issue? The first thing we can consider is statistics. The Hydrographic Basin of Los Ríos Odiel, Tinto and Piedras is a small basin, both in extension and capacity: the swamps of this Huelva basin can store a maximum of 229 cubic hectometers (HM³).
Surrounded by greater extension demarcations, such as those of the Guadiana and the Guadalquivir, the most similar basin in extension in the south would be that of the Guadalete-Barbate. However, the Cadiz basin has A capacity of 1,651 hm³.
This fact could justify the anomaly due to pure statistics: less capacity would imply greater ease for filling and emptying, more variability and with it more likely that at some time of time extreme values are achieved. The problem is that the reservoirs of this area have been maintained rather persistent as the most full of the south.
Rainfall. One of the reasons why the Odiel, Tinto and Piedras Rivers Basin attracts attention is due to the general contrast with the southern basins of the Peninsula, which in turn is associated with the fact that the South receives less rainfall than the north. Although that is true if we compare the north third with the rest of the country, the truth is that the image It is more complexand Andalusia is a good example of this.
In Andalusia it exists A marked difference in rainfall between The Eastern Zone and the rest of the Autonomous Community, especially in the West. The province of Huelva is usually among those that receive the most water, being the north of the province one of the regions that record the most rainfall.
To that we must add that 2024 was A slightly more wet year than the average in almost the entire province of Huelva. The beginning of 2025 has exacerbated this trend, with almost the entire province receiving more than double water than average During the month of January.
The “dead” reservoir. The reservoirs of the Odiel basin, red and stones receive a lot of water, yes, but also others in Western Andalusia. To explain the phenomenon of this basin, perhaps we also have to attend to emptying. And here we find another determining factor: the reservoirs of this basin do not empty the same ease.
The reason is pollution. This basin has a peculiarity, that of reservoirs useless for the contamination of its waters. To illustrate it, the El Sancho reservoir, The “Dead Reservoir” of the Odiel Basin. The waters of this reservoir are extremely acidic, with a pH value close to 3.6, to which high concentrations of toxic metals must be added, as explained In an article for The conversation Manuel Olías and José Miguel Nieto, both experts from the University of Huelva.
The reservoir is in a situation of Perennial filling at 82.76%contributing 48 hm³ of waters to the accounts of the reservoirs of a basin that add up to a capacity, remember, of 216 hm³. The problem Nor is it exclusive of the El Sancho reservoir.
Complementary explanations. None of the three explanations (size, rainfall and pollution) explains in itself the state of the reservoirs in this basin, but as a whole they can explain the anomaly of this basin and the associated problem.
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