Embalse del Cenajo - Cenajo Reservoir

In our travels through Spain we have occasionally come across a dam with its reservoir. During our visit to Murcia in early 2018 we decided to take a drive up to the Embalse del Cenajo (the Cenajo Reservoir), a drive of about 75 minutes from the city.

Spain - Dams and Reservoirs 

Before describing our visit to this particular reservoir I am taking this opportunity to look more carefully at the history (both distant and recent) of dams and reservoirs in Spain, starting with the Romans.  

It has been said that Spain has a history of dam construction that is more diversified, more continuous and more important than any other country. The dams and reservoirs are so diversified that they demonstrate the structural evolution of technical knowledge and empirical rules that were employed in the past. In addition in Spain there are strong regional differences (topography and geology) in which differing techniques were able to evolve more or less independently.

The Roman reservoirs of Proserpina (below) and Cornalvo were built in the 1st C BC and are today a World Heritage Site. Together they are part of the so-called Conjunto arqueológico de Mérida, which includes a teatro romano, an anfiteatro, a circo romano, a Puente romano, and the acueducto de los Milagros (aqueduct of miracles). The Proserpina reservoir covers some 72 hectares, has a perimeter path of about 6 km, and collects both rainwater and waters from two streams, Las Adelfas and Las Pardillas. The reservoir capacity is 4 hm3 and the dike (dam), made of regular granite blocks, is 428 m long and 21 m at its maximum height. During cleaning in 1991 they discovered that the original Roman dike was a buttressed barrier some 6 m high, and that what we see today is an extension from the 2nd C AD.


The nearby Cornalvo reservoir is somewhat bigger with a capacity of about 11 hm3, and dates from 130 AD. This dike is about 22 m long and 18 m at its maximum height. Both these reservoirs are still in use.

I have used the word dike as a description of an artificial slope or wall used to regulate water levels, but the technical term for these Roman constructions would be a kind of rock-filled gravity dam, where the weight of the stone masonry in the dam is used to hold back the horizontal pressure of the water pushing against it.

The Roman dam of Almonacid de la Cuba (below), also built in the 1st C AD, shows us how the Romans actually went about building such dams. It would have been built in three phases. The first would have been some short curved walls with vertical buttresses facing downstream. Upstream the dam face would have been flat. Then the downstream core of the dam would be made up of strong mortar (opus caementicium) mixed with small irregular stones. And finally it would be covered firstly by small stone blocks set is courses (ashlar), and then covered in larger cube-shaped blocks. The upstream wall would be a plane surface. In the 2nd C AD this dam was raised by reinforcing the downstream face with a terrace of steps, using the same technique, opus caementicium and stone blocks. Later still the buttresses were reinforce and a terrace of steps built also upstream, so the dam finally looked very similar upstream and downstream. The result was a dam 120 m long, 27 m wide, and 34 m at it highest (reservoir capacity 6 hm3). It is the highest dam known to have been built by the Romans.


However, we must also mention that this Roman dam of Almonacid de la Cuba confirmed the way the early curved walls evolved into a dam of buttresses and steps. This particular dam provided both irrigation water and urban water with an aqueduct. The aqueduct was about 0.65 m by 0.75 m cut directly into the rock and covered with stone slabs. Experts have estimated that this reservoir provided irrigation to around 7,000 hectares of agricultural land. The waters from this dam were diverted in 1787 to mills for both cereal and for ‘fulling’ (i.e. the washing of wool in the process of woollen cloth making). 


This Roman dam of Consuegra (1st C AD) was originally used for irrigation around the Amarguillo River (and was used until 1891). The remains suggest that it would have been a straight wall some 630 m long, nearly 5 m high, and constructed of concrete and quartzite masonry. 


Another impressive Roman dam, now in ruins, is that of Alcantarilla (dated to about 25 AD), which provided sewer water to Toledo. This is often quoted (incorrectly) as being the oldest Roman dam in Spain, and some reports claim that it was more than 500 m long, with a height of 17 m. 

Historians suggest that the Romans learned about dams and reservoirs from those built by the Egyptians ca. 2600 BC. It would appear that the Romans used those techniques learned from the Egyptians in building dams in Spain. Today there’re about 80 Romans dams still documented, of which 20 are in Spain.  

Dams were also built during the period of Arab domination, but none survive that exceed the capacity of the dams built by the Romans. The Azud Mayor or Contraparada dam was built between 900 and 1000 AD to divert the waters of the Segura River into the fields around Murcia. Azud is an Arabic word meaning ‘barrier’ and would best be translated as weir. The story goes that the Contraparada was built to control (or calm down) the violent flooding of the Segura River that would otherwise have destroyed the newly built city of Murcia. The Contraparada fills two main ‘acequias’ (watercourses), the Acequia Mayor Aljufía for the North area of the ‘Huerta’ (gardens), and Acequia Mayor Alquibla, for the South. It has been suggested that on the site was originally a Roman dam. The approach taken for this ‘new’ dam was to use dovetailed limestone blocks with lead protected edges (later concrete was added to stop the water eroding the stone).


By the 13th C the dam was in ruins and flooding again had become a major problem. Alfonso XI ordered the reconstruction of the dam, which in 1621 was reported to have been the largest and most expensive piece of stone and lime in Spain. This clearly was not enough since a flood in 1651 destroyed the dam, and Felipe IV again ordered its reconstruction. Interestingly during the War of the Spanish Succession (1701-1714) there was a battle called the Batalla del Huerto de las Bombas (1706). An Anglo-Dutch regiment of 6,000 infantrymen planned to invade Murcia using portable wooden bridges to cross the irrigation ditches. Cardinal Belluga (1662-1743) defended the city of Murcia by ordering the opening of the sluice gates of the dam. The flooding of the fields rendered the attack impracticable, and the forces were withdrawn.   


In 1578 (post-medieval) worked started on the Almanseño reservoir using a dam of a completely different design. Shaped as a curved wall (‘arch dam’) with a retaining wall in the form of 16 steps. The height was 20 m, the base was 16 m wide and the last upper step was 3 m wide. Construction was completed in 1584 and the reservoirs capacity was estimated at 3 hm3. Apparently it was extended in the 18th C, but then by the early 20th C it had become completely silted up. One-third of the reservoir capacity was sludge, probably due to the fact that the reservoir actually drained a large swamp area. A study in 1907 looked at the potential to re-exploit the reservoir. Purely from a technical viewpoint the reservoir could support a flow of 7 million m3 per year, which certainly justified its re-exploitation. There was a bit of a fight between the Almanseños (from Almansa) and the Alperinos (from Alpera) for the use of the water for cereal irrigation. Documents show that the Almanseños won the legal battle, but in reality the Alperinos actually won because they were physically nearer. About 600,000 m3 of sediment and sludge had to be removed. I will not delve into the depths of this major renovation project, but if you are interested check out the excellent description here


The Tibi reservoir (above) was at the time of its construction (1580-1594) the most important dam under construction in Europe, and its height of 46 m made it the most important dam in the world. This was a gravity dam with a length of 65 m (107 m at the bottom) creating a reservoir of 2 hm3. Innovations included the inclusion of intakes at different heights, and a bottom drainage channel to allow cleaning of deposits from the drained swamp land.

Both the Almanseño and Tibi reservoirs are still in service, making them some of the oldest reservoirs and dams still in use in the world. 

A famous and well documented example was the building of the Ontigola dam by Filipe II. This dam was built between 1563 and 1574 in the historical setting of Aranjuez (one of los Reales Sitios), and its actual construction was closely followed by the king.



The Elche reservoir (above) is retained by a gravity dam in the shape of an arch, and was built in the 17th C, it is often considered to have been the first arch dam built in Europe since Romans times. It was originally built to exploit the waters of the Vinalopó River, and to use them for irrigation. In 1995 the regulator gate burst, causing a large avalanche of sludge that contributed to emptying the dam. The dam has now been repaired and in March 2018 it will begin to again retain water. 

In the 17th C Spanish dam building was superior to all other civilisations. A Spaniard named Don Pedro Bernardo Villarreal de Berriz (see his biography from 2013 here) wrote the first book on designing dams in 1736. In Don Pedro's time only two types of dams were built, arch dams for narrow gaps where the foundations had good solid rocks or gravity dams when the site was wide and shallow. Don Pedro's book suggested how to design dams properly and introduced new ideas such as a multiple arch dam. Don Pedro suggested that multiple arch dams would need artificial supports or buttresses to support the arches. This theory indirectly led to the invention of the buttress dam.

The Spanish brought the art of dam building from Spain to the Americas. The idea of buttress dams was current in Spain, so many small buttress dams were built throughout the continent for irrigation purposes.

An intriguing insight into 19th C dam building is the so-called presa antigue de Zalamea de la Serena (above). This dam was built between 1780 and 1812 by the Marquis de Casa Mena y Las Matas to supply his houses. The dam is 113 m long, 17 m high and the reservoir has a capacity of 5 hm3. The dam housed a waterwheel, and on top a small chapel. 

Through to the 1800’s dams in Spain were usually of modest dimensions, and intended for providing domestic water supply, crop irrigation and some energy production. Over the last 200 years the size and number of large storage capacity reservoirs has substantially increased. The construction of dams in Spain steadily increased during the 20th C. During 1900 to 1950 about 4 dams were commission each year, but through to 1975 this figure doubled, and doubled again through to 1990. Today there are 1172 operating large dams in Spain (out of about 7000 large dams in Europe). Counting those dams under construction or in planning, this number will rise to 1230 in the foreseeable future. Below we have the modern example of the Aldeadávila dam on the Douro River near Salamanca.  

Wikipedia’s list of dams and reservoirs in Spain is incomplete, but the Sociedad Española de Presas y Embalses maintains a list of reservoirs in Spain.  

We have two topics still to review on this page. The first is our actual visit to the Embalse del Cenajo, and secondly we must look at the issue of drought in Spain, and in particular in Murcia. 

Visit to Embalse del Cenajo

Our visit to the Cenajo reservoir started with a 75 min. drive from Murcia (our return trip was via Hellin and Archena).

As we drove inland the scenery became a mix of semi-arid areas of gypsiferous soil (some protected natural habitats) and agricultural land. This type of calcium sulphate rich soil interferes with plant growth and often occurs in arid and semi-arid regions. In Europe gypsiferous soil is found predominately in Spain, and to a lesser degree in Turkey. Farms (so-called dry farming) are increasingly moving in to this region which is both cooler in summer and nearer highland water resources. The down side is that in winter it can be quite cold, and occasional freeze. Farmers focus more on cereal crops and small-grain legumes, but they are allowed to use more fertilisers than near costal waters. Here is a review of farming with gypsiferous soils. You can also see quite a variety of fruit trees, since apricots, peaches, pears, olives and grapes are known to support such soils. Very noticeable is the presence of pine trees that are known to support gypsiferous soil and helps breakup the gypsic subsoil.


Finally we arrived at the dam, which is just a ‘small’ construction blocking this entire mass of water. You can just about see the road over the top of the dam at the far-right of the reservoir.   

Below we can see that standing on the dam only a very small portion of the reservoir is visible. The white line drawn through the reservoir is in fact the frontier between the region of Murcia and that of Castile-La Mancha. 

When we finally arrived the reality was somewhat different as we saw that the reservoir was almost empty. Ten years ago the reservoir was 51.5% full (on the 5 Feb. 2008), on the same day last year it was 28.6% full, and the same day this year the reservoir was only 7.8% full. 

It is all the more shocking when we know that in years such as 1962 and 1973 the reservoir was overfull, and water was pouring uncontrolled over the top of the dam. 


We will return to the problem of drought in the region later, for now we will look in some more detail at this particular reservoir and dam. 

Everything starts with the Río Segura (Segura River). Someone once called it a ‘modest’ river, 350 km long and with a flow rate of less then 20 m3/second. It flows from its birth in the Sierra de Segura in the province of Jaén through Murcia to the river’s mouth in Guardamar del Segura near Alicante. The river has always been vital to the economic and social activity of the region. It irrigates around 65,000 hectares of the most productive land in Spain and provides work to more than 20,000 farmers. And with its different pumping stations it provided domestic water to many towns and villages. So it is not surprising that the use of the water, and the damage caused by flooding, have been a historical constant for all those people living near the river. In fact about 45% of the water flowing down the Segura comes in February-March-April when the snow melts in the upper mountains. Only about 13% of the water flows down the river in the months between June and September. In the past flooding could occur at almost any moment - April, September or October. In the period June to September the orchards need at least 100 million m3 of water, but received less than 50 million m3 from the river (thus justifying the reservoir and dam). Between 1875 and 1917 the orchards were destroyed by flooding eight times (another reason for the dam). Someone wrote at the time that the Segura was both a providence and a calamity. This report looks into some detail on the impact of the Segura has had on the orchards in the Murcia region. 

Already in 1886 there were plans to build a series of small weirs and dikes to negate the effects of flooding. It was in 1926 when plans were first put forward for the creation of the Camarilles and Cenajo reservoirs, which would both channel the water and stop the regular flooding. Some work was completed, but it did nothing to stop the serious floods of September 1943, April 1946, September 1947, and October 1948. For example in April 1946 Murcia received more than 1,000 m3/second, 50 times the maximum flood warning level for the city. 

But it was only in 1942 that it was finally decided to construct a dam at Cenajo for electricity production and for channeling the water for irrigation. The final contract was awarded in 1946 for a total cost of just over 62 million pesetas (the total cost of the reservoir and dam were estimated at 450 million pesetas). Through to the dams completion in 1960 there were 10 different sets of modifications to the project (the dam was only inaugurated in June 1963). Delays and interruptions in the work are thought to be the reason for cracks and leaks in the dam wall. 


The official story is that it required 7,700 workers, and that a small village was built near the dam for the workers (150 family homes and 1,000 individual bedrooms). Mention is made of the fact that prisoners did work on the dam in exchange for a reduction in their sentences (one day worked was worth a 3 day reduction in their sentences). 

The reality is that great hydraulic projects were important to Francoist Spain and the figure of Franco inaugurating reservoirs was an important propaganda tool. Cenajo was all the more important since it was the largest dam of the time. The 6 June 1963 was the day Franco pressed the button ‘starting’ the machinery of the dam working. 


In 1938 the Francist Spain established a Board of Redemption of Penalties for Work which one observer called “the legalisation of slavery”. In 1944 the status of ‘political condemned’ was revoked, so that all inmates were considered ‘common prisoners’, and everyone could integrate the so-called Penal Detachments in order to change days of sentence for days of work. In addition they were paid a salary, but it was just seven pesetas, and they had to pay for their clothes, food, etc. (a rural work would have earned per day between 9 and 14 pesetas). One of the reports noted that many of the prisoners working during the early years caught malaria. Also it would appear that ‘ex-‘ political prisoners were often used, with or without their explicit agreement. Some people have argued that this was just one way to eliminate or minimise the role of the Regime during both the Civil War and WW II. The objective was to smoothly integrate Spain into the international organisations that emerged after WW II, and it worked. So making political prisoners disappear was a ‘good’ idea. 

Information is scarce about the working conditions but one report did mention “dangerous work, without security measures, where dynamite was used and where the most difficult and risky tasks were reserved for the prisoners, and especially, the anarchists”. Another report called the site a ‘concentration camp’. One political prisoner was initially sentenced to death, but the sentence was later commuted to thirty years and one day. He was assigned to work in the Cenajo, but he finally committed suicide (six years later he was pardoned). Another prisoner José Vicente Ortuño worked nine months at the Cenajo. Completing his sentence in 1954 he moved to France and in 1971 he published ‘Les Racines Amères’ in which he dedicated an entire chapter to describing his work at the Cenajo.

When Ortuño arrived at the Cenajo a routine had already established. “In the morning, in the square, the corpses mutilated by bullets and the bites of the dogs used by the Civil Guard were on show. All the prisoners had to parade before the bodies, on which they were already fluttering green flies. In the afternoon, a team threw them into a tractor box and took them to the wall. The tomb was always open”. 

“The Tomb, as they called the dam, functioned as a collective burial site similar to common graves”. 

“There were differences in treatment between political and common prisoners, but the deaths of prisoner workers were not publicised”.

Below we have a few pictures of the dam as it should be (and not as it is today). 




Drought in Spain, and Murcia

Recently the press has reported on a drought hitting Spain. National Geographic mentioned the reemergence of a town called Mansilla de la Sierra that disappeared when the Mansilla reservoir was created. El País reported on the fact that Northwestern Spain has seen abnormally low rainfall for the last three years, and Spain has not experienced such a dry period since 1995. Hydroelectric generation for 2017 is down nearly 50% as compared with 2016, and this has adversely affected Spains development and use of renewable energy resources. Another El País article was titled “The drought from hell has arrived in Spain”.

What about the drought in Murcia? In an article from November 2017 the situation is analysed in detail. In Murcia the drought is real and there is a threat of restrictions to more than 2 million commercial and domestic consumers. Everyone understands that it has (or had) not rained for some months, and that the period September to November 2017 was the driest in the last 76 years. The drought has been named a ‘hydrological deficit’. Yet the winter 2016-2017 was the second wettest Spanish winter on record. Despite that the reservoirs on the Segura basin have dropped to below 13% of capacity.

The problem always comes back to the basin that feeds the medium-sized Segura River. The basin covers some 19,000 km2 in the regions of Murcia, Castilla-La Mancha, Jaén, Almería and Granada. This basin was, and still is, characterised by a year-round warm climate and a fertile flood plain, ideal for cultivating fruit and vegetables. The result is that the Murcia region produces between 20% and 30% of Spains fruit and vegetable exports. 

The only problem is that the annual rainfall is 1 mm each day, or 365 mm/year, making it the driest river basin in mainland Europe. On top of that the rain usually falls as heavy localised torrential storms, in which over half a year’s annual rainfall can fall in just 24 hours. Over the centuries floodwater channels have been formed that allow the water to run to the coast rather than be absorbed by the subsoil. In the last 500 years there have been 215 serious floods in Murcia.

Even in Roman times, 3rd C BC, weirs and irrigation ditches (‘acequias’ and ‘azarbes’) were built to control an otherwise unpredictable water supply. In the 18th C more land was devoted to agriculture, more ditches were dug, and dams and reservoirs built. In the 20th C bigger dams were built to collect the rains falling on higher land. In 1979 the Tajo-Segura water supply channel came into operation. This is one of the largest hydraulic engineering projects ever made in Spain, allowing water to arrive in the Segura basin from the Tagus River. With this the basin could again expand its agricultural production. More recently EU funding has provided for desalinated seawater, allowing another increase in agricultural production. 

The problem remains, the Segura basin is inherently a dry basin. The reality is that the ’natural water flow’ of the Segura River is about 330 m3 per person-year, whereas anything below 1,000 m3 per person-year is considered ‘catastrophically low’. Of course, to this ‘surface water’ we can add groundwater (aquifers or drought wells), water transferred from the Tajo-Segura channel, re-used or regenerated water, and desalinated water. But no matter how you do the math, an ‘inherent water deficit’ of more than 20% exists in the Segura basin. And this deficit is even more pronounced because of the draught that has hit the whole of Spain.

In fact today the Segura reservoirs are supplying less than 50% of their theoretical target. Aquifers can't replace this shortfall. Equally the headwaters of the Tajo is also supplying less than their theoretical target. Already the Segura basin is a world leader in recycling urban waste water, with 98% being re-used for agriculture. Supplies of desalinated water can be increased, but they require a huge long-term investment, and there are problems with the boron concentration in desalinated water. Despite this, desalinated water is now mixed in 75% of domestic water supplies.

So we are left with a ’structural hydrological deficit’ and a drought. 

Already in 2001 there were plans (Plan Hidrológico Nacional) for a major link between the Ebro basin down to Murcia, but nothing appears to have happened. Any investment in desalination will not solve the short-term problem. The dams and reservoirs on the Segura do a good job capturing the surface water available, and there is little use in building more reservoirs. The amount of irrigated farmland in the Segura basin has increased over the last 10 years from 166,000 hectares to 244,000 hectares, and today 84% of the water in the basin is consumed by agriculture. Can we imagine that farmers in Murcia will be willing to return to production levels of 1984?

So now we know what the problem is, but we have no idea what the answer is. Rain dancing might be an option. 

What are farmers doing?

Of course it would be wrong to assume that farmers are not trying to reduce their water consumption. We have already mentioned the move to higher ground to profit from a cooler climate and get nearer to water resources, and to the building of freezing facilities that will allow suppliers to control better ‘time-to-market’. Murcia is a major user of irrigation, but we must stress that still only 36% of its agricultural land is actually irrigated. And 77% of that agricultural land is irrigated with drip feed, often using groundwater (which brings its own problems of over-exploitation of subterranean aquifers). 


Another way to better optimise water usage is to move to covered crops (horticulture, flowers and ornamental plants) which carry the most added value per unit of water (7.8 €/m3). This is followed by vegetables with 2.1 €/m3. And in this context one important, relatively new development in Murcia is the introduction of greenhouses (tunnels, etc.). Murcia now has more than 5,500 hectares under greenhouses, 30% more than in 2012. This makes the Murcia the region with the second largest permanent greenhouse acreage devoted to vegetables, after the Canaries. For the moment the region is focussed on tomatoes, peppers, table grapes, courgettes and flowers. Cherry and papaya trees are planned for the near future. Of course, top of the leaderboard is Almeria which already has more than 32,000 hectares of greenhouses devoted to vegetables, fruit trees, flowers and nursery plants. 

The trend to move to greenhouses in Murcia will continue because greenhouse prices have dropped from 105,000 €/hectare in 2000 to 43,000 €/hectare today. And this includes automated air conditioning and drip irrigation systems with fertigation. One welcomed side effect is that the region now has a large plastics industry and is a national leader in automated fertigation systems. And the region exports annually about €60 million of greenhouses and fetigation systems (Including turnkey system) to countries such as Egypt, Azerbaijan, Mexico and Morocco.

bernard.smith@mac.com  © Bernard Smith 2017-18