Irrigation Reservoir Evaporation in Spain: How Much Water You Really Lose
Ask a farmer in Murcia or Almeria what theirreservoir loses to evaporation and most will say "a lot" and leave itthere. It is one of the few water losses in Spanish agriculture that nobodymeters, nobody bills and almost nobody budgets for. It does not leak, it doesnot break, and it does not show up on any invoice.
It is also, in most of the Spanish peninsula,larger than people expect.
The numbers, and why sources disagree
Open water in Spain evaporates somewherebetween 1.5 and 1.9 cubic meters per square meter of surface per year,depending on the region. On a one hectare reservoir, that is between 15,000 and19,000 m3 leaving through the surface without irrigating anything.
If you go looking for published figures, youwill find a wide spread, and the spread is not noise. It comes from two things:the measurement method, and the type of water body being measured.
Temez, working with Class A evaporation pansacross peninsular reservoirs, reported values for Andalusian reservoirs between1,382 and 1,982 mm per year. Later work applying FAO Penman-Monteith to a largesample of Andalusian reservoirs produced a mean closer to 1,296 mm per year.For irrigation reservoirs specifically in the Region of Murcia, publishedestimates run between 1,600 and 2,000 mm per year, while a research group atthe Universidad Politecnica de Cartagena has used a figure of around 1,400 mm forreservoirs in the Levante.
That is a spread of roughly 40% between the lowand high published values, and any supplier quoting you a single number withoutsaying which method it came from is skipping the most important part.
Why a farm reservoir evaporates more than a dam
The lower figures in the literature generallydescribe large, deep reservoirs. Those have thermal inertia: they store heatthrough the day, lag behind air temperature, and the surface stays cooler thanyou would expect from the weather alone.
A farm reservoir behaves nothing like that. Itis shallow, it is small, and it has a high perimeter relative to its surface.It tracks ambient temperature with almost no lag, and it sits surrounded bydry, hot ground that pushes warm dry air across the water, an advective effectthat adds to evaporation rather than moderating it.
So when a technician tells you thatPenman-Monteith gives 1.30 for Andalusian reservoirs and your supplier isquoting 1.85, both can be right. They are describing different objects. Spanishhydrological planning applies different pan coefficients precisely for shallowwater bodies for this reason.
The part that actually hurts: seasonality
Annual totals understate the problem, becauseevaporation is not spread evenly across the year.
Roughly two thirds of annual evaporationhappens between May and September. That is the same window in which thecrop is drawing water, in which the seasonal allocation is already committed,and in which there is no realistic way to refill.
On a 10,000 m2 reservoir in Almeria, those fivemonths account for something in the order of 12,000 m3. That is not water lostin February when the reservoir is full and it does not matter. It is water thatdisappears in July, when every cubic meter is deciding whether a plot makes itto harvest.
This is why evaporation control behavesdifferently from most water efficiency investments. Its performance peaksexactly when demand peaks, and it does so with no pumps, no electricity and nooperator.
Why the euros-per-cubic-meter calculation fails in Spain
Here is where most evaporation controlproposals lose the room.
In a Spanish irrigation community, water may bebilled at a few cents per cubic meter. Multiply 16,000 m3 by five cents and youget 800 euros a year, which will not justify covering a reservoir on anyspreadsheet. The calculation is arithmetically correct and strategicallyuseless, because it measures the wrong variable.
The binding constraint on a Spanish reservoiris not the price of water. It is the availability of water: the licensedconcession volume, what you managed to store before the season started, andwhat remains after the river basin authority applies a restriction in a dryyear.
Framed that way, the question changes. It is nolonger how many euros you save on the water bill, but how many hectares you canstill irrigate in August of a dry year, and how many growing seasons you do notlose to a restriction.
There are exceptions where the direct financialreturn works on its own, and they are worth identifying early: desalinatedwater, deep-lift boreholes with a real energy cost per cubic meter, marketwater purchased in a dry year, and high-value greenhouse crops where a singlelost season dwarfs the cost of the cover.
The options, honestly compared
Shadestructures and suspended covers
Effective, and also the most expensive by awide margin. They need engineered supports, foundations and periodic structuralinspection. They make sense on small, high-value tanks, rarely on a hectare ofirrigation storage.
Continuousfloating membranes
High coverage and good performance, but theydemand careful design for rainwater drainage, gas venting and access, andrepairs are a specialist job. They also complicate maintenance and inspectionof the underlying liner.
Floatingphotovoltaic
Growing fast in Spain and genuinely attractivebecause it generates revenue rather than avoiding a loss, which is a mucheasier case to take to a general assembly. Coverage of the water surface isusually partial, so the evaporation benefit is real but secondary to the energybenefit.
Modularfloating covers
Individual floating elements that self-arrange,follow the water level as the reservoir draws down, and need no structure,anchoring or civil works. They install without draining the reservoir andwithout interrupting service, which for a shared reservoir in season is oftenthe deciding factor.
How to evaluate a modular cover
If you go down the modular route, thedifferences that determine real performance are not the ones printed in thecatalog. Four questions separate the products.
Is the figure geometric coverage or measuredreduction? Theseare not the same thing, and the gap between them can be enormous. In anindependent three-year comparative trial of four floating cover systems, oneproduct claiming over 95% reduction measured 28% once corrected. Separately, aSpanish university field trial on a modular floating cover over a realagricultural reservoir measured a reduction of 77.5% over a full year. Thoseare the kinds of numbers worth asking for.
Does the element carry ballast? An unballasted element floats highwith its center of gravity above the waterline. Elements that carry waterballast in a sealed chamber sit lower and hold formation. This is the singlebiggest predictor of whether a cover stays where you put it.
How much of the element sits above the water? Everything above the waterline issurface for wind to push against. Bulky shapes with protrusions or projectingfins catch more wind for the same covered area, and combined with littleballast that is what makes a cover pile against one bank and expose open wateron exactly the windiest days.
What polymer, and what warranty in writing? HDPE with carbon black at around 1%is the same UV specification used in buried HDPE pipe rated for decades. Askfor the polymer, the UV package and the warranty document, not the brochure.
Funding
Grant lines for irrigation modernization andwater saving have been the real engine of water technology investment in Spainfor the last decade. Floating photovoltaic is the clearest precedent for how awell used funding window can transform a market in two seasons.
Evaporation control can fit as a water savingmeasure within modernization projects, in regional calls funded by EAFRD andthe Spanish CAP Strategic Plan and in works promoted through the stateagricultural infrastructure company. Each call defines its own eligible items,deadlines and solvency requirements, so the practical work is making sure thetechnical documentation is written in the format the call scores against, withsavings expressed in cubic meters and as a percentage of allocation.
Where to start
Before comparing products, get an evaporationfigure for your own reservoir. Not a catalog percentage, but a calculationusing climate data for your location, the geometry of the reservoir and itsreal filling and drawdown regime through the season. That number is what tellsyou whether this is worth pursuing, and it is also the number a grantapplication will be scored on.
→ Learn more about floating covers for evaporation control: /balsas
