Do wind turbines make droughts worse? A new study says they do not

Can wind turbines exacerbate a drought, or strip moisture from Australia’s arid soils?

This idea has circulated among Facebook groups that are sceptical of renewables for about two years, and it’s roaring back again on the heels of a new German study. 

The problem is, the most recent study doesn’t actually say what many social media observers think it does.  

The “bombshell” study showing “potentially serious consequences for agriculture” is in fact a computer simulation designed to test how turbines influence surface temperatures and soil moisture in central Europe.

It does include some scary numbers, but with a careful proviso that the scenarios it’s testing are not possible in the real world.

What climate and energy engineer Sampath Weerappulli found was that under the right circumstances a small turbine with a 57m hub height could cause as much as 0.73ºC of surface warming and a small drying effect in the top layer of soil.

But that was an extreme outlier; the next highest figure was 0.34ºC, with two measurements showing cooling.

The highest soil moisture change was 0.002316 per cent.

“Our study does not demonstrate that wind farms cause long-term drying of agricultural land,” Weerappulli said in an email to Renew Economy.

“The changes in soil moisture that we observed were very small and, in my view, are not large enough to give cause for concern about substantial additional drying.”

Weerappulli said he would also be “cautious” about mapping the results directly onto dry regions such as southern Australia, or applying them to much larger modern turbines which are now the norm for southern hemisphere wind projects. 

“Assessing potential effects under drought conditions would require simulations with different soil-moisture states, realistic Australian meteorology, soils, and vegetation, and substantially longer simulation periods,” he says. 

Unfortunately, there is no local data to feed into Weerappulli’s model.

Juan Zaracho, a research fellow at James Cook University in Queensland, says the German study is “legitimate, careful science” but there’s no Australian data to anchor it either way.

“Your question about Australian data. I quickly looked, and I think there’s essentially none,” he told Renew Economy.

“I couldn’t find a published Australian field or satellite study on this… nobody on either side can point to Australian measurements.”

Zaracho says Australia’s wind farms are mostly on windy ridges and coasts where natural turbulence is already high and turbine-added mixing matters less. Inland cropping country with still, clear nights is where he’d most expect to see a change, if it happened. 

Warmer nights, cooler days

Weerappulli set out to uncover how the turbulent air that eddies from turbines alters microclimates close to the ground and soil moisture. 

There is already a small amount of observational research in China, Europe and the US showing turbines sometimes create a small warming effect at night and a small cooling effect during the day, as blades mix colder surface air with warmer layers higher up. 

The bigger the wind farm, the bigger the effect, but the change is measured in tiny increments – 0.72 per cent over a decade at one large wind farm in Texas, and just 0.07-0.1ºC over grasslands and crops in China. 

To smooth out a lack of European field data and to compare identical scenarios, Weerappulli’s paper used a computer model to simulate ideal conditions over a fixed 4.5 hour period.

The model looked at nine atmospheric conditions, from strongly unstable through to neutral and strongly stable. 

It was under strongly stable – no wind, no mixing of air layers – night time conditions that the model predicted a small but measurable drying effect in the top millimetres of soil. 

The model showed that a simulation of a 5×5 array of 25 turbines on perfectly flat, uniform European farmland over a 4.5 hour period, could – under the right circumstances – cause as much as 0.76ºC of surface warming and a small drying effect in the top layer of soil.

The results came with the proviso that “such stationary conditions do not occur in nature” – scary numbers that amount to snapshots from an idealised model.

The data also shows turbines can also cause a colder microclimate or create no measurable change at all.

“Although these effects may appear small on average, even subtle shifts in microclimatic conditions can influence plant growth, frost risks, and soil water retention factors critical to sustainable agricultural productivity,” Weerappulli wrote in the paper, published in the journal Boundary-Layer Meteorology in late July.

But strongly stable is “a rather extreme” scenario, Weerappulli says. It’s not a condition that would occur at every moment all night long, for example. 

In a real-world scenario, there is likely to be some wind, which lends itself to smaller surface temperature changes and soil moisture effects. 

“In real conditions, where atmospheric stability and wind direction continuously change, these effects would not necessarily persist in the same direction or over the same location,” he says.

“Changing wind directions would also distribute any potential long-term effects over a larger area, making them less pronounced locally. 

“Overall, one of the key takeaways from our study is that although wind turbines can measurably modify near-surface turbulent exchanges, the resulting soil-moisture effects were very small and did not indicate a significant risk of increased drying of agricultural land.”

The Australia question

Although the study was designed to look specifically at central Europe, Weerappulli says Australia’s dry climate makes the soil moisture question “particularly interesting”.

“Our simulations showed that turbine-generated turbulence can modify latent heat flux and therefore soil moisture loss,” he says.

“For Australian wind farms, the frequency of the relevant stability regimes would… be just as important as turbine size.”

And could turbines exacerbate a drought? Weerappulli says no.

Extra atmospheric mixing can’t strip out moisture that isn’t there. 

But looking into surface temperatures during drought is one direction Weerappulli’s work may go next.

Turbine-maxxing

Developers in Australia have been turbine-maxxing for years, building bigger and bigger machines to capture stronger winds higher above the earth.

But bigger turbines, that interact with more atmospheric layers, don’t necessarily mean hotter eddies or bigger surface effects.  

“I would not assume that a larger turbine automatically produces proportionally greater effects at the surface,” Weerappulli says. 

“Hub height, rotor diameter, atmospheric boundary layer depth, the strength of the nighttime temperature inversion, wind shear, turbine spacing, and land-surface conditions would also matter. 

“Other than that, due to changing wind directions, the same area is not continuously affected by turbine wakes. As a result, any long-term effects are distributed over a larger area and are therefore likely to be weaker at any specific location.”

Extrapolating any of these results to Australia, be it from a model designed for Europe or satellite images from China, is fraught because there isn’t any similar data here.

It’s not possible to scale up the effect of a turbine on temperature or soil moisture by scaling up a turbine, says Zaracho. 

“The near-surface effect depends on whether the rotor actually reaches down into the shallow, cold, stable layer near the ground,” he told Renew Economy. 

“So [a computer model based on a smaller turbine] genuinely can’t tell you what a 140m machine does. If anything, that’s a reason to be sceptical of anyone using a 57m-hub German simulation to predict effects for the much taller turbines built here.”

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