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El Niño is Here and That Means Droughts… But It’s Not What You’d Expect

Posted by on October 3, 2015 in Earth & Space, Environment, Environmental Hazards with 1 Comment

Michael Roderick | IFL Science

Drought

With an El Niño looming large this summer, it is very likely we will start to hear quite a bit about drought over the coming months.


Inevitably, at some point you will hear someone say that the warmer temperatures are making the drought worse. This is guaranteed to cause confusion because it’s not actually how droughts work.

For a drought-prone country, it is quite surprising how few Australians really understand them. Most believe it is a simple equation: rainfall goes down and the temperature rises, leading to more evaporation and an increasingly parched landscape.

But the truth is much more interesting and complex, and the idea that increased evaporation is responsible for common droughts is completely wrong. In fact the issue of how droughts develop is a bit of a chicken-and-egg situation.

Related Article: California Drought Worst in 1,200 Years: Study


The first part of the general idea about drought – that it is kicked off by a decline in rainfall – is correct. But it is the cascading set of changes that occur after this that are often misunderstood.

Why evaporation declines

In a dry region like the Australian wheatbelt, a decline in rainfall also means a decline in evaporation, not an increase as many people suppose.

When we talk about a decline in evaporation in this context, we are talking about evaporation over large regions like wheat fields, sheep paddocks, woodlands, grasslands and so on, not just the evaporation from a small farm dam.

When rainfall declines, the availability of water to be evaporated in the soil also declines. Quite simply, if there is little water in the soil then evaporation can’t increase.

So less rainfall means less evaporation, which in turn means less evaporative cooling of the land and the air immediately above it. To appreciate how evaporative cooling works, think about the principle behind the Coolgardie Safe or those old-school hessian water bags that have made a recent comeback in some hipster shops. Or if you prefer, think about how much cooler a lawn is than a concrete driveway on a hot sunny day – that’s because the lawn is cooled by evaporation.

Related Article: Organic Farming: Resilience in the Time of Drought

In a drought, then, the relative lack of evaporative cooling means the land surface and the air just above it tend to get warmer still. Scientists call this a land-surface feedback. But that is just one part of the cascade that increases temperatures.

Where there is less rain, there are also typically fewer clouds and more sunshine. Extra sunshine means extra heat and this has to go somewhere. In normal circumstances, it would go towards evaporation from plants and soil. But with little available water it heats the surface of the land, which makes it even warmer again.

With everything pushing the temperature in the same direction, the net effect is even warmer daytime temperatures. So the lack of rainfall drives the temperatures up, not the other way around.

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  1. 235487769905222@facebook.com' Kitty Tyler says:

    anyone like this post as much as i like ?

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