Sunday, June 29, 2014

Straw albedo mitigates extreme heat

Peter Ruegg in a press release from ETH Zurich: Fields that are not tilled after crop harvesting reflect a greater amount of solar radiation than tilled fields. This phenomenon can reduce temperatures in heat waves by as much as 2 °C, as researchers from ETH have demonstrated in a recent study.

Wheat fields are often tilled immediately after the crop is harvested, removing the light-coloured stubble and crop residues from the soil surface and bringing dark bare earth to the top. Post-harvest tilling is a widely practised and common management technique in Europe. However, ploughed fields can have a negative effect on the local climate during a heat wave. This effect was addressed in a recent study conducted by researchers at ETH Zurich led by Edouard Davin, senior lecturer at the Institute for Atmospheric and Climate Science, and Sonia Seneviratne, professor of land-climate dynamics, which has now been published in the scientific journal PNAS.

Unploughed stubble is lighter in colour and reflects more solar radiation than tilled surfaces. Measurements taken show that approximately 30% of sunlight is reflected back due to the albedo effect – the albedo is a measure of the reflectance capacity of reflective surfaces. Ploughed fields reflect only 20% of incoming solar radiation. Model simulations have shown that this difference results in a 50% higher level of reflection in unploughed fields and that this in turn has a significant effect in extreme heat. In the event of a heat wave, such as the one in Europe in 2003, unploughed farm fields could reduce the local temperature by as much as 2 °C.

The hotter it becomes, the greater the albedo effect and the resulting temperature reduction. "Cropland albedo management has more effect during heat waves because there is almost no clouds during these events and more radiation can be reflected back into space", says first author Edouard Davin. However, this effect is only short term and local — perhaps at the most regional, but never trans-regional.

...The scientists also report that the cooling effect of an unploughed field cannot be attributed solely to changes in albedo values. Crop residue acts as an insulating layer that holds back moisture from deeper soil strata and releases it only slowly — this long process of evaporation also helps reduce the air temperature during a heat wave. In a ploughed field, on the other hand, moisture evaporates more rapidly and almost completely in extreme heat. Thus, there is an additional cooling effect of no-till farming through slow evaporation....

Harvested wheat field with round straw bales. Shot by Arne Hückelheim, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license

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