Friday, November 30, 2012

The timing of precipitation

Traces of cumulative precipitation (standardized across years) for Konza.  Date range is 145-250 (May 25 - Sep 7), which is the precipitation timing critical climate period.

When rain falls might not seem to be important, but in many ecosystems it is often more important than how much rain falls.

In annual grasslands, the germination of different species if dependent on when rains begin. And the end of the rains set the end of the growing season. Same could be said for many tropical grasslands.

Many deserts are structured by when rain falls as shrubs with deep roots benefit from winter rains and grasses summer rains.

But what about temperate grasslands where the timing growth is dependent more on temperature than rainfall? How important is timing?

Put another way, if climate change causes rains to fall later (or earlier), will it matter.

There has been little work to directly test the importance of timing of rainfall.

I recently dug through the Konza climate data to see how much the timing of precipitation varied among years. Quite a lot. For May-September rainfall, the timing of when the average unit of precipitation fell varied by over six weeks over a 25-y period. In some years, half the rain had already fallen by early June. In other years, it took to mid August.

Does it matter? If plants aren't running out of water and it all goes into the same bucket, how much impact can there be of the bucket filling early or late?

Turns out a lot.

Variation in the timing of rainfall explained as much variation in grass productivity as temperatures and about half as much as the total amount of rain.

From these calculations, shifting precipitation by just 1 week later in the season can reduce productivity by 5-10%. 

For reference, a decrease in productivity of this amount is enough to put most ranchers out of business, all other things equal. 

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