Mark Hunt weather corner; 109

Mark Huntin Science

The climate framework within which we manage turfgrass is undoubtedly changing. Climate experts can debate the theories behind this change until the cows come home, but greenkeepers and groundsmen now manage the reality on a day-to-day basis.

One of the changing climate trends that is clear concerns rainfall. Warmer air holds more moisture and so we have an increasing potential for more intense rainfall events. Dove-tail that in with changes in our jet stream dynamics, which can lead to slower-moving rainfall events, and we have (pardon the pun) a perfect storm. Climate scientists predict more extreme rainfall events will become (and I'd argue, already are) more common.

Measuring and understanding this change in the dynamic of rainfall events and the consequence for turfgrass management is key. It isn't of course just turfgrass management that is affected by extreme rainfall events. All well and good in having a greens rootzone that can cope with 100mm per hour rain rates when the fairways, bunkers, members car park and local roads flood at 30mm per hour.

Nowadays, you hear more and more on the news, quotes of "a month's rainfall in a day" and the focus is on the amount of rain in mm (or old style inches), but what about how that rainfall is falling?

This is described by the rainfall rate in mm (or inches) per hour, there is a classification scale for rainfall rate ;

Light rain Less than 2.5 mm/h (<0.1"/hr)

Moderate rain Rain rate of fall is 2.6 to 7.5 mm/h (0.1 to 0.3"/hr)

Heavy rain Rain rate is greater than 7.6 to 50 mm/h (0.3 to 2"/hr)

Violent rain Rain rates greater than >50 mm/hr (>2 in/hr)

As we see warmer air temperatures in the summer and autumn, the potential for more violent rainfall events is clearly growing. In mid-June this year, we saw thunderstorms cross the U.K & Ireland, with significant rainfall events. So what degree of rain rate in mm per hour did we see?

Below is a graph using data from a Davis Vantage Pro weather station, showing a rainfall event in my home town of Market Harborough, Leicestershire. In that event, we had a total of 18mm of rain and it fell over the course of 2.5 hours between 16:45 and 19:15 and caused significant local flooding. All well and good if we just focus on the amount of rainfall that fell. The reality from an infiltration rate perspective is that the storm peaked between 17:00 and 17:15, during which time 14.5mm fell at a rain rate peak of 256.8mm per hour (just over 10" per hour).

It was that 15-minute period with its associated high rain rate that caused the flooding issue.

Modern day facilities, whether that is tees and greens rootzones, bunkers and cricket outfields, need to be able to cope with such extreme rainfall events and particularly those at the pinnacle of our profession under the spotlight of the tv camera. At the very least, we need to be able to collate and communicate this type of data so members / customers understand the capability of our facility and maybe how we need to invest in the future to improve it.

Mark is well respected in the turfcare community and welcomes your questions.
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