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  • Writer's pictureMark Hunt

May 30th, 2023

Hi All,

The GIF above courtesy of tells the story of this week's weather pretty well with high pressure dominating our weather picture for the second week in succession. Dependent on your location, you will pick up gentle easterly winds ( Scotland and Ireland), cool north easterly winds (England) or slightly warmer north easterly winds (Wales). May has been a curious month from a wind direction perspective with north or north easterly winds dominating. This means as soon as the sun goes down or sits lower in the sky, the temperature drops like a stone.

As you may know I love my fly fishing and because I fish dry flies (that sit on the surface), I am constantly looking at what insects are hatching on or near the water, whether that be Crane Flies, Damsel Flies, Buzzer / Midges, Mayfly or Sedges. All of these species are abundant at the moment during the day but come the evening, the hatches stop because the temperature drops so quickly. This is the opposite to what you'd normally expect at this time of year.

From a turf management perspective, soil moisture is one of the other parameters that has done a passable impression of dropping off a cliff as we have gone from an abundance to a scarcity seemingly in the blink of an eye. Scotland has its first water scarcity order already and this week it will experience the highest temperatures of the year so far for the U.K. E.T levels are tipping 4-5.0mm daily currently and last week took turf down into single figure moisture levels, super-quick. You might be sitting happily at 20% vmc (volumetric moisture content) at 06:30 in the morning, but with moisture loss ramping up, they could be close to single figures by mid-afternoon. A lot of this concerns how E.T builds during a day dependent on cloud cover, sunshine levels, humidity and wind strength. I'll take a look at this later in the blog.

General Weather Situation

So this week is a pretty simple forecast with the only key variable being wind direction and strength and its consequent effect on cloud cover and temperature. Scotland will have the gentlest winds and be closer to the centre of the high pressure so here we can expect temperatures to push up and maybe through the mid-twenty barrier with the early part of the week warmest before temperatures drop back to the low twenties for the second part of the week. Night temperatures will sit between 8-10°C, but will soon increase during the day as the force of the sun kicks in. Coming further south to England, it's the east coast that will pick up more in the way of cloud cover off The North Sea and consequently it is the east coast that will sit cooler with temperatures only in the high teens during the day and dropping back to 7-8°C at night. More westerly locations will be warmer as they'll experience less cloud cover and be further away from the cooling effect of The North Sea so The South West (Hi Steg) and Wales will be sitting some 5-6°C warmer than central and eastern areas with temperatures in the low twenties.

Ireland will see the same effect as the U.K, with eastern-facing coasts seeing more in the way of cloud cover and stronger, cooling winds, so here I'd expect temperatures in the high teens. Now, head over to Kerry, Galway, Mayo or Sligo and we are talking significantly warmer with 21-24°C more likely (cue a mass exodus to the west !)

As we go through the week, that high pressure shifts west and loses intensity so the warmest temperatures are more likely in the first part of the week and the wind will ramp up across eastern coasts later in the week. I expect this scenario to last through the weekend into next week.

Weather Outlook

Now for some you'll be enjoying your course / pitches drying out nicely after a wet spring but for others (particularly across the east of England) we will be over 3 weeks without rain by the end of this week.

So, any chance of rain in next week's forecast ?................................Maybe.....

The GFS situation is delicately balanced for next week as you can see from the GIF above with low pressure sitting in The Bay of Biscay and another across the north east of the continent. Much depends on the behaviour of these two low pressure systems and whether they continue to be divided by high pressure in the middle. The projection currently is for the BOB low to track towards Ireland during next week and introduce showers from mid-week as it does so. Some of these showers will push across to the south of the U.K later next week but it really depends on what that high pressure system does between the two lows. It could quite easily build and form a heat plume and pull very hot air up from Africa or the two lows could merge and put us into some cooler and wetter weather. If it does so we will be in drought conditions before you know it across central and eastern areas of the U.K and Scotland of course. Amazing really when you think of how wet the spring was.

Agronomic Notes

So with managed turf areas drying out quickly last week and forecast to do this week as well where we have high wind strength, air temperatures and low humidity, I thought it would be a good idea to look at how E.T actually builds in a day.

The data below is taken from a golf club in the north west of Ireland and it surprised me to see that the total moisture loss was close to 5 mm for the day.

That is higher than other weather stations I monitored across the U.K on the same date and opened my eyes a bit because I always had a hunch that E.T levels across Ireland don't reach the same heights as the U.K because temperatures tend to be slightly lower out west.

Whilst this might be true in the heat of mid-summer, in early summer it appears E.T levels are equal to anything we see across The Irish Sea and that maybe throws a bit of light on disease activity from a plant stress perspective. (more on that later)

So above we have two graphs that describe some of the key factors that drive E.T loss across a 24-hour period. The top graph plots solar energy (measured in Langley - ly units), air temperature and wind strength.

So we see solar energy start to build as the sun comes up at 05:30 and as it does so, the air temperature rises as well. As the heat warms the air, we start to get convection and the wind strength begins to increase. There is about a 5-6 hour lag between sun up and wind increasing in this case. Of the 3 parameters measured we see them peak during the mid to late afternoon and evening and that really gives us a clue to how moisture loss develops across a 24-hour period. In most cases peak moisture loss isn't occurring in the middle of the day, rather it is later into the afternoon. As day length extends into June, this period will also extend later and later into the day.

The bottom graph highlights this as peak E.T occurs on this day between 14:00-15:00 but continues to be high through the latter part of the afternoon and into the evening. Indeed moisture loss continues right up until the sun begins to set at 21:00. So the later you hand water, the better.

If we add up the hourly moisture loss this is how the day pans out E.T-wise ;

06:00 - 09:00 Total E.T loss = 0.26mm

09:00 - 12:00 Total E.T loss = 1.09mm

12:00 - 15:00 Total E.T loss = 1.78mm

15:00 - 18:00 Total E.T loss = 1.40mm

18:00 - 21:00 Total E.T loss = 0.43mm

So the main part of the E.T loss for the day occurs from 12:00 - 18:00 (64%)

Now if you consider you may be irrigating at say 04:00 and taking soil moisture levels at let us say 06:00, then peak E.T is occurring some 9 hours later and after you have probably packed up for the day. So peak stress on the grass plant is occurring mid way through the afternoon at let us say 15:00 and irrigation wont be occurring for another 12 hours or so, if you are irrigating during the early morning.

This isn't in any way a criticism of current practice in terms of when you irrigate, test for soil moisture and hand water, more so it is an explanation as to why areas dry out so fast and it provides an insight into plant stress because it's likely that in the summer, the plant is under maximum moisture stress from 14:00 through to sun down and possibly beyond if the wind keeps up into the night.

Ideally, I think the most useful practice from a soil moisture monitoring practice would be to test first thing in the morning (so you assess if the level of irrigation has raised them sufficiently) and then last thing before you head home (so you can see how much moisture has already been lost). If you have a weather station you can then dovetail these figures in with measured E.T levels to build up a picture of how best to manage soil moisture.

Implications for disease

There are a number of diseases that are linked to plant stress with Anthracnose and Dollar Spot, the two that are front and centre in my mind.

When you look at the E.T levels measured at just one location in the west of Ireland (above), then it is perhaps easier to understand why Anthracnose and Dollar Spot are on the increase in Ireland Plant stress levels are higher, earlier in the year and so last for longer across the summer season. If we plot out the daily E.T readings from that same weather station we can see we exceeded that critical 4mm E.T in the middle of April and then on 9 days in May. Food for thought.

If we look at Scotland, we can see that they typically have warmer / hotter weather earlier in the year due to the formation of high pressure systems close to the country. This week some areas of Scotland are forecast to hit 26°C, which is above the air temperature point that we know Anthracnose spores will germinate. So this week will represent their first Anthracnose temperature trigger. Now the fact that humidity levels ae low currently may mean that fungal development will be sub-optimal.

Across England and Wales, it is the west that will experience the highest E.T and temperature readings this week due to the prevalent wind direction and cloud cover / lack of cloud cover. Plant stress will therefore be higher across the west this week though having said that, the driest areas are to the east where the last meaningful rainfall was back on the 12th of May, so it isn't that clear cut.

This dry spell that is presenting to us in May and extending into June is mirroring 2018 when we last had a cold, wet start to the year. Next week I'll graph this year our vs. 2018 and 2022 from this perspective and we can see how we actually are faring.

All the best.

Enjoy the sun in the west and wrap up well in the east !

Mark Hunt


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