Well this is the last blog of May, didn’t that pass by quickly as a month ?
Maybe it is the phenomenon of lock-down or maybe I’m just getting old but time just seems to race by nowadays ?
Above is the readout of my Netatmo weather station for May 2020 so far….
Perhaps it is stating the obvious that it has been one hell of a dry month, to date here in arid Market Harborough we have had a rip roaring 0.4mm !
You can also see that May has been dominated by high pressure and with that comes rapidly alternating day and night temperatures as we lose temperature at night with predominantly clear skies and then gain it during the day. So dry, alternating dry-cold cycles and to boot it has also been a very high E.T month, can’t remember a May like it so later I’ll be delving into my (or rather Sean’s) archives from The Oxfordshire to see how it compares with past years.
I have a very good friend who lives out in Breckenridge, Colorado. He’s a Brit but after many years he’s slowly being assimilated into the American way which means he’s losing the understanding of the sense of irony and sarcasm that goes with most conversations over here 🙂 (Mr Scott please don’t take offence as you’re exempt !)
I digress because whilst we are looking at an extremely dry, sometimes hot Spring, this is what he woke up to the other morning, a very late in the year snowfall !
That’s some view though eh ? (yes Bigtop I am jealous…)
General Weather Situation
So will May finish true to type or has that trough system kept on track as projected last week to bring us a cooler, wetter interlude ?
Well the short week we have coming up is one of those rare ones where it is pretty much the same weather for everyone all week give or take a bit more cloud to the west by mid-week and a smattering of rain over north-west Ireland and Scotland, Wednesday p.m into Thursday a.m. So I’m afraid high pressure has won the day this week with no trough interlude pattern, no cool weather and most importantly no real rain for any of us.
Does that mean that all is lost from a change in the weather perspective ?
Well no because there is a change on the way, it’s been delayed a little but I do think we are edging closer to that trough pattern and some meaningful rainfall. First to this week though, so high pressure wins the day and that means, bright, sunny and warm with the south of England and Wales popping up into the mid-twenties. Scotland and Ireland will only be 2-4°C down from this, with similar fine, sunny and warm weather all week. In difference to this type of weather in the middle of summer, the nights will be refreshingly cool, down into high single figures / low double figures so not bad, all in all, unless you’re desperate for rain that is.
From Wednesday, the wind will switch round from its current north-westerly perspective to easterly and there it’ll stay probably until the end of the weekend / early part of next week. Unlike the end of last week / weekend, it won’t be as windy with moderate to light winds, abating at the end of the week. That means the E.T will drop back a bit as well this week as we approach the weekend, thankfully.
So dry, settled, warm during the day with some very long sun hours. That means high U.V and a requirement for sunblock and looking after your skin.
So above is the projected GFS output for next Monday, the 1st of June.
As you can see we do have some sort of a change going on with an Atlantic low pressure system trying to push in to the west of Ireland. It is this system that will be the catalyst for a change in our weather so that’s the one we need to keep an eye on. If you believe in the GFS output (and it has to be said that the European projections on ECMWF are different), next week will start a little cooler than this with the wind changing across Ireland to a south westerly during Monday, heralding the arrival of cooler, wetter conditions. The U.K will probably be similar to this week, stable, dry and warm, perhaps with more in the way of cloud cover across the west. Rain if it happens is likely to be be across Ireland and western-facing coasts of the U.K later on Monday into Tuesday with The South West, Wales, the north west of England and west of Scotland picking some up as well.
Now where the two models differ is in the tracking of this low pressure system with one model suggesting it will stay west of the U.K and sink south so any rain will be confined to Ireland and the west / south west of England. The other model (GFS) shows it tracking more eastwards and that’ll bring rain to most areas for Tuesday and Wednesday next week. If the former happens it means next week will continue the dry spell, maybe a little cooler than this week with more cloud cover and a change to a more northerly wind aspect.
I have to say that I have little confidence as it stands now of rain across the U.K early next week because the outlook has changed every day and if anything the prospect of rain has diminished.
It’s like waiting at a station for a train that gets progressively more delayed the closer it gets to the station so you end up wondering if it’ll ever arrive.
I haven’t totally given up hope though because the ECMWF model still shows a trough pattern forming but just later than the GFS output so my gut feeling is ‘a change is a coming’, it’s just a matter of when.
E.T is a moving target….
So the first aspect I want to focus on is evapotranspiration (E.T) because that dictates (or should dictate) your irrigation input. It remains to me one of the strongest arguments for a facility having access to a professional weather station in that an understanding of moisture loss by daily E.T is essential to efficient turf management. As mentioned earlier, I can’t remember a May where we had such a prolonged dry spell accompanied by strong sunlight, winds and consequently high E.T.
Now before I go on it is worth reminding ourselves that the term evapotranspiration actually covers two processes of moisture loss to the atmosphere. The first is the one we probably most readily associate with E.T, that of evaporation of moisture from the surface of the earth, so that’s from the soil, from water features / bodies, vegetation and hard surfaces like a road for example. The second concerns moisture loss by transpiration which occurs when water inside the plant leaf is vapourised and lost as a gas through the stomatal pores of the plant. Now in our case, we are hopefully maintaining a scenario where full grass cover is the norm and therefore water loss by transpiration is the key driver to E.T, this is known as the reference E.T and denoted by E.To. Now from hereon in it starts to get complicated because the equation to calculate E.T, known as the Penman-Monteith Evapotranspiration equation (FAO-56 Method), relies on a number of factors.
I have listed them below but to see how they relate to the actual formula and to get properly confused, I suggest you click on the link here to a very detailed paper on the calculation of E.To.
Now if you look at the equation you’ll see it takes into account the following….
ETo= reference evapotranspiration, mm day-1
Rn= net radiation at the crop surface, MJ m-2d-1
G = soil heat flux density, MJ m-2d-1
T = mean daily air temperature at 2 m height, ºC;
u2= wind speed at 2 m height, m s-1
es= saturation vapor pressure, kPa;
ea= actual vapor pressure, kPa;
es-ea= saturation vapor pressure deficit, kPa;
î = slope of the vapor pressure curve, kPa ºC-1
³ = psychrometric constant, kPa ºC-
The problem with calculating E.To is that there are a number of assumptions and as you’ll see many of them (above) relate to weather parameters measured at 2 metres height whereas we are concerned with what is going on at grass height. I know from my work on disease modelling that the wind speed, temperature and humidity at ‘grass height’ is very different to 2 metres up in the air and that’s where the first inaccuracy comes into play when we look at calculated E.T, rather than actual E.T.
You then need to apply a conversion factor that calculates the ‘crops water usage rate’ often known as the crop coefficient. Again there is a really easy to understand paper on this produced by the USGA Record here.
There is an easier way…
In practice the widespread use of moisture meters has obviated the requirement to do all of these calculations but in my experience knowing the daily E.T allows you to apply your own conversion factor to your situation and then you can check the results with a moisture meter. When I first started out doing wetting agent trials back in 2005, I knew none of the above, so when I was asked by the course manager how much irrigation needed to be applied to the trial plots, I answered, just replace the E.T loss. (as measured and calculated by an on-site weather station). The next time I looked at the plots that were saturated to the point of filled capacity. I then realised that irrigating by E.T didn’t involve the seemingly (to me) straight forward process of replacing what the calculation said you had lost !
My rule of thumb (remembering moisture meters weren’t freely available back then) was to replace 60% of E.T loss to give a healthy grass plant, 50% to run it on the edge and 40% to apply stress.
In practice I’ve found this to work well.
So has May 2020 really been a high E.T month ?
Well to answer this question fully we have to take into account not only measured E.T but also measured rainfall and see how the two shape up because after all you can have a high E.T / high rainfall month which effectively cancel each other out. This could occur when we have two or three days of heavy rain followed by a prolonged dry spell and then more rain for example…
So first off I decided to look at one of the windy, warm days we had over the weekend to see how the E.T mapped out and I used the data from the Davis Weather Station at The Oxfordshire.
So here’s how the E.T panned out yesterday on an hourly basis on the 25th May ;
So we began to lose moisture from the turf canopy between 6 a.m. and 7 a.m. in the morning as the wind began to pick up bearing in mind ‘sun up’ was an hour or so before. The hourly E.T then commenced a steady increase through the morning reaching a peak value of 0.64 mm between 12 noon and 1 p.m. and the same value for the next hour. It then began to decline slowly and even at 6 p.m. was at 50% of maximum hourly E.T. The weather station still measured a very small E.T loss right up to midnight. For the whole day, the total calculated E.T loss was 5.42 mm. This means that working on my ‘back of a fag packet’ rule of E.T, I’d need to replace 3.25 mm to keep the turf healthy. Of course we also know that irrigation requirement isn’t uniform and some areas would be over-watered at this replacement rate (sheltered greens for example) and so in practice we may replace 50% and use hand-watering to top up on the dry areas. (All well and good if the majority of your staff aren’t still under furlough !).
So yesterday was a pretty punishing E.T day, with of course no rainfall.
I then looked back at the month of May for the last 5 years to see how E.T and rainfall for the month have shaped up and whether my hunch that May 2020 is a bit of a b***h of a month was justified…Of course the month isn’t over yet so I used data up until today and then Meteoblue’s daily projected E.T (which isn’t bad ) for the rest of the month.
Here’s how the data looks ;
So there we have it…
May 2020 looks to be one of the highest E.T May’s we have experienced and when you add in that it will also be one of the driest (difficult to get much drier than 1 mm rainfall for the entire month), you can see that the calculated deficit from a rainfall vs. E.T perspective is the worst we have experienced at this location in recent years. Now I don’t have historical E.T info going back years unfortunately so can’t compare with back in the 70’s or 80’s, but I think this comprehensively shows that we are up against it from a moisture perspective.
Dovetail that in with only 6 days of recorded rainfall before this month stretching back to March 19th and you can easily see why this year has been a real case of ‘Taps On’ followed by ‘Taps Off’ !!!
So if your Poa annua is still seeding profusely (I think we are just getting over the hump) and looking pretty sorry for itself or areas that thinned over the winter haven’t knitted in on outfield turf that doesn’t receive irrigation, this is why…
Here’s hoping I’m sitting here next week with a trough pattern firmly in the weather sites and another week of what now is pseudo lockdown behind us…
All the best, stay safe and healthy…