Mark Hunt Articles
Mark Hunt weather corner; 128 - What record evapotranspiration means for the future of turf management
This summer has broken many records so far and it isn’t over yet. The hottest May and June temperatures and now the driest, sunniest July on record. Away from mainstream media, there are other records being broken that impact on turf management more directly.
At our default location (Thame, Oxfordshire), July 2026 was also significant in recording the highest monthly moisture loss by evapotranspiration (154.74mm) since we started measuring climatic data with a Davis weather station back in 2005. That is only over the last 21 years, but bear in mind that according to the UK Centre for Ecology and Hydrology, 5 out of the 10 driest and warmest summers have occurred since that year (2006, 2018, 2022, 2025 and surely 2026). That figure didn’t just break the previous highest total (133.4mm – May 2020) by 1 or 2%, it exceeded it by 16%!
It got me thinking, why was the E.T so high in July 2026?
As discussed before, E.T is not a measured parameter, it is a calculated using the Penman-Montieth equation. This equation takes into account solar radiation, air temperature, humidity and wind speed.
It follows then that in July 2026, one or more of those parameters must have been significantly higher to result in such an increase. So, I took a deep dive into the Davis weather station data to look for the cause.
The results are shown below, comparing July 2026 with 2025, 2024, 2023 and 2022 for Thame, U.K.
We can see from the summary data above that July 2026’s mean temperature was significantly higher than previous July’s. In combination with this, the mean humidity was lower and, most significantly, the mean and total solar radiation received was appreciably higher, some 33% up on the previous year for the latter.
The biggest driver then was a significantly higher level of solar radiation, meaning more of the sun’s energy reached the ground. This energy is able to change the state of water from a liquid to a gas by the process of evaporation, so more water is evaporated with a higher level of solar radiation. In combination with this, the atmosphere was drier, which makes it easier for that water to evaporate. At a higher atmospheric level, that drier air meant we had less cloud formation which acts to filter/reduce levels of solar radiation reaching the ground.
Finally, we have the wind effect, not a general one because high pressure systems tend to be associated with low wind speeds, but a convective one. As the ground warms during the day, heat rises in highly-localised areas and this causes mixing between warm and cold air, giving rise to convective winds through the afternoon and into the evening.
These winds further drive the process of evapotranspiration. Above is a snapshot of moisture loss by E.T from Prodata Reports software, shown on an hourly basis through one day in July.
You can see right how the hourly E.T rate increases in the afternoon and continues late into the evening.
This effect has important implications for irrigation efficiency and scheduling, adding to the challenges faced by turf managers.
As summers continue to push climatic boundaries, the future maintenance of golf courses, winter sports pitches, tennis courts and racecourses is becoming an increasingly important topic for discussion.
