Today was sunny at Bangkok, and I decided to do an experiment on the lawn at the apartment complex where I live. For more on this topic, see this post on why I would not try to cool grass by syringing. That post drew a mixed and sometimes heated response.

It was was of the most viewed posts on my blog, ever, and some people wrote to say they agreed with me, and others wrote to say that I was wrong. With that mixed reaction in mind, I wanted to make some measurements myself. Here's what I did today:

• the air temperature was 35°C, wind was negligible, there were no clouds in the sky, and the photosynthetic irradiance was 1970 micromoles m^-2 s^-1
• this area of the lawn was in full sun and is primarily manilagrass (Zoysia matrella) with a bit of purple nutsedge (Cyperus rotundus)
• the height of the turf was 5 cm
• I marked 4 points on the lawn with a golf tee
• I measured the surface temperature on both sides of each marked point using an infrared thermometer
• Starting at 13:18, water was applied as a fine mist (a syringe) to one side of each marked point, with the water coming from a spray bottle calibrated to apply 40 mL m^-2 to an area of 0.159 m²
• Immediately after the water application, and again at 5, 10, 15, 20, 25, and 30 minutes after application, the surface temperature was measured on a location within the syringed area and on a location outside the syringed area
• The temperature of the water applied to the grass was 31.2°C

Here are the temperature measurements. Time is in minutes, with time 0 immediately before water was applied, and all other times in minutes after the syringe application.

##    Time Syringe No Syringe
## 1     0    33.2       36.2
## 2     0    36.6       34.6
## 3     0    34.4       37.2
## 4     0    35.0       34.4
## 5     1    33.4       32.8
## 6     1    30.4       36.6
## 7     1    31.0       37.2
## 8     1    32.8       34.2
## 9     5    33.2       34.6
## 10    5    38.4       35.4
## 11    5    33.4       37.0
## 12    5    37.2       36.2
## 13   10    34.6       30.2
## 14   10    35.6       33.6
## 15   10    34.6       35.8
## 16   10    33.2       33.0
## 17   15    33.4       31.2
## 18   15    33.8       33.4
## 19   15    32.0       37.6
## 20   15    33.4       33.4
## 21   20    30.4       29.6
## 22   20    33.6       34.4
## 23   20    33.2       34.8
## 24   20    32.4       31.2
## 25   25    31.8       33.8
## 26   25    35.6       34.0
## 27   25    34.0       37.4
## 28   25    36.8       32.8
## 29   30    31.4       29.0
## 30   30    34.6       32.8
## 31   30    31.2       36.4
## 32   30    31.8       32.4

In summary what happened was this:

1. The surface temperature before application of water was about 35°C, the same as air temperature.
2. 1 minute after water application, the surface temperature had dropped to 31.9°C, which was close to the actual temperature of the water. There was a 3°C reduction in temperature immediately after applying water, but keep in mind that the water temperature was 31.2°C.
3. Using a t-test to compare the mean value of syringed vs. non-syringed turf at 1 minute after water application, the probability that such a difference in temperature was due to chance was 0.04. Clearly, the water application reduced temperature for about a minute, maybe a little more.
4. At 5 minutes after application the surface temperature was back up to more than 35°C, a higher temperature than before water was applied. And then at 10, 15, 20, 25, and 30 minutes after application, the surface temperatures ranged from about 32 to 35°C, but there was never a significant difference in surface temperature between the syringed and non-syringed turf.

These results are remarkably similar to those I described in the controversial blog post. Today's data were collected on a lawn, and the higher mowing height increased (I think) the variability in the measurements. If the weather stays like this, I will try to collect data from a putting green to see what type of syringing effect I can measure.