Friends, after a day of seminars by several researchers, I feel pretty comfortable telling you that we do not know what effects climate change will have on evapotranspiration. ET is the part of water that goes through the plant (transpiration) or evaporates off the plant surface or dirt. It is a huge part of the water balance, so if you are hoping to know how much water will be in rivers or in groundwater in a few decades, you’d sure like to be able to put a good number for ET into your model. The verdict was that several things will matter:
The growing season will be warmer, and therefore shorter, so crops might use less water from start to finish.
When CO2 is readily available, plants do not need to open their stomata so much to take it in, so less water escapes. Transpiration goes down.
But it will be hotter, so plants will need more water.
If the growing season is short enough, growers might add a second or third crop into rotation, raising the amount of water needed for that acreage.
Basically we don’t know, and we definitely don’t know well enough to settle on a number to put in the big water models.
I have to say, after seeing presentation after presentation, my doubts about the usefulness of large scale models have returned. I do understand that you have to make assumptions to be able to do calculations, or that you dump all your unexplained phenomena into a closure term. But seeing them presented in model after model (because of course the researchers were ethical enough to present them) made me remember that they aren’t remotely close to real life. Then researchers agreed that they have to refine those terms. A part of me just thinks they should skip the model part and present their gut opinions. I think those might be just as good.
Finally, I got to hear a presentation about super high tech laser ET measurements. This guy! I saw these stories about fancy-dancy laser measurement of ET in the news a couple weeks ago. I didn’t pay it much mind. Hmmm, I thought. “It looks like another tool, like soil moisture probes. That could be useful, I suppose.” Then I moved on to contemplating lunch. But that was before I saw pictures, with neato telescope-looking gadgets that shoot lasers a couple kilometers to a receiver and then figure out the water content in the air by refraction or optics or physics or science or something. They can do a good job getting moisture in the air for the transect, but it is expensive to get your fancy-dancy lasers out to the field. BUT GET THIS!
They’re working on calibrating the transects from the lasers to satellite thermal imaging. The satellites come over every couple weeks and take pictures at 30m resolution, but the fancy-dancy lasers can refine that. If those can be calibrated, the Landsat images can cover a lot of ground and the fancy-dancy lasers can improve the resolution and then we’ll know everything about ET everywhere! It’ll be great!