Howdy Steve and WeatherCat gardeners,
A question on the VP2 ET calculation: Are you doing any calculation in your script to display and store the P-M ET results, or is it calculated in the Console?
Actually all I do is grab the value that Stu gives me and then I do things like keep track of it over multiple days. I do grab the value at 11:59 instead of exactly midnight, but I can't imagine the value would change in the seconds around midnight.
You can get your console to display the running ET value that is it computing. Just do <2nd> <Bar> on the console and it will replace the daily rainfall value with the daily ET value. That way you can check.
The reason I ask, is that over time I've found discrepancies in my comparison of the Davis VP2 ET from your script and that from the CoCoRaHS ET Gage.
My CoCoRaHS ET Gage is working just fine, giving good results with other stations across Ohio. I know the weather can vary a lot across the state, and that shows in comparisons. But we've had a lot of hot, cloudless days this month, and those are all about the same.
The comparison to the VP2 is interesting. Firstly, the VP2 is shaded later in the morning and earlier in the evening than the CoCoRaHS ET Gage, so the VP2 sees less direct sunlight. So on really sunny days, the CoCoRaHS gauge sees a little higher evapotranspiration. On other days it is a toss-up, but they will be close.
That seems reasonable to me though. The Penman?Monteith is an estimate and certainly if the solar sensor is getting less sunlight than your CoCoRaHS ET Gauge then the estimate should be less.
On any day we have rain, the numbers get weird, and significant rain leads to a zero ET on the VP2, even if the day was mostly sunny with a heavy shower during the day. I'm wondering if the VP2 calculation is not really displaying ET but instead is showing the day's water balance, or net ET (daily ET minus daily rainfall.)
Hmm, the script does compute the "net" ET for what it displays in the Growls and Emails, but it is exporting the raw ET value into the spreadsheet. Which value are you using for your graphs? If you are transferring the values from the email or Growl - yes indeed it will have the rainfall subtracted from it. The spreadsheet values won't have that though.
If you are using the spreadsheet data, then perhaps the rainfall is causing the CoCoRaHS ET Gauge to behave differently. Isn't that gauge getting wet during a rainstorm? I thought the principle of the gauge was some sort of wicking mechanism? If the wick is getting wet from the rain, that certainly would change the results.
On the other hand, Penman?Monteith estimate doesn't take into account anything about falling rain. You can look at the formula on
Wikipedia. So it could be that the CoCoRaHS ET Gauge is taking into account something about the rainfall that the Penman?Monteith doesn't deal with at all.
Does that seem like a possible explanation?
Cheers, Edouard
![[cheers1]](https://athena.trixology.com/Smileys/default/food-smiley-004.gif "Cheers")
