ET Evapotranspiration / Evaporation - calculation

[Xant]

After getting hands-on with WeeWX and Belchertown Skin, changed PWS to acquire UV and Radiation. Not a Davis yet, which (at a cost) can provide ET by hardware. Still wondering if WeeWX per Radiation, time and the usual wx data, can calculate ET.

Already placed "ET = software" in weewx.conf and publishing through Belchertown, but only "zero".

Is this possible, or is this the correct way? If so, any else? (ie, must extend database?)

Thanks,

Xant

[Thomas Keffer]

ET is in the database schema.

To calculate it, you need radiation, wind, temperature and relative humidity. If you have all that, it should be calculated automatically. Check your database to see what's in it.

This will show ET for the latest 20 archive records:

sqlite3 weewx.sdb

sqlite> select dateTime, datetime(dateTime, 'unixepoch','localtime'), ET from archive order by dateTime desc limit 20;

Keep in mind that ET will be zero at night.

-tk

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[gjr80]

Also be aware that ET is not calculated for loop packets, only archive records. So services using loop only data will not see calculated ET.

Gary

[Xant]

Thank you Thomas & Gary for prompt support.

Will investigate both paths (daytime).

Best, Xant

[Liz]

On Tue, 20 Aug 2019 19:05:29 -0700
Thomas Keffer <tke...@gmail.com> wrote:

> Keep in mind that ET will be zero at night.

If so, that is in error.
Even in 1992 it was clear that nocturnal ET can be high
https://www.researchgate.net/publication/239351598_Night-time_evapotranspiration_vs_daytime_and_24h_evapotranspiration

and as a physical process, could still be quantified in 2019
https://www.researchgate.net/publication/331942685_Quantification_and_Prediction_of_Nighttime_Evapotranspiration_for_Two_Distinct_Grassland_Ecosystems

Liz

[Thomas Keffer]

Sorry. ET can of course be non-zero at night. It's just a lot less than in the day.

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[Xant]

Thank you Thomas. Your database trick worked and indeed shows ET at a very low value, which I then must better understand and figure-out (or a multiplier for display).

root@raspberrypi:/var/lib/weewx# sqlite3 weewx.sdb
SQLite version 3.27.2 2019-02-25 16:06:06
Enter ".help" for usage hints.

sqlite> select dateTime, datetime(dateTime, 'unixepoch','localtime'), ET from archive order by dateTime desc limit 20;

1569094200|2019-09-21 15:30:00|0.00113383667160128
1569093900|2019-09-21 15:25:00|0.00115460974957007
1569093600|2019-09-21 15:20:00|0.00117362977424164
1569093300|2019-09-21 15:15:00|0.0011904398413829
1569093000|2019-09-21 15:10:00|0.00120682836842174
1569092700|2019-09-21 15:05:00|0.0012226234610386
1569092400|2019-09-21 15:00:00|0.00123687715492316
1569092100|2019-09-21 14:55:00|0.00125017435348036
1569091800|2019-09-21 14:50:00|0.00126219404937789
1569091500|2019-09-21 14:45:00|0.0012752201510339
1569091200|2019-09-21 14:40:00|0.00128731319969464
1569090900|2019-09-21 14:35:00|0.0012972602173087
1569090600|2019-09-21 14:30:00|0.00130148004764894
1569090300|2019-09-21 14:25:00|0.00130622115073571
1569090000|2019-09-21 14:20:00|0.00131200388265713
1569089700|2019-09-21 14:15:00|0.00131627804628626
1569089400|2019-09-21 14:10:00|0.00131874529135357
1569089100|2019-09-21 14:05:00|0.00132100745579488
1569088800|2019-09-21 14:00:00|0.00132049046840312
1569088500|2019-09-21 13:55:00|0.00132219669245463
sqlite>

Thanks, Xant

On Tuesday, August 20, 2019 at 10:05:59 PM UTC-4, Thomas Keffer wrote:

ET is in the database schema.

To calculate it, you need radiation, wind, temperature and relative humidity. If you have all that, it should be calculated automatically. Check your database to see what's in it.

This will show ET for the latest 20 archive records:

sqlite3 weewx.sdb

sqlite> select dateTime, datetime(dateTime, 'unixepoch','localtime'), ET from archive order by dateTime desc limit 20;

Keep in mind that ET will be zero at night.

-tk

On Tue, Aug 20, 2019 at 6:56 PM Xant <armando...@gmail.com> wrote:

After getting hands-on with WeeWX and Belchertown Skin, changed PWS to acquire UV and Radiation. Not a Davis yet, which (at a cost) can provide ET by hardware. Still wondering if WeeWX per Radiation, time and the usual wx data, can calculate ET.

Already placed "ET = software" in weewx.conf and publishing through Belchertown, but only "zero".

Is this possible, or is this the correct way? If so, any else? (ie, must extend database?)

Thanks,

Xant

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