Flow meter - Get different readings each calibration run

I set up my flow meter towards the beginning of July, and calibrated all of the zones once everything was up and running. It came up with the following numbers, comparing original calibration to one I ran yesterday, compared to high flow alerts i’m getting today.

Is this expected to have that much variance in the GPM readings from one day to the next? And for the GPM to be that much lower earlier in the morning compared to afternoon / evening?

Zone GPM - Orig. Cal. - 7/21 @6:20 PM GPM - 2nd Cal. - 8/1 @ 6:20 AM GPM - Today Warnings Change from Orig. to 2nd Change from Orig to today
Tree Lawn East 3.4 3.1 3.4 -9% 0%
Tree Lawn West 2.4 2.1 2.5 -13% 4%
Front Yard East Near 3.5 3.1 3.8 -11% 9%
Front Yard East Side 6 5.3 6.5 -12% 8%
Side Yard East 5.9 5.6 -5% N/A
Back Yard Far 6.3 5.7 -10% N/A
Back Yard Mid 7 6.2 -11% N/A
Back Yard Near 6.1 5.3 -13% N/A
Side Yard West 4.6 3.9 -15% N/A
Front Yard West 7 6.1 7.5 -13% 7%
Front Yard East Far 6.8 5.9 7.3 -13% 7%

Very possible depending on water pressures and demand on the system. I’d be curious to see if you experience large pressure swings throughout the day.

I wonder that too, any ideas on how I could monitor the water pressure over a day?

You would need a pressure data logger … they are NOT cheap.

Your pressure is most likely lower in the mornings because everyone else is irrigating their yards too.

You could buy a cheap hose bibb gauge from Home Depot and just manually check at various times of the day.