For as long as I can remember, I've used the easy 1 mm = 1 L = 1% soil volumetric water content (VWC) relationship when converting between water use by grass, ET, irrigation, and soil VWC. Bill Kreuser shared the simple calculation to work this out in inches, for any rootzone depth.

The post discussed here is https://www.asianturfgrass.com/post/soil-water-content-turf-hacks/

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