Flex Daily just about gives you want you need already. Crop Coefficient is multiplied by the daily FRET (Forecast Reference EvapoTranspiration) value, plus shade factors, to calculate evaporation per day based on current weather conditions. FRET is a daily or weekly value in inches to indicate how much water evaporates from a standard size open tray, and is the basis for all these irrigation calculations. Using a Crop Coefficient of 1.0 uses the basic FRET value without any correction for crop type, which wouldn’t be necessary here. It wouldn’t include spillage due to splashing or leaks, but should be pretty darn close.

Then, all you’d have to do is measure your water flow to the pool in GPM and pool size. Let’s try an example: Say water flow is 10 GPM, and pool size is 24’ x 40’. From that you can calculate the Nozzle Inches per hour: 24 x 40 = 960 sq ft x 144 = 138,240 sq in = 138,240 cubic inches / 231 = 598.4 gallons per inch of water. So it takes 598.4 / 10 = 59.84, say 60 minutes to raise pool level 1". That is a Nozzle Inches per Hour value of 60/60 = 1.00" per hour. Surprisingly close to a standard sprinkler value (a coincidence, as size of pool and flow rate both change it). So, a set flow rate in this case of 1" per hour and Crop Coefficient of 1.00 should give you about what you need for your pool, on a daily or weekly basis, accounting for the weather. Flex Daily also accounts for the rain, so it’s a no-brainer here.

The only real question then remaining is how often you want it to water, or rather, how low do you want the pool to get before being filled. This is exactly analogous to setting the Soil available water, the crop root depth, and the allowed depletion. Those 3 values multiplied together gives you the amount of water applied at one time. And that value must be /at least/ the amount evaporated per day in hot weather. A value of about 0.5" is pretty good, so if you set root depth to 2, available water to 0.5 and allowed depletion to 50%, that gives you 2 x 0.5 x 0.5 = .5" water. Set the Efficiency to 100% (as you don’t waste any water and it has to be even), and I’m pretty sure you’ve got a Pool Filling Program.

If, over time, it starts lowering, set the efficiency lower. If, over time, it overfills (not because of rain), then increase the Nozzle value a bit. Voila!

This may sound silly, but keeping a pool filled or a yard filled is not really a different problem; it just has different values. If anyone wants to try it and are unsure how to calculate it or proceed, ask and we’ll figure it out.

Oh, and yes, it would be pretty simple for Rachio to add a Pool Program by just doing the above. They’d just have to ask you your pool size and pump GPM, and it’s done. But until then, try the above.