Estimating water needs

I want to estimate my water needs for August so that I can then compare with the actual usage. Does this look right (the two columns on the left are actual local ET0s):

ET0 Calculations
8/2/2021 0.3 Warm Season Grass Kc: 0.71 from reference
8/3/2021 0.29 Warm Season Grass ET (month): 2.6 =[Total ET0]*Kc (3.71 * 0.71=2.6)
8/4/2021 0.27 Root Depth (in): 6 Rachio zone setting
8/5/2021 0.27 Total water replacement (in): 15.8 =[Monthly Warm Season Grass ET]*[Root Depth] (2.6 * 6 = 15.8)
8/6/2021 Area (sq. ft.): 1000 measured
8/7/2021 0.26 Area (sq. in.): 144000
8/8/2021 0.23 Replacement water volume (gal): 9852 Area * Total water replacement (144000*15.8/231=9852)
8/9/2021 0.26
8/15/2021 0.25
8/16/2021 0.25
8/17/2021 0.24
8/18/2021 0.06
8/19/2021 0.19
8/20/2021 0.2
8/21/2021 0.06
8/22/2021 0.16
8/23/2021 0.19
8/24/2021 0.23
Total ET0 3.71

I couldn’t way without knowing where you are located, as far as ET goes. But and ET of 2.63 for a month averages less than 0.09 for the day, which sounds awfully low for any place in the U.S.

Also, I don’t know what you mean by Total Water Replacement. And what are the numbers immediately after the dates? They look like daily ET values, but with a total of 3.71 for the 17 days listed, that comes out to an average 0.22, far higher than the ET that results from the calculation above. But closer to what I would expect this time of year in most places.

It is true that 15.8" of water over 1000 square feet takes 9852 gallons, but I’m not sure what that represents.

Daily ETs (numbers after the dates) should be accurate (i got them from here: California Weather Data: Set Dates and Variables–UC IPM ( It’s the other calculations that I’m not sure about.

Total Water Replacement is the how much water will need to be applied to compensate due to the loss of over ET in the month of August.

I also add comments to the table to explain the calculations

Well, 15.8" of Total Water Replacement means an average of 15.8/30 = 0.53" per day ET. OMG, I hope that isn’t true! You averaged 0.22 for the days you listed. 0.53 is well over double that! Which is why the number made no sense to me.

Ah, I see:

ET does not get multiplied by root depth. ET is unrelated to root depth. It is the water used by the plant/lawn and evaporated by the plant/lawn and ground. Root depth is proportional to the water stored by the ground and available to the plant.

If you want to know the total water replacement for a month you’d total the ET for each day of the month (let’s say that’s about 0.22 x 30 = 6.6" per month), then multiply by area and convert to gallons to get 4114 gallons per month for that zone. Of course that will vary each month as the ET values vary.

Again, ET per month makes no sense to me. 2.6 is about right for 12 days. I don’t know what the 3.71 is.

I may be wrong, but I think there’s apples and oranges mixed in there.

I see. I thought ET numbers were normalized to 1" of soil depth.

I grabbed the data from Aug 1st to Aug 27th, but it looks like the data has gaps in it. In effect, there is only data for 17 days (and that totals to 3.71). So, I need to extrapolate from this: 3.71/17*30=6.5. So the August ET0 is 6.5.

I’ve rerun the numbers (using 1" root depth, so I don’t have to change formulas in Excel):

Total ET0 for August: 6.55
Warm Season Grass Kc: 0.71
Warm Season Grass ET (month): 4.65
Root Depth (in): 1.00
Total water replacement (in): 4.65
Area (sq. ft.): 1,000.00
Area (sq. in.): 144,000.00
Replacement water volume (gal): 2,897.71

If the calculations are correct, then it should take ~3,000 gal to water a 1,000 sq. ft. lawn in inland Southern California. It seems this month I’ve used at least 5 times the volume on my lawn zones. Now I have to look for where all that excess water is going.


No, all ET is total water used by the plant during a day (or whatever).

That makes sense. The the 2.6 and 15.8 calculations do not, to me.

Okay, looks like you’re taking the ET values and multiplying them by the Kc. That’s backwards. The ET is calculated by the FRET value to get the ET. Either way, ET is the actual water you’re using, except for efficiency, each time you water.

Using your total ET for August on 1000 square feet of area:

Gallons per month = 6.55 x 1000 x 144 / 231 = 4,083 Gal/Mo

To take efficiency into account, let’s assume 80% efficiency, so flow is divided by:

 Efficiency Factor = 0.4 + 0.6 X Effiency = 0.4 + 0.6 x 0.8 = 0.88, so

Actual Gallons per Month = 4083 / 0.88 = 4,640

Now remember, that is the water needed for the particular month time you measured for each 1,000 square feet. If you actually have 3,000 square feet total, you’d multiply it by 3.

I was sloppy with terminology. The date table shows reference ET. So the 6.5 number is the ET0. It was my understanding that ET0 has to be multiplied by Kc.

Well, if ET0 is equivalent to FRET, then that is true. I was considering these ETs to be comparable to Rachio’s, which already has Kc included.

You still need to account for efficiency and total square feet, though.

True, I neglected accounting for the efficiency setting.

Having gone through this exercise, though easy enough, I wish Rachio would make the total water volume for a month easily available for referencing with water meter. They already have all the data, just need to make it visible. Better yet, have a screen where those of us without an integrated water meter can regularly enter the readings.

Thanks for the help

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