Consideration to make the calculation more precise

[acarlo79]

The calculation can be affected by the type of grass. Different types of grass have different water requirements, which are accounted for by the crop coefficient (Kc). The crop coefficient varies depending on the type of grass and its growth stage. This coefficient adjusts the reference evapotranspiration (ET0) to reflect the actual water needs of the grass.

Understanding Crop Coefficient (Kc)
Cool-season grasses (e.g., fescue, bluegrass):
Typically have a higher Kc value, often ranging from 0.7 to 0.9 during their peak growing season.
Warm-season grasses (e.g., Bermuda, zoysia):
Generally have a lower Kc value, usually ranging from 0.6 to 0.8 during their peak growing season.

Adjusting the Calculation
To account for the grass type, we need to adjust the ET0 with the appropriate Kc value.

Daily ETc (Crop Evapotranspiration):
ETc = ET0 × Kc
Calculate the effective water requirement:
Adjusted Daily ET = 3 mm/day × Kc

Example Calculations
Cool-season Grass (Kc = 0.8)
Adjusted Daily ET:

ETc = 3 mm/day × 0.8 = 2.4 mm/day

We should take into consideration also the type of soil:

Determine Soil Water Holding Capacity:

Understand the available water capacity (AWC) of your soil. For loam soil, this is typically 150-200 mm/meter.
Calculate how much water your soil can hold in the root zone of your grass (usually 15-30 cm deep).
Example: For a 30 cm root zone in loam soil with 200 mm/m AWC: 200 mm/m * 0.3 m = 60 mm.

To avoid overwatering and encourage deep rooting, water deeply but infrequently.
Determine the allowable depletion level. Often, watering is done when 50% of the available water has been used.
Example: If the AWC in the root zone is 60 mm, you would water when 30 mm has been depleted (50%).

Look at this example:

Steps to Determine Watering Needs:

Evapotranspiration Rate (ET):
Daily ET = 3 mm/day.

Total Rainfall:
Rainfall over 2 days = 100 mm.

Soil Water Holding Capacity:
Clay soil typically has an available water capacity (AWC) of about 200 mm/meter.

Root Zone Depth:
Assuming the root zone depth for a lawn is 30 cm (0.3 meters).

Calculate the Available Water in the Root Zone:
AWC for clay soil in the root zone = 200 mm/meter × 0.3 meters = 60 mm.

Consider Effective Rainfall:
Rainfall received = 100 mm.
Since the soil's AWC is 60 mm in the root zone, any additional water will likely lead to runoff or deep percolation. Therefore, effectively, the soil will only use up to its capacity of 60 mm.

Calculate Depletion Rate:
Daily ET = 3 mm/day.
Days until the soil moisture is depleted = Available water / Daily ET = 60 mm / 3 mm/day = 20 days.

However, considering the rainfall:

Adjust for the Rainfall:
Rainfall far exceeds the soil's AWC. We consider only the 60 mm that the soil can effectively utilize.
Days provided by effective rainfall: 60 mm / 3 mm/day = 20 days.

Total Days Without Watering:
With effective use of rainfall and the high daily ET, you can go up to 20 days without watering, considering that the soil's water holding capacity is the limiting factor, not the total rainfall.

Considering Grass Type (Cool-season vs. Warm-season)

Cool-season Grass (Kc = 0.8)
Adjusted Daily ET:
ETc = 3 mm/day × 0.8 = 2.4 mm/day.

Effective Rainfall Contribution:
Total AWC used: 60 mm (limited by soil capacity).
Days provided by effective rainfall: 60 mm / 2.4 mm/day ≈ 25 days.

Warm-season Grass (Kc = 0.6)
Adjusted Daily ET:
ETc = 3 mm/day × 0.6 = 1.8 mm/day.

Effective Rainfall Contribution:
Total AWC used: 60 mm (limited by soil capacity).
Days provided by effective rainfall: 60 mm / 1.8 mm/day ≈ 33 days.

Conclusion
For cool-season grass (Kc = 0.8), you can go approximately 25 days without watering after 100 mm of rainfall, considering the soil's water holding capacity.
For warm-season grass (Kc = 0.6), you can go approximately 33 days without watering after 100 mm of rainfall, considering the soil's water holding capacity.
These calculations assume optimal conditions where all rainfall is effectively utilized and there is no additional water loss due to runoff or deep percolation.

So, to summarize, I would:

1. Create predefined buckets that match the soil type and this would be the max bucket size ( Clay soil = 60mm, Sandy = 22.5mm)
2. Possibility to select cool season grass vs warm season grass when doing the final calculation and taking in account the time of the year to adjust Kc accordingly

I think such options will further enahnce this tool.

[jeroenterheerdt]

Very cool, thank you. Any chance you could make a PR for this? I have very limited time - down to an hour per week - to work on this so would like to get some help.

[acarlo79]

I will see what I can do....not much time on my side as well :)

[acarlo79]

After some further testing, I believe you can leave everything as it is and just improve the documentation with something like this:

Max bucket size reccomanded based on type of soil:
Clay soil = 30mm
Sandy soil = 12mm

The above is reccomanded based on the following:

Determine Soil Water Holding Capacity:

Understand the available water capacity (AWC) of your soil. For loam soil, this is typically 150-200 mm/meter.
Calculate how much water your soil can hold in the root zone of your grass (usually 15-30 cm deep).
Example: For a 30 cm root zone in loam soil with 200 mm/m AWC: 200 mm/m * 0.3 m = 60 mm.

To avoid overwatering and encourage deep rooting, water deeply but infrequently.
Determine the allowable depletion level. Often, watering is done when 50% of the available water has been used.
Example: If the AWC in the root zone is 60 mm, you would water when 30 mm has been depleted (50%).

Multiplier reccomanded based on grass type:

Cool-season grasses = 0.8
Typically have a higher Kc value, often ranging from 0.7 to 0.9 during their peak growing season.
Warm-season grasses = 0.7
Generally have a lower Kc value, usually ranging from 0.6 to 0.8 during their peak growing season.

Also I would reccomand to water based on an automation that will be triggered and check the following conditions:

Trigger: every day at 5am
Condition: duration is > 0 and day of the week Monday (this will trigger the automation once a week and if water is needed.
or when bucket is < -25mm

[jeroenterheerdt]

thanks - i will add this to the readme!

[jata1]

This is very interesting. Thank you @acarlo79

Does this only apply to grass? I am using smart watering to water a tropical garden with a range of different plants - not grass.

[acarlo79]

the soil rule would be the same, but the multiplier for the plants would be differrent based on the plant type.

[lynxchat4]

Good day!
Thank you for your research - it was very useful and informative and these are the questions that have come to mind.

But I think that the approach to the "bucket" needs to be radically changed!
We should water in the range of moisture saturation from 50% to 100%.

But when we work with a bucket in the range of both "-" and "+", it turns out that the maintenance works correctly only if the rains do not overflow the bucket more than 0.

Let's look at an example:
Bucket limitation 30 mm, irrigation once every 4 days. Water consumption 3 mm per day.

First step - irrigation until the moisture saturation level is 100%, the bucket = 0.

If was no rain for a period of 4 days, then everything will work: the moisture will be taken away exactly as much as the smart watering calculates: 3mm * 4 days = 12mm

But if the next day after watering there was a 10mm rainfall.
The calculation will not work correctly:

The calculation will not work correctly:
The bucket state day by day:
After irrigation: 0mm
1 day: bucket = -3mm (Evapotranspiration)
2 day: bucket = -3mm(1 day state) + 10mm(rainfall) -3mm(2 day Evapotranspiration) = 4mm
But if we consider that after irrigation the moisture saturation was 100%, 6mm of rain will go to compensate Evapotranspiration, and 4mm will flow into the drain.
3 day: bucket = 4mm(2 day state) - 3mm(3 day Evapotranspiration) = 1mm
4 day: bucket = 1mm(3 day state) - 3mm(4 day Evapotranspiration) = -2mm

Irrigation will add 2mm of precipitation to make the bucket 0.
But to make the moisture saturation 100%, 6mm is required.
It turns out that we did not receive 4 mm of precipitation, which went into the drainage after the rain.

After some time we will get a moisture saturation balance at the level of 0%-50%.
BUT "We should get range of moisture saturation from 50% to 100%."

Therefore, I propose to remove such a parameter as Maximum bucket and make a new parameter moisture capacity.
Next, make a variable that will take the values ​​0 or 1 and say that it is time to start watering in this zone (when the moisture saturation is about 50%).
And make a threshold adjustable in percentage of the moisture capacity, when this variable will change. To make it convenient to work with soils where more than 40% of moisture saturation can be lost in 1 day.

Now I'm implementing this method with triggers, but if it's built-in, it will be much more convenient.

Thank you for your attention. Translated via Google, so if the idea is not very clear, write to me.

[lynxchat4]

Со временем, я уверен, это как-то сбалансируется.

maybe in a cold climate it is not so noticeable.
In our summer we had evapotranspiration of 6 mm. I have watering on Monday and Thursday. When there was a heavy rain on Tuesday, the watering does not work until the following Monday, and the soil dries out. It turns out that the moisture capacity with such a high evapotranspiration is not enough for 5 days.

[acarlo79]

You could set bucket max size to 0 and then via automation water every 2 days or if bucket reach -30mm. Obviously, you want change the 30mm depending on soil type.

[lynxchat4]

You could set bucket max size to 0 and then via automation water every 2 days or if bucket reach -30mm. Obviously, you want change the 30mm depending on soil type.

Yes. It is a best easy way.
I make a more complex trigger for myself so that I don't have to water every 2 days. Otherwise, moss starts to grow.
I'll post it here when it's ready.

[yestrdave]

Guys I'm down here in South Florida trying to make this program make senses. Im guessing it has to do with my lack of understanding . We are dealing with St Augustine grass (Floratam) it has a typical root depth of 4-6 inches. Holding Capacity of the soil here is about 1 inch per foot which would make the bucket size about 0.5 inch. The drainage is a big variance here 1.0 inch per hour is on the low side. I am using that. I have also found the Kc Values for each month in my area December is .45 Max in May .90 . I cant see how to change this in the program and made a general assumption I could reduce the multiplier since there is a service call to do so. The watering values seam really high. Any help would be appricated helping me get this set up properly. I'm irrigating 5.5 acres and consume a ton of water. I am feeding the data from a Davis Vantage Vue with solar radiation. I have 18 zones showing 3 of the better ones. I just manually calculated values about 39 points since last night and a rare occurrence we got about .16 inches of rain last night. Can you guys comment on my set up?

[jeroenterheerdt]

Guys I'm down here in South Florida trying to make this program make senses. Im guessing it has to do with my lack of understanding . We are dealing with St Augustine grass (Floratam) it has a typical root depth of 4-6 inches. Holding Capacity of the soil here is about 1 inch per foot which would make the bucket size about 0.5 inch. The drainage is a big variance here 1.0 inch per hour is on the low side. I am using that. I have also found the Kc Values for each month in my area December is .45 Max in May .90 . I cant see how to change this in the program and made a general assumption I could reduce the multiplier since there is a service call to do so. The watering values seam really high. Any help would be appricated helping me get this set up properly. I'm irrigating 5.5 acres and consume a ton of water. I am feeding the data from a Davis Vantage Vue with solar radiation. I have 18 zones showing 3 of the better ones. I just manually calculated values about 39 points since last night and a rare occurrence we got about .16 inches of rain last night. Can you guys comment on my set up?

I'd start with getting rid of the drainage rate fully and adding it back in increments of 0.1 to see if you end up with a comfortable level.