Cool season grass crop coefficient *new* default 80%

[franz]

When we rolled out the ability to adjust crop coefficients in the 2.5 release we set the default cool season grass crop coefficient to 70%. After some more research and empirical evidence the new default for cool season grass is 80%.

How does this affect existing users? Existing cool season grass zones will not get this new default unless manually adjusted.

Zone --> Advanced Settings --> Crop coefficient.

How does crop coefficient affect flex scheduling?

The best lever for adjusting watering frequency. Each crop type has a coefficient. Crop coefficients are properties of plants used in predicting evapotranspiration (ET). The coefficient is the ratio of ET observed for the crop. If ET is our zone checkbook debit, crop coefficient will offset ET slightly and not remove as much from our account.

Example: ET for the day is .2 inches. Crop coefficient for cool season grass is 85%. The debit for the day is .17 inches (.2 x .85)

Increasing crop coefficient increases watering frequency, decreasing crop coefficient decreases watering frequency.

Recommend moving the crop coefficient lever +/- 10% increments and see if that gets watering to the correct cadence. Crop coefficient can be modified from Zone Menu > Advanced Settings > Crop Coefficient

[bug99]

Although I am no expert, I felt it would help others to clarify what I believe the Crop Coefficient to be and how it is implemented here (slightly different).

The crop coefficient is a single variable correction or adjustment if you prefer to “correct” or take into account the variances between different plant’s moisture absorption (and then evaporation) relative to a standard baseline plant ET rate, at it’s various stages of vigor. That baseline plant is normally chosen to be cool season grass, which is the ET0 provided by weather/crop services for a day/month/season and year. Multiply ET0 by the crop coefficient K to get the crop ET of ETc.

ETc = K* ET0

As you would expect a plant uses more water when it is healthy and vigorous compared to young or poorly growing. A higher K is needed for a more vigorous plant. Typical values for this coefficient throughout the year (relative to cool season grass) vary from 0.2 to 1.2.

I believe that Rachio presently only allows the crop coefficient to vary from 0% to 100%. This should be extended to 120% in future releases to better fit with standard crop tables, and to give better adjustability with this powerful irrigation variable. Picking the right constant is further complicated in that the cool season grass constant is defaulted to 80% vs 100%. By having it at 80% and making another crop at 100%, would effectively give the full range (.2 to 1.2) as in the crop tables, but not allowing easy data entry as would be the case if based (normalized to cool season grass as is the standard.

Increasing the crop coefficient results in more water per week, by increasing the frequency without reducing the duration for a calculated ET from the weather data.