This is a followup to what I posted previously related to a “crop quality” offset to the new Dynamic CC feature. Texas A&M is looked to as the horticulture expert in our area as they have been involved in agriculture research in Texas longer than anyone. Below is their statement on Crop Coefficients, in Texas, for turf grasses. (See ET and Weather Data → Turf Coefficients at https://texaset.tamu.edu/ for source.)
First, note that the starting Tc of 0.6 is a lot lower than the one picked by the Rachio Dynamic Coefficient feature for my area (0.8). Second, note the statement that the Tc remains constant throughout the growing season for established lawns. A&M recommends no water during the dormant season for warm weather grasses in my area. Rachio might want to somehow work with Texas A&M when developing their “proprietary” settings.
Lastly, see the quality offset (Qf). This is the offset that could be added as a feature. Using this table, I set my Crop Coefficient at 0.36 for my Bermuda/St Augustine lawn during the growing season. This seems to pretty much put me in line with recommendations from A&M’s “Water My Lawn” update service.
Below is from Texas A&M:…
Turf Coefficents
Potential Evapotranspiration, ETo (also abreviated as PET), is an estimate of the water requirements of a 4-inch grass growing in a deep soil under well-watered conditions. A turf coefficent (Tc) is needed to relate pET to the water requirements of a specific turf. For established lawns, the Tc remains constant throughout the active growing season of the grass. For warm season grasses, such as St. Augustine, the Tc is 0.6 throughout much of the year, while the Tc for cool season grasses, such as rye, is 0.8.
However, we seldomly apply this amount of water to lawns. Why? Because we do not want maximum production of grass clippings. Instead, we want to maintain a healthy, attractive turf with as little water as possible. Thus, we modify the coefficent by specifying a Quality Factor.
The complete equation is:
ETo x Tc x Qf = turf water requirement
Turf Coefficient Values (Tc)
| Warm Season | 0.6 |
|---|---|
| Cool Season | 0.8 |
Quality Factor (Qf)
| No Stress | 1.0 |
|---|---|
| Low Stress | 0.8 |
| Normal Stress | 0.6 |
| High Stress | 0.5 |
| Very High Stress | 0.4 |