| Tule Actual ET Sensors | Reference ET | Weather Station | Soil Moisture Sensors | Sap Flow Sensors | Pressure Chambers | Porometer | Aerial Imaging | Models | “Look & Feel” | |
| Basic Operation | Measures actual evapotranspiration in your field | Measures evapotranspiration of a well-watered grass field nearby | Measures environmental factors | Measures soil water availability | Estimates movement of water through trunk | Measures leaf and stem water potential | Measures stomatal conductance | Broad scale visible and thermal images | Simulates potential crop water use and/or water stress based on input parameters | Look at plant appearance or feel soil |
| Provides Quantitative Measurement | Yes | No | No | No (unless rigorously lab calibrated) | No (unless rigorously lab calibrated) | Yes | Yes (calibration required) | No | No (unless rigorously calibrated each season) | No |
| Measures Plant Water Status | Yes | No | No | No | Yes | Yes | Yes | No | No | Yes |
| Monitoring Labor and Frequency | Automatic, Daily | Automatic, Hourly | Automatic, Hourly | Manual or Automatic, Hourly | Automatic, Hourly | Manual, Infrequently | Manual, Infrequently | Automatic, Weekly or less | Automatic, Weekly (although requires continual manual calibration) | Manual, Infrequently |
| Measurement Zone | 1 to 10 acres | Nearby, not in your field | Many acres | 2" from sensor | Single Plant | Single plant | Single Leaf | Many acres | Per Block | Single plant |
| Replacement & Maintenance | Included | None | Annual calibration required | 3-4 years, some require annual removal | Included | Minimal | Daily calibration required | None | Minimal | None |
| Challenges | Minimum measurement area is 1 acre. |
ET is not measured in the grower’s actual field. Crop coefficients can be inaccurate. Does not measure crop stress. |
Weather factors alone do not tell the grower about the water use or status of the crop. Reference ET calculated from local weather factors provides inaccurate estimates for crop types other than well watered grass. |
For crops with an extensive root zone (i.e orchards or vineyards), soil moisture measured at a particular depth may not be representative of available water. Multiple devices required to represent field conditions. |
Not applicable for all crop types. Multiple devices required to represent field conditions. |
Labor intensive. Measurements can only be taken during narrow time windows. Multiple measurements required to represent field conditions. |
Multiple measurements required to represent field conditions. Measurements highly influenced by weather conditions. |
It's too late. Once there is a decrease in greenness, there will be a reduction in yield that is unrecoverable. Without turbulence measurements, imaging alone cannot tell the grower the water status of the crop. |
Only as accurate as the data put into the model. Recurring in-field measurements required to confirm model is on track. |
It's too late. Visual cues occur after growth has slowed. Cannot reliably reproduce timing and degree of stress year after year. |
