| Method | Principle | Advantages | Limitation | Level of Adoption by Industry |
| Historical potential evapotranspiration | Weather data from the past 30+ years are averaged to estimate ETo | IFAS recommended method crop water use (ETc) simply calculated as ETc = Kc × ETo, where Kc is the crop coefficient | Year to year variability may be ±20% of the historical average most Kc values available are for bare-ground production | None |
| Real time potential evapotranspiration | ETo is computed daily using site-specific, current weather data | Data more available as the FAWN system expands Increasingly attractive as the cost of small, on-farm weather stations keeps decreasing Crop water use (ETc) simply calculated as ETc = Kc × ETo, where Kc is the crop coefficient. Variable Kc allows daily irrigation adjustment depending on crop age and weather demand. Likely to be part of BMPs | Most Kc values available are for bare-ground production | Currently limited, but with real potential |
| Class A pan evaporation (Ep) | ETo is related to water loss from a free water surface | Crop water use (ETc) simply calculated as ETc = CF × Ep, where CF is the crop factor. For practical purposes, CF and Kc can be interconverted Principle can be used with pans other the expensive class A pan Variable Kc allows daily irrigation adjustment depending on crop age and weather demand Possible alternative BMP method | Most CF values available are for bare-ground production Old method that was not adopted widely | Virtually unused; should be replaced by the method above |
| Atmometers | Water loss from a ceramic plate with a canvas cover mimics ETo | Simple principle: water loss from a small surface closely estimates ETo Units are rather inexpensive | Calibration data usually not available | None |
| Empirical methods | Rely on experience and individual knowledge to estimate irrigation needs | Simple to implement Most farmers’ favorite | Based on experience, rather than science Typically results in over irrigation early in the season, and sometimes under-irrigation during peak demand periods Likely to be insufficient in the BMP era | Industry standard |