A recent irrigation seminar from the University of California and the Almond Board discussed various methods to determine when/how to irrigate crops. There are 3 main methods: weather-based, soil-based, and plant-based.
The most widely used method is weather based evapotranspiration (ET), which estimates the amount of water used by the plants. ET serves as a great ballpark number to start thinking about irrigation, particularly because the information is readily available and free. We highly recommend using ET to help guide your irrigation decisions. Still, we caution growers to verify and fine-tune ET information with other kinds of measurements, either from soil or plant measurements. Unfortunately, ET has various drawbacks as explained in the University of California slide. The crop coefficients, critical to use of ET for scheduling, were developed in the '80s and '90s with infrequent irrigation methods and old-school management practices that produced lower yields. 30 - 40 years later, grower practices have changed dramatically and the crop coefficients previously developed no longer apply. In our experience, irrigation based solely on ET tends to dramatically over-water the plants, leading to suboptimal crop health and yields.
We thus highly recommend you check out soil sensors, a pressure chamber, or the FloraPulse automated plant sensors. Your call!