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Different plants have different water requirements, so they have different ET rates.
Since there are thousands of cultivated plants, we have tried to simplify matters by establishing a standard ET rate for general reference and use. This is the potential ET since we are assuming the crop is in a deep soil and under well watered conditions. The standard crop we are using is a cool season grass which is 4-inches tall. ETo depends on the climate and varies from location to location. The water requirements of specific crops and turf grasses can be calculated as a fraction of the ETo.