My nephew recently visited me and asked for advice on lawn irrigation. He has a couple of soil moisture sensors placed all over his lawn in the root zone (about 10 cm deep) and is wondering how to adjust the threshold for irrigation. My first simple advice was just to observe the lawn and identifiy the water content when the leaves are getting brown. Brown leaves are an indication of water stress. My nephew was not really satisfied with this answer and said: "Grandma, you have such a long experience in gardening, you have a so-called green thumb, but I am a real novice. Could you please help me so that even I can adjust he irrigation threshold without observing the leaves". So did a survey and found this guide from the University of Florida on Smart Irrigation This is the sum up of their recommendations:
In any case I would always observe the lawn and see if it is in good shape with this irrigation threshold setting.