If the soil is drier than this, tree and fruit growth may decrease. If moisture demand by transpiration is greater than the roots can supply, water shortage in the tree will occur. Moisture then will be transferred from the fruit to satisfy the tree's demand. Fruit size decreases and, in extreme cases, the fruit may shrivel when dry trees are forced to pull water out of the fruit. Also, roots cannot pick up nutrients readily if the soil is too dry.
Trees vary in size and vigor, and the crop size varies from year to year.
Also, temperatures and humidity vary during any one month and between years. Therefore, it is impossible to predict the exact amount of water your trees will need. It does not take into account inefficiencies such as runoff, deep percolation, evaporation, etc. Start irrigation before trees exhibit effects of water shortage. To avoid wasting water or under-irrigating, find out how much water has been drawn from the soil before you irrigate.