In most environments, trees will be thrive on natural rainfall alone. Trees will need supplemental water when first planted, when young and during droughts. Some areas are prone to droughts on a regularly and should have watering systems in place at all times. According to Colorado State University, the root systems of most trees are between 6 and 24 inches deep so, that is where you must aim to get the water. When considering the best water system for trees think size first.
Young trees, recently planted trees, need the most help with extra water, even with normal rainfall. These are trees with a circumference of less than 7 inches. The best methods for watering these trees are automatic drip, delayed drip (bucket method), soaker hose and soil needles. The automatic drip is a long hose perforated at regular intervals of about 1 inch along its length laid out in a circular pattern around the tree, starting at the base of the trunk and ending at the edge of the canopy shade and hooked up to a water source for a constant trickle. The roots of a tree at any age usually extend underground as far out as the branches reach out overhead, creating the canopy. The delayed drip method consists of three or more large 5-gallon buckets set on the ground in a circle under the tree canopy. Each bucket has several holes punctured in its bottom so water can drip out slowly.
Medium trees have a circumference between 8 and 15 inches. They have more resilience and usually survive on rainfall, with help during droughts or periods of minimal rainfall. The same methods used above with small trees work well for medium trees, along with the spray method, which is simply hooking a spray nozzle up to a hose, connected to a water source, and giving the tree a good healthy drink manually. The soil needle is another method to get water directly to the roots. The soil needle is a spike hooked up to a water source that is planted at the base of the tree so the water is carried down by the needle and released at the roots.
Trees with a circumference over 16 inches rarely need extra water, except in the direst circumstances. In times of severe drought, or areas where severe drought is a regular occurrence, large trees do best with a soil needle or direct spray to ensure they're getting enough water.