Water is essential to all plant life -- but trees, the giants of the plant world, require exceptional amounts. A tree large enough to provide shade usually is well established with a good root system and has a large area upon which to draw to satisfy its thirst. If it doesn't rain for quite a while, the tree doesn't suffer much. But a newly planted tree may take four or five years to develop a good root system in the new location; it needs to be watered every week there is less than an inch of rainfall to make up the difference. If it suffers from drought for the first three weeks of a month, it may not respond if the rainfall catches up to normal during the last week of the month. How you water is critical, according to Robert L. Tate, Rutgers University professor of forestry. Never water more than once a week, and when you water do it generously in one application, letting the moisture move downward through the soil to moisten the ground to about one foot in depth. Tree roots also need oxygen to survive, and constantly soggy soil will cause problems. If the soil surface is quite dry, you may have to supply the water in two phases: lightly to soften the surface so you won't waste water by runoff, then heavily enough later to get the water to the root zone. With a critical water shortage in many parts of the country, there are several things that can be done to conserve water and still provide an adequate supply to the tree roots. First, keep track of rainfall accurately on a week-by-week basis yourself. Amounts of rainfall are usually measured for specific locations, which may be miles from your tree. One side of a city may have had plenty of rainfall while the other may have been missed by the same storm. Home rain gauges are inexpensive and accurate enough for your purpose. If the tree is planted on the lawn, eliminate grass from a 12-foot circle around it. Then the tree roots won't have to compete with the grass roots for water and nutrients. For tree roots to grow and increase in length, they must invade soil already occupied by grass roots. There is competition for space, and the grass roots have the advantage because they are all well developed and able to hold their own for a year or two or even longer. Mulching newly planted trees will help prevent water loss and keep the soil cooler which promotes root development. In addi- tion, the mulch will enable you to use less water.