Watering Pecan Trees              Alternative Orchard Irrigation Methods
If you don't do anything else 'by the book' when it comes to caring for your pecan trees, water...water...water.
I will be the first to tell you that it's ok to under-fertilize, to miss a zinc application, to let insects get a little out of hand, but I will not give you my 'thumbs up' when you decide not to water your trees properly.  If you water as you should, and do nothing else to your trees the entire growing season, chances are good that you will produce an acceptable pecan crop.
To calculate the amount of water needed, various factors need to be taken into account.  Naturally, the size of your tree(s) plays a factor.  The time of year, of course, must be considered.  The type of soil your tree(s) are planted in.  It's a no-brainer that smaller, younger pecan trees will require less water than mature trees, however, do not underestimate their water requirements.  A mature tree has an extensive root system to harvest moisture from a substantial area while a young tree's roots may only spread two or three feet from the trunk.  A safe starting point when determining water needs of a newly planted tree during its first growing season is one gallon of water per day.  This is not to say you should pour a milk jug of water on your tree daily, but when you do water (weekly is good for young trees,) water sufficiently to carry your tree through till the next watering.  For the first three years of your tree's life, I recommend at least one gallon of water per day for each year of your tree's age.  From age four to age seven, double that requirement, except during the hottest months of the growing season.  Then you should double what you just doubled.  Lost?  At age three, your tree should receive three gallons of water per day. (twenty-one gallons per week or forty-two gallons if you water bi-weekly.)  At age four, your tree should receive eight gallons of water per day, except when the temperatures approach ninety-five degrees or higher (or winds deplete the soil moisture.)  Once hot weather sets in, your four-year-old tree should be receiving around sixteen gallons of water per day.
By age seven, your pecan tree is beginning to reach 'puberty,' that is, it is in its pre-production stage of growth.  Your tree is larger (taller, wider) so more energy is required.  Your tree is approaching the nut bearing stage in its life which will require substantial amounts of water.  You must now determine how much water it will take to keep the entire area moist beneath the canopy area of your pecan tree...to a depth of four feet.  Your initial watering every spring should be a deep watering to saturate the soil beneath your tree.  After that, you need to water whenever 60% of the initial water has been depleted.  To judge when to water, you must take the soil type into consideration.  Sandy soil can only store approximately one inch of moisture per one foot of soil depth whereas clay soils store three times as much.  So, if you receive a one-inch rain, and your tree is planted in sandy soil, only the first foot is storing any moisture.  It is important to understand that for the most part, rain just doesn't count when you are talking about watering pecan trees.
I think you should get into the mindset that you probably aren't watering your pecan trees enough.  A producing pecan tree over the age of ten will need upwards of 150 to 300 gallons of water per day during the hottest part of the growing season.  From April thru June, we generally water our trees ten hours per week. (Our 'drip irrigation' system has been modified to more of a 'water hose running full blast' at the base of our trees.)  During July thru September, we water sixteen to eighteen hours per week per tree.  When harvest comes, we can still identify trees that didn't receive sufficient water.   They may have dropped their nuts early, produced smaller pecans, less pecans, or the pecan hulls failed to open properly (sticktights.)
Whether you are contemplating planting a pecan orchard or purchasing an existing one, water availability should be a major factor in your decision.  A pecan orchard must have an unlimited source of water if it is to survive  AND..you must make a commitment to see to it your trees receive the amount of water they will need to produce a sufficient crop of pecans.