Here is some information from UGA Extension Pecan Specialist Dr. Lenny Wells:
Some early pecan varieties have reached shell hardening and others are still approaching. This means that the water use requirements of pecan are nearing peak demand and will remain so through kernel filling. Growers should have irrigation systems running at full capacity at least by next week for most mid-season varieties like Stuart, Desirable, Schley, etc. This means 3600-4000 gallons per acre per day. Solid-set sprinkler irrigated orchards should be applying 1.5″-2″ per acre week, depending on soil type (the sandier the soil, the more water). Ideally, systems should be designed to meet these water requirements within 12 hours.
Its amazing how much water a pecan tree with a heavy crop load can use at this time. The heavier the crop, the more water required. Sure, you can still make some nuts and have a decent harvest with less water but this will maximize percent kernel, which is required for good pecan prices. If the trees don’t have a crop, you won’t have to water this much because it is the kernel-filling process that creates the enormous demand for water. Adequate irrigation at this time will also relieve undue stress on the trees, which helps them to to return a better crop the following year.