This information was taken from Dr. Lenny Wells' Pecan BlogThe nut sizing period normally occurs from May 1 through August 15. Although not a critical water use stage for pecan, serious drought conditions during this period can affect yield. The most common visible effects of an extended drought during this period are excessive nut drop and “shell hardening” on small nuts. Lack of sufficient water during the nut sizing period causes small nuts and may lead to water stage fruit split, resulting from a sudden influx of water during the nut filling stage in some varieties.
The nut filling stage occurs from about August 15 to the first week of October, depending on variety. The most critical period for water use is during the first two weeks of September. Lack of sufficient water during the nut filling stage will lead to poorly filled nuts, which will result in poor nut quality.
Reports from other areas of the country indicate that as much as 350 gallons of water per day can be required by each tree during the nut filling stage. Based on this recommendation, if a mature orchard has a plant density of 12 trees per acre (60' x 60' spacing), then 4,200 gallons per acre per day may be needed. For a density of 20 trees per acre (46.5' x 46.5'), 7,000 gallons per acre per day may be needed.
Pecans can have high water requirements, using as much as 60 inches of total water (including rainfall) during the growing season. Drip and micro-irrigation system design capacity for a mature pecan orchard should be 3,600-6,000 gallons of water per acre per day.