LSZ, that is absolutely right. That's why pecans grow best in areas with shallow water tables and bottom land around creeks and rivers. Here's some interesting numbers from the Noble Foundation website.
"If we say a mature pecan tree requires 55 acre-inches per year, and most of this water is used during the growing season (April through October or 225 days), the daily water use can be predicted as follows:
55 acre-inches per year x 27,154 gallons per acre-inch = 1,493,470 gallons per acre per year
1,493,470 gallons ÷ 225 days of growing = 6,638 gallons per acre per day
6,638 gallons ÷ 35 trees per acre = 190 gallons per tree per day
The standard recommendation for water requirements of pecan trees is 1 to 2 inches per week and can be calculated into daily water requirements like this:
1 acre-inch x 27,154 gallons per acre-inch ÷ 7 days = 3,879 gallons per acre per day
3,879 gallons per day ÷ 35 trees per acre = 111 gallons per tree per day
(Stein, 1994)
Remember, trees get water from the soil, and the deeper the soil, the greater the water-holding capacity. For example, a tree growing in a soil 7 feet deep has the potential of 9,000 gallons of available water per tree compared to a 7-inch-deep soil, which has the potential of only 800 gallons of available water per tree. Therefore, for a mature tree that uses 100 gallons per day, the 7-foot soil can supply water for 90 days while the 7-inch soil can supply water for only eight days (McEachern, 2006). Another factor that plays into water-holding capacity is the soil type. Sandy soils have less waterholding capacity than loams or clay soils. So, a 7-foot loamy soil holds more water than a 7-foot sandy soil."
