Water Needs of Pecan Trees: Revisiting McFarland & Worthington’s Lysimeters
If the water is not limited every day of the growing season, then you should achieve the desired results of producing a crop of good quality pecans and making exceptional vegetative growth. If water is limited over enough days, the effects may be seen in reduced shoot growth or even shoot dieback; reduced nut size; poor kernel development; or inadequate shuck opening at harvest. This year, the pecan crop is sizeable in many pecan orchards in Texas, so appropriate watering will be pivotal to the final size and overall quality of the crop.
What is enough water for a pecan tree? Extension fact sheets and reports for pecan irrigation management provide recommendations in acre-inch values. Our own Texas Pecan Handbook chapter on irrigation (authored by Drs. Larry Stein and Jody Worthington) suggests targeting 1 acre-inch per week from budbreak to August, followed by 2 acre-inches per week in August, September and October (or well split-open shucks). New Mexico State University’s Guide H-636, by Ted Sammis and Esteban Herrera, describes the consumptive use for mature orchards as ranging from 1.75 inches per week in early season to 3.5 inches per week in the water stage of nut development and beyond. This same guide shows annual rates of delivery from 35 inches per year to 57 inches per year, depending on tree size and soil water-holding capacity.
On the other hand, with the University of Georgia, pecan specialist Lenny Wells, Ph.D., provides a pecan irrigation schedule at the UGA Extension web that projects pecan tree irrigation rates increasing from rates as low as 0.14 inches per week in April to 1 inch per week in August and September. In another guide, Charles Rohla, Ph.D., with the Noble Research Institute recommends irrigation rates of 1 to 2 acre-inches per week from June to October for Oklahoma and north Texas. So from these four sources, we can determine that there are within-season differences in pecan orchards, with the nut fill period being the time of highest need and rates of delivery from less than a quarter inch upwards to 3.5 inches per week.
Taking the above-cited estimates of water delivery per acre and using a medium-high tree density of 27 trees per acre, one can back-calculate that single pecan trees need anywhere from 20 gallons per tree on the low side in Georgia to 503 gallons on the high side in New Mexico. Dictated by the tree and row spacing, these values move up or down as there are greater or fewer numbers of trees per acre.
With such a range of tree sizes, spacing, soil types and environmental condition differences, how can a grower know whether their point-in-time water delivery amount is adequate, too little or too much?
An important study was conducted in the 1980s at the Stephenville Agricultural Research and Extension Center by agricultural engineer M.J. McFarland, Ph.D., and horticulturist Jody Worthington, Ph.D., to answer the question of daily and weekly water use of fruit trees, including pecans (1983). They constructed a twin-weighing lysimeter apparatus to hold and precisely weigh a live tree and soil in an 8-foot diameter, 5-foot deep cylinder that is positioned below ground and fitted to collect and weigh incoming and outgoing amounts of water with an accuracy of “several liters.”
A lysimeter is a weighing device that accounts for two major losses of water in a rooting zone: evaporation of water from the soil and transpiration of water by the tree. Evaporation is affected by the wind, humidity and temperature at the site. Transpiration, what the tree withdraws from the soil and returns to the atmosphere, is affected by those same factors, in addition to sunlight. Stomates, the pores on leaves that allow water to diffuse out, have greater turgor and opening in high light environments, thus transpiration rates can be higher. Las Cruces, New Mexico—part of the Chihuahuan Desert—is a hot, dry and high sunlight environment, wherein pecan trees would be expected to experience relatively much higher evaporation and transpiration rates than say Central Texas, Southern Oklahoma or Middle Georgia. By knowing the starting weight of the tree-root-soil-water conglomerate in the lysimeter, reductions in weight can be attributed to evapotranspiration (ET), thus providing a high-resolution understanding of daily tree water use.
McFarland and Worthington’s studies were conducted with mature-sized peach trees (15-foot canopy diameter) and similar-sized young, non-bearing pecan trees. By monitoring lysimeter changes in weight as well as other environmental factors at the site over time, they found that pecans use water at a rate proportional to Class A pan evaporation rates.
A Class A pan is a standardized tub, similar to a #2 washtub. With a screen over it to prevent animal consumption, water level loss can be measured daily or more frequently to reflect drying conditions for that orchard. For pecan trees, the lysimeter study found pecan water use in May and September equaled 50 percent Class A pan evaporation, in June it was 80 percent, in July it was 100 percent, and in August it was 90 percent. These measurements reflect the influence of seasonal weather changes.
Class A pan evaporation rates vary by location and are higher in the western states than in the Southeast. While areas in West Texas in July may have evaporation rates of 0.35 to 0.50 inches per day in July, the rates in coastal areas like Alabama and Florida rarely exceed 0.25 inches per day. Current evaporation data can be found at sites like the National Weather Service’s Climate Prediction Center. Historical tables can be found as well as seen in the sample table for four Texas cities in Figure 1.
Worthington would go on to develop a formula for daily pecan water use that is a valuable estimator to discern effects of tree size and weather (Fig. 1). It is a simple multiplication of the following factors:
- Canopy diameter (squared)—The larger the canopy diameter, the more leaves there are on the tree, and thus, the greater the potential for transpiration. This factor is calculated by measuring limb spread across the trunk from east to west or north to south (or both to obtain an average). In calculation A in Figure 1, it can be seen for 30-foot trees for June in Fort Stockton; the daily water use is 169 gallons per day, while the same location with 10-foot trees is only 19 gallons per day. Tree size matters greatly to how much water is used every day.
- Evaporation (inches per day)—For Fort Stockton, June evaporation rates per day are about 0.48 inches per day. By changing the location to Waco in 2011, the evaporation rate goes up to 0.65 inches per day and increases tree daily water use 89 additional gallons. As conditions favor more evaporation, tree water use goes up significantly.
- Evaporation factor (0.5, 0.8, 0.9, 1.0)—This number should be adjusted for the appropriate month as discussed in the text above. The weakness of Worthington’s formula is what to do about moderate to heavy crop loads. His lysimeter work was done with non-bearing trees. For moderate to heavy cropping situations, the evaporation factor could be adjusted upward 25 to 40 percent above the monthly rate to account for the water needed by the developing nut crop.
- 49 is mathematical constant in the formula for converting area covered by the canopies.
Irrigation systems are an important investment for any pecan grower. Water delivery deficiencies at any point in the growing season create tree stress, nutritional deficiencies, reduced energy production, and ultimately less vegetative growth and kernel production. New orchards should investigate the potential for tree water demand at their geographic location by studying both precipitation and evaporation averages.
Extremes happen, such as in the example shown for 2011—a year that many pecan trees died in Texas. Looking back and analyzing potential tree water demand, we see that Worthington’s formula accurately shows that a system designed to deliver 2 acre-inches would have run short in August of 2011, certainly not enough to cause tree death but kernel quality likely suffered.
As the acreage of commercial pecan production expands across the United States, it is possible that pecan growers will face scrutiny for water resource management and conservation. A class A pan is a very simple device to maintain and monitor in any pecan orchard. This simple assessment tool may help growers seize opportunities to reduce water delivery.
There is such a thing as too much with pecan tree irrigation, especially on soils that have internal drainage limitations. Reducing irrigation run times when evaporation conditions are lower can not only save growers money in energy cost but also reduce the potential for overwatering stress that is equally detrimental to pecan tree health as not enough water.