Pecan Nut Yield and Tree Growth as Influenced by Irrigation

The objective of this study was to develop methods to schedule irrigations on pecan orchards. The first section of this report presents a description of an irrigation scheduling model for pecans that is based on a water balance approach. In the model a crop curve is used to reduce potential evapotranspiration (Fp) calculated from Penman’s Equation down to non-stress transpiration. The model then reduces transpiration as soil moisture stress occurs. Evaporation and transpiration are extracted from the soil and irrigation and rainfall are added and the new soil moisture content determined. Excess water application goes to deep drainage. The model was tested on a border at Salopek’s Farm and the model tracked the measured soil moisture very closely. It showed that the application exceeded the evapotranspiration (Et) requirement and resulted in an irrigation application efficiency of 0.76 and 0.87 in 1983 and 1984 respectively. The model has the capability of utilizing actual time weather data or simulated weather data and the model simulated seasonal rainfall over 20 years within 14 percent of measured values. When used to simulate seasonal potential evapotranspiration, the model was within 7 percent of the computed Ep using 1983-1984 weather data. The irrigation scheduling model uses a water production function to convert seasonal evapotranspiration to nut yield. All irrigation scheduling procedures that are based on a computer simulation process need to be verified in the field through direct observations. Data is presented on infrared measurements which are used to determine the crop water stress index and relative evapotranspiration which is another method used to schedule irrigations. The relative Et computed with the infrared measurements are linearly related to the measured values in pecan trees growing in small barrels with a coefficient of determination of O.78. The infrared measurements indicated a greater relative evapotranspiration compared to the water balance technique on drip irrigated pecan trees receiving different irrigation levels. Information is presented to show how leaf diffusion resistance measurements change throughout the day during the growing season under moisture stress conditions. Also information on, and how leaf area index is related to the basal area of the pecan trees, is presented. This information can be used in understanding the growth rate of the tree under irrigation.