Model of water and nitrogen management in pecan trees under normal and resource-limited conditions

Nut production from pecans, almonds, and pistachios figures heavily in the economies of California, Texas, and New Mexico, as well as several other states, and surface irrigation water supplies have been reduced in low runoff years in the western United States. Water and nitrogen management in tree crops is constrained through lack of information and inability to provide targeted management. The goal of this research was to develop an improved management pecan model to monitor and predict water and nutrient demand and nutrient status in pecan trees, along with the interaction of nutrient and water stress on nut yield. The pecan nut tree model developed by Andales et al. (2006) had a nutrient uptake and allocation and nutrient stress subroutine added to the model to predict the interaction of water and nutrient stress (nitrogen and potassium). The nitrogen submodel presented simulates the interaction of nitrogen transformation, soil temperature, water, and nitrogen uptake to describe nitrate distribution in the root zone of a growing pecan tree for the entire growing season. The nitrogen submodel follows the nitrogen transformation equation developed by Jones and Kiniry (1986) for the CERES-maize model. The nitrogen root uptake submodel follows the approach developed by Yanai (1994), which is a model of solute uptake that accepts root growth, water uptake, and soil solution concentrations as time-varying inputs that interactively link plant and soil processes. The model was tested against a nitrogen–potassium water stress experiment conducted by Smith et al. (1985) in Oklahoma from 1978 to 1983 where the pecan trees received varying rainfall amounts, the only source of water, and four levels of nitrogen application. The measured yield response represented a water nitrogen stress response. The model over-predicted the yield by 42% compared to the measured yield, but the model under-predicted yield by 21% in 1981 with 0kg/ha of applied N and under-predicted yield by 13% when 224kg/ha N was applied that year. The current pecan model appears to simulate water stress reasonably well but may overestimate the nitrogen uptake by the pecan trees and underestimate the reduction in yield caused by nitrogen stress. The model also may overestimate yield through lack of an insect damage submodel.