Physiological model of light interception and water use in pecan trees

Evapotranspiration and light interception in unstressed pecan trees were evaluated at branch and whole stand scales for a commercial pecan orchard near Las Cruces, NM during 1996, 1997, and part of 1998. Field instrumentation and on-site measurements of water inputs, soil matric potential, change of water storage, average leaf irradiance and sap flow were performed as part of this study. Measurements of canopy architecture, foliage density, photosynthetic capacity, and leaf microenvironment were also taken to develop a computer model based on physiological parameters of stomatal control.; Simulations of light interception and transpiration at branch scale were run for short campaigns during fall 1997 and 1998. Light interception was also simulated as a function of tree size and orchard geometry during fall of 1998. Predictions of daily and cumulative transpiration for the crop season 1996 and 1997 were also run out to test the model accuracy against measured values and against values estimated by the Penman-Miyamoto model.; The proposed model simulated mean values of leaf irradiance close to the measured in the branch, following the daily trend of leaf irradiance at branch scale. However the results showed that predictions of light interception might be more accurate for whole trees than for a single tree segment. Light intercepted by tree foliage increased as size of the tree increased. However, total irradiance intercepted per ground area did not depend on tree spacing but on the spacing ratio (tree radius/tree spacing), increasing as a function of surface coverage.; Transpiration rate and peak water loss for a tree branch scale were very close to those obtained from the heat balance measurements. The model showed the capability to predict transpiration as response to environmental changes within a day, daily mean and total seasonal water use. Field measurements for water balance in this orchard, demonstrated that greatest density of roots of irrigated pecan trees seem to reside in the first meter of soil depth. It was also observed that in frequently irrigated pecan orchards, steady downward water flow may occur; this drainage may be possible to calculate by assuming gravity-flow. Seasonal water use predicted by the Physiology-Transpiration model agreed with field measurements.