Numerical Modeling of Water Vapor Fluxes in the Unsaturated Zone of Mature Pecan Orchards in Arid Southern New Mexico.

Information is still limited about the coupled liquid water, water vapor and heat transport associated with transient variation of soil water content and temperature in the unsaturated soil of irrigated pecan orchard grown under arid climate of southern New Mexico. Field experiments were carried out at two pecan orchards in Las Cruces, New Mexico, sited on a sandy loam (site 1) and a silty clay loam (site 2) soil. Three pecan trees at each site were chosen to monitor diurnal soil water content under the canopy (approximately half way between trunk and the drip line) and outside the drip line (bare spot) along a transect at the depths of 5, 10, 20, 40, 60 and 80 cm using TDR sensors. TDR calibration at each sensor depth was found to fit the gravimetrically determined volumetric water contents data reasonably well with R² ranging from 0.9 to 0.95. Soil temperature sensors were installed under the canopy and at bare spots to monitor soil temperature data at the depths of 5, 10, 20, and 40 cm. At each site, core and bulk soil samples including within the canopy of selected trees and bare spot were collected to determine soil hydrological properties up to 80 cm depth. HYDRUS-1D model was previously calibrated and validated under arable field conditions of the site 2 planted to onions. We are attempting to apply HYDRUS-1D to mature pecans to evaluate various transport mechanisms associated with temporal variations in soil water content and soil temperature in the 0-40 cm unsaturated zone within canopy and bare spots. The model will be applied using measured soil water content and soil temperature at 5, 10, 20, and 40 cm. Measured soil hydraulic and thermal properties, and meteorological data will be used in model simulations. The research will provide quantitative estimates of water vapor fluxes under open and closed canopy.