Crop Water Stress Index of Pecans

Abstract

RESEARCH was conducted in New Mexico to develop the baselines needed to calculate the crop water stress index (CWSI) for pecans {Carya illinoensis (Wang) K. Koch) using infrared thermometry. Leaflet temperature, canopy temperature, vapor pressure deficit and leaflet diffusion resistance (rs) were measured on six-year-old trees in an open canopy that received four trickle irrigation water application rates. Similar measurements were made at a second site on 14-year-old trees flood irrigated in a closed canopy, and at a third site on eight small trees growing in 113-liter barrel lysimeters. The CWSI lower baseline relates canopy temperature minus air temperature, (Tc-Ta) to the vapor pressure deficit (V), for trees that are not stressed for soil water. This relationship was linearly related with a coefficient of determination of 0.75, and was independent of canopy tree size. The CWSI upper baseline for non-transpiring trees was independent of V but was affected by the canopy cover and had a Tc-Ta value of 6.0 and 4.0C for trees growing in an open and closed canopy respectively. During three dry-down cycles, where soil water stress caused by soil moisture depletion occurred to the pecans in the barrel lysimeters, the calculated relative evapotranspiration (1-CWSI) was linearly related to the measured relative evapotranspira-tion (Et/Etmax) with a coefficient of determination of 0.78. Pecan tree rs was curvilinearly related to CWSI. During diurnal cycles, pecan tree rs decreased early in the morning as light intensity increased, remained steady during the midday and increased late in the afternoon because of decreasing illumination. An error of 3.5% was made in estimating pecan tree rs from measurements taken only on the abaxial leaflet surface. Leaflet diffusion resistance measurements taken on the edge of the canopy can only be used to represent an average value for the tree having open canopies.