Crop coefficients of open-canopy pecan orchards

Pecan [Carya illinoinensis (Wangehn.) K. Koch] production is an essential component of irrigated agriculture in arid southwestern USA. Pecan consumptive water use is high compared to other crops. For irrigation purposes, consumptive water use (ETc) or evapotranspiration is estimated from ETc = Kc ETr where Kc is the crop coefficient used to adjust the known reference crop evapotranspiration (ETr). The Kc of pecans, the ratio of ETc and ETr, varies according to tree age, tree spacing, growth stage within a season, and local weather conditions. The equation for Kc that is applicable to closed-canopy pecan orchards (i.e., for Kcmax) is available from the literature. However, there is a lack of a method to calculate open-canopy pecan Kc according to tree size and spacing (varying effective canopy cover, ECC). The objective of this study was to derive a regression relationship in the form Kc = Kcmax × f(ECC) or Kc/Kcmax = f(ECC) for open-canopy pecan orchards assuming that Kc/Kmax is affected mainly by ECC. ECC was measured for orchards with different canopy covers using image analysis. Images were taken from a balloon and by satellite. Corresponding ETc values from the various pecan orchards were measured or estimated using four methods: (1) one propeller eddy covariance (OPEC) technique, (2) eddy covariance (EC) system, (3) remote sensing technique, and (4) simulations from a climate-based physiological tree model. Kc/Kcmax was calculated for different ECC values as ETc/ETcmax, where ETcmax is the closed-canopy ETc. A regression equation for open-canopy pecan orchards was obtained using data from OPEC, EC, and remote sensing techniques (Kc/Kcmax = 1.33ECC, i.e., Kc = Kcmax × 1.33ECC) and was found to be statistically significant (R2 = 0.96, F = 2487, P < 0.001). The equation is consistent with the results from the physiological model and related literature. The Kc equation from this study can help pecan farmers and researchers get more accurate estimates of pecan irrigation requirements for open-canopy orchards.