Comparative analysis of evapotranspiration (ET), crop water stress index (CWSI), and normalized difference vegetation index (NDVI) to delineate site-specific irrigation management zones in almond orchards

Abstract

Management zones are an important aspect of precision agriculture as they help to define spatiotemporal areas that share homogenous attributes for site-specific management such as irrigation. In this study, we evaluated the potential of delineating irrigation management zones in almond (Prunus dulcis) orchards using three different variables: evapotranspiration (ET), crop water stress index (CWSI), and normalized difference vegetation index (NDVI). Multispectral and thermal images were collected on 11 days in June and July of 2020 using an instrumented aircraft flown over an almond orchard in the Central Valley of California. Obtained images were used to compute ET, CWSI, and NDVI. An unsupervised k-means clustering algorithm was used to delineate the field into management zones, and silhouette width was used to determine the optimum number of zones. Regardless of the input variable and collection date, the optimum number of irrigation management zones was identified as two. ET- and CWSI-based management zones addressed higher spatial variability in the field (up to 73.6 %) than NDVI (up to 68 %). Similarly, the level of agreement between management zones delineated using ET and CWSI was strong (kappa coefficient: 0.84 to 1.00). ET- and CWSI-based management zones also showed a trend (p < 0.1) in distinguishing the difference in the irrigation application and almond yield between the two delineated zones. This study shows that CWSI can be as effective as ET in delineating irrigation management zones, and both inputs sensitive to capture the vegetation responses to irrigation management during the summer growing season. Results from this study can be useful for growers and decision makers to practice precision irrigation management in woody perennial cropping systems.