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

Authors: SAPKOTA, ANISH - University Of California, Davis; Roby, Matthew; PEDDINTI, SRINIVASA - University Of California, Davis; FULTON, ALLAN - University Of California - Cooperative Extension Service; KISEKKA, ISAYA - University Of California, Davis

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2024
Publication Date: 1/1/2025
Citation: Sapkota, A., Roby, M.C., Peddinti, S.R., Fulton, A., Kisekka, I. 2025. 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. Scientia Horticulturae. 339. Article 113860. https://doi.org/10.1016/j.scienta.2024.113860.
DOI: https://doi.org/10.1016/j.scienta.2024.113860

Interpretive Summary: Precision agriculture is a management approach that seeks to optimize irrigation and nutrient use to meet the requirements of specific areas within farms. In this study, we used imagery collected by a research aircraft to classify a commercial almond orchard into distinct irrigation management zones based on three variables: evapotranspiration (ET; which reflects plant water use and soil evaporation), crop water stress index (CWSI; a metric of plant water stress), and normalized difference vegetation index (NDVI; a measure of plant greenness). Irrigation zones were classified based on spatial maps of ET, CWSI, and NDVI that were generated using data collected during 11 flights in summer 2020 over a 12-year old almond orchard near Corning, California. All three methods determined that irrigation would be optimized if the orchard was classified into two different management zones. Results indicated that ET and CWSI were more informative than NDVI when classifying irrigation management zones. There was strong agreement between the irrigation management zones defined using ET and CWSI. These results show that CWSI, which is relatively simple to calculate, can as effective as ET in delineating irrigation management zones. show that CWSI, which is relatively simple to calculate, can as effective as ET in delineating irrigation management zones. These results may help growers and farm managers use thermal imagery to implement precision irrigation management in orchards.

Technical 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 (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 showed sensitivity to the plant response within the short growing cycle. Results from this study can be useful for growers and decision makers in upgrading existing irrigation systems in order to practice precision irrigation management in woody perennial cropping systems.