Calculating Vegetation Index-Based Crop Coefficients for Alfalfa in the Mesilla Valley, New Mexico Using Harmonized Landsat Sentinel-2 (HLS) Data and Eddy Covariance Flux Tower Data.

The goal of this study is to investigate the usefulness of the relatively new 30 m spatial and <5.7-day temporal resolution Harmonized Landsat Sentinel-2 (HLS) dataset for calculating vegetation index-based crop coefficients (K_cVI) for estimating field scale crop evapotranspiration (ET_c). Increased spatial and temporal resolution ET_c estimates are needed for improving irrigation scheduling, monitoring impacts of water conservation programs, and improving crop yield. The crop coefficient (K_c) method is widely used for estimating ET_c. Remote sensing vegetation indices (VI) are highly correlated to Kc and allow the creation of a K_cVI but the approach is limited by the availability of high temporal and spatial resolutions. We selected and calculated sixteen commonly used VIs using HLS data and regressed them against field-measured ET for alfalfa in the Mesilla Valley, New Mexico to create linear K_cVI models. All models showed good agreement with K_c (r² > 0.67 and RMSE < 0.15). ET_c prediction resulted in an MAE ranging between 0.35- and 0.64-mm day⁻¹, an MSE ranging between 0.20- and 0.75-mm day⁻¹ and an MAPD ranging between 10.0 and 16.5%. The largest differences in predicted ET_c occurred early in the growing season and during cutting periods when the spectral signal could be influenced by soil background or irrigation events. The results suggest that applying the K_cVI approach to the HLS dataset can help fill in the data gap in remote sensing ET tools. Future work should focus on assessing additional crops and integration into other tools such as the emerging OpenET platform. [ABSTRACT FROM AUTHOR]