Using time series of high-resolution planet satellite images to monitor grapevine stem water potential in commercial vineyards

Spectral-based vegetation indices (VI) have been shown to be good proxies of grapevine stem water potential (Ψ_stem), assisting in irrigation decision-making for commercial vineyards. However, VI-Ψ_stem correlations are mostly reported at the leaf or canopy scales, using proximal canopy-based sensors or very-high-spatial resolution images derived from sensors mounted on small airplanes or drones. Here, for the first time, we take advantage of high-spatial resolution (3-m) near-daily images acquired from Planet's nano-satellite constellation to derive VI-Ψ_stem correlations at the vineyard scale. Weekly Ψ_stem was measured along the growing season of 2017 in six vines each in 81 commercial vineyards and in 60 pairs of grapevines in a 2.4 ha experimental vineyard in Israel. The Clip application programming interface (API), provided by Planet, and the Google Earth Engine platform were used to derive spatially continuous time series of four VIs-GNDVI, NDVI, EVI and SAVI-in the 82 vineyards. Results show that per-week multivariable linear models using variables extracted from VI time series successfully tracked spatial variations in Ψ_stem across the experimental vineyard (Pearson's-r = 0.45-0.84; N = 60). A simple linear regression model enabled monitoring seasonal changes in Ψ_stem along the growing season in the vineyard (r = 0.80-0.82). Planet VIs and seasonal Ψ_stem data from the 82 vineyards were used to derive a 'global' model for in-season monitoring of Ψ_stem at the vineyard-level (r = 0.78; RMSE = 18.5%; N = 970). The 'global' model, which requires only a few VI variables extracted from Planet images, may be used for real-time weekly assessment of Ψ_stem in Mediterranean vineyards, substantially improving the efficiency of conventional in-field monitoring efforts.