Luca Brillante,* Matteo Ramagli, and Khushwinder Singh
*California State University Fresno, 2360 E Barstow Ave, Fresno, CA 93740 (lucabrillante@csufresno.edu)

Modeling grapevine water status on hillslopes is challenging. This research demonstrates for the first time that it is possible to obtain daily estimates of grapevine water status at the estate scale by re-elaborating routine measurements with digital technology. This information can be used to optimize irrigation scheduling, drive selective harvest decisions, and cluster vineyard variability. Data was collected during two consecutive seasons in a ~40 ha (100 ac) wine estate located in Paso Robles, CA. The topography was very diverse, with large variation in slope grade (0 to 30%) and exposure (0 to 359). One hundred experimental units of Cabernet-Sauvignon, Cabernet franc, and Petit Verdot were identified by a maximum dissimilarity sampling algorithm based on environmental attributes derived from a digital elevation model and a soil map. Grapevine water status was monitored by weekly measurements of stem water potential, Ψstem, and carbon isotope discrimination of musts, δ13C, at harvest. The grape composition during ripening was assessed by measuring primary components and skin phenolic composition with high-performance liquid chromato- graph-diode array detection. Vegetation indexes were derived from ~3 m resolution CubeSat satellites. Irrigation amounts were provided by the grower and weather data was obtained from three on-site stations. Ψstem was modeled from weather data, irrigation amounts, vegetation indicies, topographic attributes, and soil type using a gradient-boosting-machine algorithm. The model predicted plant water status with <0.1 MPa assessed error through several validation methods. External validation of the model was carried out by correlating predictions with δ13C. The model allowed obtaining high-resolution daily mapping of Ψstem at the estate scale. Time-series of grapevine Ψstem correlated significantly with the content of total soluble solids of musts, grape anthocyanin amounts, and the ratio of tri-hydroxylated to di-hydroxylated compounds at harvest and were mapped. Spatial clustering of grape anthocyanin composition was obtained from Ψstem model-estimates and could be used to guide harvest selectively.

Funding Support: Daou Family Estates