Vitis vinifera L. is a plant species that depends highly on temperature and water availability. Excessively hot and dry conditions can undermine growth and threaten grapevine performance. In these situations, grapevines activate self-regulating mechanisms to respond to water stress by prioritizing their survival through stomatal control and water redistribution. The monitoring of these mechanisms, through the measurements of the trunk diameter fluctuations and sap flow, was made for ‘Touriga-Nacional’ planted in the Douro Demarcated Region (NE Portugal), during the 2017 growing season. Seasonal and trend decomposition of the acquired data, as well as the assessment of the potential influence of meteorological variables was carried out, using locally estimated weighted regression and scatterplot smoothing. The objective behind this decomposition was to assess if the individual analysis of the periodic and inter-daily variations of the grapevine's trunk diameter fluctuations and sap flow could improve the understanding of their response to abiotic stress. The results have shown the methodology is efficient in extracting the different components and that their analysis is informative. It was possible to determine that the delay between the daily trunk diameter and sap flow periodic variations became shorter in time, suggesting the loss of water by transpiration is more easily observable under increasingly hotter and drier conditions. Furthermore, longerterm, inter-daily variations at the trunk are highly correlated with those of relative humidity, evidencing the impact of air moisture on their water status. Such findings justify the implementation of locally weighted regression and scatterplot smoothing (STL) in the operational processing of sap flow and trunk diameter time series in the control of grapevine water status, in the case of optimization of vineyard management by wine growers
