Site-specific determination of plant water status in a steep sloped vineyard using a microclimatic monitoring system in combination with a water balance model and UAV-based thermal and multispectral imagery

Climate change poses fundamental challenges to viticulture, such as more frequent droughts in Central Europe. This development requires precise, site-specific methods to determine plant water status. Especially in steep sloped vineyards, the spatial variability of drought stress can be high and depends on different factors such as slope, aspect and soil characteristics. Most established methods for determining plant water status are destructive, labor-intensive, or provide point-in-time measurements, or e.g. non-destructive modeling approaches need to be well referenced. UAV campaigns using thermal and multispectral imagery, as well as in-field sensor networks, provide non-destructive solutions with high spatio-temporal resolution. This study aims to combine both solutions to measure the high spatial and temporal variability of drought stress in a steep sloped vineyard. The goal is to develop a continuous, cross-scale, and resource-efficient method that can be used directly for irrigation scheduling or as a reference method for cross-scale modeling approaches at high spatial resolution. During the growing season of 2022, UAV campaigns were conducted every two weeks to generate thermal and multispectral imagery over a vineyard of 1 ha in Saxony, Germany. The vineyard was divided into five management zones (MZ), which differ in terms of slope, aspect, soil characteristics and grape varieties. A monitoring system has been established in each management zone to continuously collect data on local climate, as well as soil and plant water properties. Simultaneously with the UAV campaigns, the water status and physiological stage of the vines were determined as reference measurements. Therefore, predawn leaf water potential (pd) was measured using a Scholander pressure chamber. Based on the processed aerial images and the in-situ sensor-based measurements the Crop Water Stress Index (CWSI) was computed and then validated by comparing its values to in-field reference measurements such as soil water status and pd. Weather and plant physiological in-situ measurements were also integrated into a grapevine water balance model to derive quantitative information on plant and soil water status. In-situ measurements of plant and soil water potentials correlated well with the results of the modeling approach. This was a good representation of the spatial heterogeneity of the vineyard, especially the differences in plant water availability between MZs. CWSI values from the UAV campaigns will be compared with the in-situ measurements in terms of spatial variability, and also temporal variability to reproduce drought and other seasonal events. The combination of sensor data, simulation modeling and UAV-based thermal and multispectral imagery offers great potential to provide site-specific information with high spatio-temporal resolution about the plant water status. In particular, the inclusion of UAV campaigns can help to optimize the implemented sensor network and minimize the number of in-situ reference measurements. However, this cross-scale method also depends on a large number of influencing factors that need to be considered and discussed in depth in order to allow a valid assessment of drought stress dynamics and to set thresholds for irrigation or other management measures.