Relationships Between, Soil Water Content, Irrigation, and Evapotranspiration Measurements in a California Drip-irrigated Pinot Noir Vineyard

The Central Valley of California relies on irrigation for food production, but water resources, particularly groundwater, have reached critical stages due to extended drought periods and overuse by irrigated agriculture. The purpose of the Grape Remote sensing Atmospheric Profiling and Evapotranspiration eXperiment (GRAPEX) project is to improve efficiency in vineyard irrigation through intensive sampling and modeling efforts that make use of remote sensing data. The purpose of this portion of the project is to analyze the ground measurements to find patterns that may inform future irrigation recommendations. The study location consists of two adjacent Pinot Noir vineyards in the California Central Valley (38.29 N, 121.12 W) where eddy covariance towers, soil heat flux plate transects, and volumetric soil moisture profiles (up to 90 cm depth) have been in place since March 2013. Irrigation records for this period have been supplied by the growers. The resulting data used for this analysis are reference evapotranspiration (ETref), actual evapotranspiration (ETa), mean daytime soil water content (SWC), precipitation, and irrigation, which were aggregated on a weekly basis. The relationship between SWC and the ratio of ETa/ETref changes throughout the spring and summer months due to advancing phenological stages and management practices. In early spring, ETa is strongly correlated with SWC due to the inter-row cover crop being the only source of ET. As the vine canopy grows in, the relationship breaks down due to the vines having access to water beyond the depth of the soil moisture sensors. As the soil profile dries out during the summer, correlation between ETa and SWC once again emerges once the vines become strongly dependent on irrigation. This dynamic interaction between the upper and lower root zone profile and evaporative demand means that the key to understanding water status using shallow SWC is to use it in conjunction with ETa measurements: these two tracking together indicates a lower overall soil water status than if ETa remains high when shallow SWC is low. Therefore, delaying the initiation or ramping up of irrigation until weekly SWC and ETa both decrease may both reduce the total amount of water used and give the grower more control over vine growth and grape quality affected by water status.