Deficit irrigation has evolved as a tool to reduce water use in viticulture because of increasing water scarcity in many agricultural regions. The accomplishment of this objective depends on the accurate knowledge of both vineyard water requirements and vine water status. The improved surface renewal method (SR) is a bio-meteorological technique that can be used to accurately measure crop evapotranspiration (ETa) at a vineyard scale. A deficit irrigation experiment was carried out in a commercial vineyard of the North Coast Viticultural Region, CA.
The objective of the study was to determine the impact of irrigation practices on the relationships of SR estimates of ETa and vine water status parameters, such as leaf water potential (ΨLEAF) and stomatal conductance (gs). Three irrigation treatments were applied from veraison to harvest: Wet Control: vines were irrigated at 100%ETc (as calculated by Williams & Ayars, 2005); Medium-Wet: vines were irrigated between 70-80%ETc; Moderate Deficit: vines were irrigated between 40-50%ETc. SR provided with reliable estimates of daily ETa compared to the eddy covariance, which is regarded as the reference bio-meteorological method for measuring ETa.
Although ΨLEAF was consistently higher in the wet control than in medium-wet and moderate deficit treatments, the medium-wet block showed the highest ETa and gs values. Surprisingly, vines from the wet and the moderate deficit treatments often exhibited similar ETa values, regardless of large differences in applied water and ΨLEAF. A quadratic relationship between ETa/ETo and ΨLEAF indicated that the maximum water demand was reached at mild levels of water stress (~0.9 MPa). Conversely, vines under wet (>-0.7 MPa) or severe water stress (<-1.4 MPa) conditions showed similar ETa/ETo values. These results showed that the SR technique can be used to better understand the extent of the effect of irrigation practices on both vineyard water use and vine water status in winegrape production.