Rain-Fed and Irrigated Vineyard Yields in Spain: Regional Differences and Associations with Annual Hydrological Conditions

Regional yield comparisons can complement vineyard experiments but cannot identify causal irrigation responses. We linked Spanish agricultural and hydrological statistics for 70 community-year pairs in 14 communities during 2019–2023. Equal-weight paired differences and joint regressions with regime-specific community and year effects were analyzed using community-level wild cluster bootstrap tests. Confidence intervals used approximate CR1-t inference. Mean irrigated yield exceeded rainfed yield by 1677 kg/ha (95% CI 785–2568; bootstrap p = 0.0013). Per 0.1-unit increase in annual precipitation/reference evapotranspiration (P/ETo), estimated rainfed and irrigated slopes were 307 and −73 kg/ha. Their direct difference was −380 kg/ha (95% CI −1223 to 462; bootstrap p = 0.6306), changing to 104 after excluding Galicia 2023 (bootstrap p = 0.3782). The positive mean yield difference persisted under winsorization and leave-one-community analyses. Evidence for different hydrological slopes remained imprecise and sensitive to influential observations. Annual regional P/ETo does not measure vine water stress, and absent irrigation volumes, management and quality data limit interpretation. The findings establish a descriptive regional yield difference but do not demonstrate physiological buffering, causal irrigation benefits, or greater yield stability.