Rainfall timing drives rainfed durum wheat yield in western Iran

Rainfed agriculture is critically dependent on seasonal precipitation, yet the relationship between rainfall amount, its temporal distribution, and crop yield remains poorly understood—particularly in water-limited, Mediterranean-type environments. This study examines whether increased rainfall translates into higher grain yield in durum wheat ( Triticum turgidum L. var. durum) under rainfed conditions, leveraging an extensive 22-year (2002–2024) dataset from the Dryland Agricultural Research Station of Sararood, Kermanshah, Iran. The analysis encompasses 2,736 genotypes across 88 field trials, grouped into four experimental series, and is supported by high-resolution on-site meteorological records. Results reveal pronounced interannual variability in seasonal rainfall (159–783 mm), with no consistent linear association between total annual precipitation and grain yield. Although total rainfall showed a moderate positive correlation with yield across series ( r = 0.48–0.59, P < 0.05), high-rainfall years frequently failed to deliver high yields when precipitation was poorly distributed, especially when moisture deficits occurred during critical phenological stages (e.g., tillering in February, stem elongation in March – April, and heading in May). Conversely, the highest yields occurred in years with near-average total rainfall (≈ 430 mm) but optimal intra-seasonal distribution, notably 2006–07, 2009–10, and 2023–24. Multiple regression and PCA analyses further confirm that rainfall in February, April, and May—coinciding with tillering, pre-heading, and heading—is the strongest positive predictor of yield, whereas early-season (October – January) and late-season (June) rainfall exhibit significant negative effects, likely due to waterlogging or phenological interference. Yield uncertainty, as captured by regression prediction intervals, increases markedly outside the 300–550 mm “optimal rainfall window,” underscoring heightened production risk from both moisture deficit and excess. These findings demonstrate that rainfall timing, intra-seasonal distribution, and synchronization with key developmental stages are more decisive than total seasonal volume for durum wheat productivity in Mediterranean rainfed systems. The study calls for a paradigm shift in breeding and management—from maximizing yield under high rainfall to optimizing water-use efficiency, reproductive-stage drought/heat tolerance, and phenological adaptability under increasingly erratic precipitation regimes driven by climate change.