Assessment of the precipitation deficit for rain-fed winter wheat in the Mediterranean coastal zone using the Agricultural Standardized Precipitation Index

Agricultural drought assessments based solely on meteorological data often provide a limited depiction of actual drought conditions and related hazards, as they do not account for crop water demand and precipitation-related water deficits. This study aims to determine the extent to which agricultural drought analyses based on meteorological data can represent precipitation deficits (PD), a key indicator of agricultural drought occurring during crop phenological stages. In this context, analyses were conducted for rain-fed winter wheat sown on different dates using the Agricultural Standardized Precipitation Index (aSPI). Daily crop evapotranspiration (ETc) values during the vegetation period and long-term monthly precipitation data (1970–2023) were used as a base material. Effective precipitation (Pe) was calculated using the USDA-SCS method. Monthly PD maps (based on median Pe values) and aSPI maps (based on median aSPI values) were generated for the wheat growth periods corresponding to each sowing date. The aSPI method showed good agreement with the spatial distribution of PD-related agricultural drought during the early phenological stages—November through February—when effective precipitation was sufficient to meet ETc. However, its ability to reflect drought severity during the critical late growth phases (March to harvest)—including stem extension, heading, and milk development—was limited due to increasing water demand and decreasing effective precipitation. Consequently, reliance on median monthly effective precipitation for drought monitoring during the wheat growing season may lead to underestimation of drought severity and a potential risk of yield reduction. Therefore, it is recommended that agricultural drought assessments integrate plant-based indicators to better account for crop-specific water demands throughout the growing cycle.