Intra-seasonal prediction of irrigated wheat yield using a regional water balance index, (Case study: Qom province)

Frequent droughts and soil moisture anomalies, cause significant yield reductions both for irrigated and rainfed crops. In the current study, attempts have been made to correlate regional production of irrigated winter wheat in Qom province with drought conditions during 2006-2017, using a water balance index namely the Standardized Precipitation Evapotranspiration Index (SPEI). Due to the lack of sufficient synoptic stations in the region, the reanalysis data consisting 30 grid points covering the whole province were also used. Moreover, the multivariate principal component analysis (PCA) was used to identify the spatio-temporal drought patterns. According to 117 years (1901-2017) reanalysis data, it was revealed that the dominant pattern of drought in the region explains more than 80% of existing temporal variations in grid points. Furthermore, this pattern indicated that extrem droughts (of all types including meteorological, agricultural, and hydrological) have occurred during recent two decades in the region. Findings showed that the meteorological drought index (SPEI1-PC1) of March was strongly correlated (r coefficient greater than 0.9) to final yield of wheat harvested three months later (June). Using the Jacknife approach, all the multiple regression models were independently validated and the results confirmed the predictability of the final yield amount.