tIn this study variation of six climatic indices including accumulated precipitation (P), accumulatedpotential evapotranspiration (PET), accumulated actual evapotranspiration (AET), accumulated cropevapotranspiration (ETC), accumulated water stress (S) and climatic water deficit (D), was investigated.Climatic indices and their variation were calculated during seven growth stages of wheat in five locationsin the northeast of Iran from 1983 to 2008. Principal component analysis (PCA) technique was applied toexplore major modes of variation in the regional climatic indices during different crop growth stages. Theprinciple component obtained for each region was correlated to the regional winter wheat yield. Finallythe regional amount of water and precipitation use efficiency (WUE and PUE) were analyzed in orderto assess any possible association with wheat yield. The results showed that the highest precipitationoccurred during the tillering stage and spatially decreased from north (Bojnord) to south (Birjand) andfrom east (Mashhad) to west (Sabzevar). The difference between the highest and the lowest precipitationacross all locations was 2.5 of standard value. The variation pattern of AET, compared to other indices,showed more similarity to variation of precipitation at different growth stages and the highest AET (morethan 2 of standard value in all locations) occurred during the tillering stage. The PCA indicated that effec-tive components varied in different locations. The most positive and effective components were typesof evapotranspiration that are associated with crop (ETC and AET) and precipitation. However none ofthese effective PCs showed a significant correlation with final yield. The PUE and WUE analysis indicatedthat PUE provides more information to interpret the relationship between total amounts of precipitationand the final yield.

Keywords