Assesment of drought impacts on rainfed wheat yield using SPI and Agricultural-SPI indices (Case study: North Khorasan province)

In this study, a modified version of standard precipitatıon index (SPI), namely Agriculture-SPI,or aSPI was used to assese the impacts of drought on rainifed wheat yield in North Khorasan province,northeast of Iran. In the aSPI index, the effective rainfall is used instead of precipitation. The USDA-SCS, method was used for effective rainfall calculation. Both indices were worked out , using 30-years (1989-2018) monthly rainfall and temperature data. Rainfed wheat yield data for the period of 2004 to 2018 were also retrived from local oranizations and standardized for comparisons in different SPI time windows of 1, 3, 4, 5, 6, 7, 8, 9 and 12 months .The obtained annual yields were correlated with SPI and aSPI values in selected timescales. The results showed that in Bojnord station, the rainfall druring November-May period, in Shirvan, Faruj and Esfarayen stations April-June, in Maneh-Semelgan station, November-December period and in Jajarm station, October-June period rainfall, are the most signifcant variables affteting the rainfed wheat according to drought indices used in study region.