Drought Analyzing in Birjand with emphasis on Calibrated Palmer Drought severity Index and Relative Yield of Rain-fed Wheat Crop

Drought is a natural and continuous climatic phenomenon which usually occurs in all areas of the world, but its characteristics, type and intensity is different regionally. It is also a progressive phenomenon so that its start and end is not clear and its influence intensity is low but, it encompasses more areas compare to other disasters such as flood, earthquake and etc. which is finally caused more losses rather than other disasters. Drought threshold considering spatial condition is different, therefore, point statistics of one station as an index station of each region is useable for removing spatial effects. In this study, by use of a software which is designed for calculating Palmer Drought severity Index, this index is calculated and presented for Birjand synoptic station during 1985-2014 statistical period in annual and monthly scale. Also, drought process is studied by use of Mann- Kendal and Petit statistical test. Spi drought index is also presented and an analysis has taken concerning drought indexes and relative yield of rain-fed wheat crop relationship by use of Birjand rain-fed wheat yield data during foregoing statistical period. While the air temperature and precipitation data were homogeneous, Based on statistical tests, it was identified that the 30-year period is enough for drought monitoring and analysis. Results showed that sudden changes of annual and monthly spi time series are significant at 5 percent and its change point is May 1998. Also, about annual and monthly pdsi index, results indicate that sudden changes of annual pdsi time series are significant at 5 percent and its change point is February 2000. Spi and pdsi indexes also had a trend consistent with the relative yield crop index of rain-fed wheat and after change point; the pdsi index correlation coefficient was obtained for rain-fed wheat relative yield equal to 0.7 and the spi index correlation rate for rain-fed wheat relative yield equal to 0.55.