تحليل و پيش بيني آماري خشكسالي و دوره هاي خشك كوتاه مدت در استان خراسان

Analysis and Predict of Statitical Drought and Short Period Dry Spells in Khorasan Region

The present study aims to investigate the drought phenomena and its short occurrences in Khorasan region. To achieve these goals, two methods have been utilized. To study the drought conditions of the region; monthly precipitation statistics of 34 stations from 1968-1997 was taken into account. In the next stage, to obtain short period occurrences of the drought, Two State First Order Markov chain has been used. In this method, the daily precipitation data obtained from five representative stations in different parts of the province was helped to derive climatic characteristic associated with occurrences of short thy and wet spell such as stationary and simple probability, day frequencies, the length of wet and dry spells and weather cycle. Then, having computed the frequency of dry period, probability of occurrences of these periods and their return periods. The analysis results of the process of monthly precipitation trough method cleared the condition of different years in terms of drought and wet year. The analysis showed that 57 percent of stations in 1970 suffered from severe drought. On the contrary, the recived precipitation of all stations in 1991 was higher than normal level. It should pointed out that 76 percent of these stations spent a wet year with different intensities. The exact determination of probability of two sequential dry days have, differences between simple and stationary probabilities, the occurrences of wet and dry days and also the frequency of dry spell can be accounted among the significent result of this study. This study also showed that the probability of susequent dry spell fluctuated. from 71 to 98 percent. No noticeable difference was observed between the simple and stationary drought of the stations. Interms the frequency of dry days, the average number of dry days will be 20 or more during the whole seasons.