Analysis of hydrological drought in karoon river basin by daily discharge data

Drought is a national calamity that has bad effects on ecological، social and economical divisions. Growing of crowd and increasing of water demand aggravate the effects of drought. Analyzing of drought and attention to it in development programs has much importance، especially in Iran that is an arid and semiarid country. “Hydrological drought” means time periods that discharge of river is not sufficient to supply planned demands. In this research the flow duration curve (FDC) established using Pole-Shaloo station daily discharges for 1957-2001 years. Next، the hydrological drought periods extracted by truncation level method and analyzed. The “interior criterion” method (IC) used to eliminating minor droughts and pooling dependence droughts. Concluded that Johnson and Generalized Pareto distributions was best cases for annual maximum series (AMS) include “duration” and “deficit volume” respectively، based on Q70 truncation level so، type 3 Pearson and Generalized Pareto distributions best cases for annual maximum series (AMS) include “duration” and “deficit volume” respectively، based on Q90 truncation level. Therefore the return periods calculated for historical droughts. The most drought’s return periods was less than 20 years except 1963-1964 drought. For example The 2001 drought’s return period، that enveloped most of regions in Iran، was 20 years based on analyzing of deficit volume. Also drought risk calculated for return periods and water resource projects life. The maximum drought deficit volume is related to primary 11 years and 5 years at the end of time، so that in primary 11 years 57 and 50 percent of droughts was happened in lieu of Q90 and Q70 respectively. Also in 5 years of end of studying period 40 and 20 percent of droughts was happened in lieu of Q90 and Q70 respectively. The minimum amount of drought deficit volume was related to 1966-1974 period، so that equal to zero and 20 percent in lieu of Q90 and Q70. In final part of research، the annual maximum series of deficit volume extracted with respect to 10، 20،30،60،90،120،150 and180 days durations، and the best probability distributions appointed for these. Consequently، the severity-duration-frequency curve furnished.