investigation of meteorological and hydrological drought using drought indices in qarehsou river basin

water crisis and drought are among the most important issues to which the human has been faced particularly in the recent years; hence, investigation of drought is so important in optimal water management. drought has been known as an environmental phenomenon which is inseparable from climate changes which can occur in any geographical region. various definitions have been presented since yet. by a general definition, drought includes abnormal shortage of precipitation in a long-term period so that, it causes soil moisture shortage and reduced current waters; therefore, human activities and natural life of plants are disturbed. qarehsou watershed is among the places which are affected by drought issue. considering the major role of this watershed on water supply of kermanshah province as well as recreational value of qarehsou river; the occurred droughts can cause economic challenges and ultimately, social crisis in the region. the objective of the present study is to determine meteorological and hydrological wet and drought periods and investigating the relationship between them of which the results can be used for more appropriate water resources management in qarehsou watershed. in the present study, the statistics of five rain-gauge stations, five hydrometric and 20 piezometric wells existing in the watershed were used for drought analysis. drought indices rai and siap were used to extract meteorological drought periods and also the indices sdi and swi were used to analyze hydrological drought. according to the obtained amounts by each drought indices, the severest drought has occurred in 2007; hence, the hydrological and meteorological drought zoning maps of the year 2007-2008 were drawn. the results showed that, the drought severity has been increased gradually since 1999 and the severest drought was in 2007 and the most durable one has occurred during 2007 to 2012.the results indicated that, hydrological drought has been occurred since the cropping season 2007-2008 with one-year delay relative to meteorological drought. also during the recent years, one or two years of delay has been observed between the occurrence of hydrological drought of underground waters and meteorological drought. the results demonstrated that, based on the index rai, the maximum frequency was related to the stations mahidasht, pol kohneh and ravansar by 27% at a very severe drought class. also based on the index siap, the maximum frequency was related to the station doabmerk by 23% at a moderate drought class. based on the index sdi, the maximum frequency was in the station sarasiab by 45% at a moderate drought class and finally, based on the index swi, the maximum frequency was related to the piezometric wells of hashilan, qarehtapeh and kahriz by 45% at a moderate drought class. zoning maps of meteorological drought showed that, meteorological drought has occurred in all the region. according to the zoning map of hydrological drought, hydrological drought of the surface flows is at a moderate class throughout the watershed while, hydrological drought of the underground water resources at the north east and south west and somewhat central parts of the watershed is at a very severe class. generally, results of the present study indicated that, duration and severity of the droughts and particularly hydrological drought are considerable during the recent years in qarehsou watershed. hence, results of the present study can be useful for optimal management of water resources and water demand and supply planning, as well as the managers, lanners and experts who can be enabled to provide required strategies and practical solutions.