The using of SPI and SPEI indices in evaluating the effect of drought on quality of surface water resources(Case study: Kashafroud river)

Drought is a phenomenon that in addition to the lack of water resources, causes decrease in the quality of surface water. The purpose of this research is to investigate the effect of drought on the water quality of Kashfroud river. In order to, precipitation, temperature and quality data of six meteorological and hydrometry stations were used during the 30-year statistical period (1989-2019). Calculated SPI and SPEI index, drought and wet were selected among all stations. The changes of water quality parameters were investigated in the statistical period and the drought and wet period. The results showed that the average quality parameters of water have changed significantly during drought. In all stations, the concentration of TDS, EC, Ca, Mg, Na, SO4-, HCo3-, and Cl- have increased in the drought period compared to the long-term average. Its maximum for Cl-parameter is 7.66 mg/l at Olang Asadi station. On the other hand, during the period when drought indicators increased and drought occurred, the value of this parameter decreased by 32.73%.Due to the increase in temperature in drought period and the increase in EC value, the highest percentage of changes is in Olang Asadi station with 70.73% and the lowest in Golmakan station is 1.75%. Based on the results, the average concentration of cations and anions in Olang Asadi station shows an increase of about 5 times compared to Golmkan station.Finally, it can conclude that the decrease in rainfall, increase in temperature and drought, the water quality has decreased, especially in the downstream stations.