drought and its effects on groundwater resources quality in sistan and baluchestan province

there are different materials and compounds in water that determine its chemical and physical quality. as the result of special conditions such as increase and decrease in discharge, water quality can vary. one of these conditions is drought that decreases discharge, and water quality changes. in this study, droughts of sistan and baluchistan province have been analyzed using standardized precipitation index (spi). on the basis of this index, sistan and baluchistan province has experienced alternately different droughts with various intensities in the studied period (1976-2005). most droughts have been classified as weak with a frequency of 26.7 percent and medium droughts with 20 percent frequency.  then, dry and wet years were identified. finally, the period of 2000-2004 were selected as representative of dry years. in respect to water quality, results of 4 hydrometrics station for sianchah, hirmand, bampur, and bahokalat rivers were studied for a period of 10 years (1997-2006). most changes in cations and anions concentrations were observed at payab-e- kohak dam hydrometric stations, which were about 22 percent. ec also changed in drought index period as compared with long–term average, increasing in payab–e- kohak dam, daman, ladiz and pirdan hydrometeric stations by 12.7%, 2.5%, 0.95%, and 0.03%, respectively. in fact, decrease in river base flow and temperature increase caused increase in ec in drought period. also, sar increased in all of the studied stations by about 5.5 percent in the drought period.