Abstract Groundwater quality assessment is important for the management of the agricultural industry with the aim of using these resources in agriculture. Purpose of this research is studying the effect of decline the water table on groundwater quality of Marand plain using geographical information system (GIS) for water needed of Agriculture. In this research, successive periods of Hydrogeological drought were detected using SWI index in 6 periods (6, 9, 12, 18, 24 and 48 months) in Marand plain in West Azerbaijan. For this purpose, water quality data of 70 wells and monthly water levels data in 23 wells in the period of 2001-2011 was used. The maps were produced by kriging in Arc GIS software. Results showed that groundwater drought has not happened a in studied area randomly and its focus in the West of aquifer is more than the East. Results of study based on electrical conductivity (EC), Sodium Absorption Ratio (SAR) and Chlorine showed that water quality decreases from the center of aquifer toward its outlet so that at the end of aquifer , water quality become too salty. This situation is intensified in periods that are increased water withdrawal. Evaluation of water quality for agricultural purposes based on Wilcox diagram showed that in the drought 2010- 2011 of Marand plain about 44 percent of groundwater had more salty.

Reducing the groundwater level due to irregular exploitation of these resources has caused land subsidence in most plains of our country. Study of this reduction in the groundwater level and trend of changes is essential for groundwater resources management and prevent adverse effects of irregular exploitation. In this paper, the indices of groundwater resource index (GRI) and the water level standardized (SWI) as two important index in hydrological drought were studied. To this goal, data from 40 pizometric wells located in Sari – Neka plain was used during the period of 30 years. In this study, first indicators of GRI and SWI in a year using the existing relations in the software Minitab 17 and M.S. Excel 2016 were calculated. And then, Using Mann - Kendall and Sen's slops in the application environment Makesens trends were examined. The results showed that both indices at the level of 99% has been declining and drought severity based on these two indicators has increased with time. As the drought severity in two indices for annual scale in 1394 and best wet for two indices for 1369, The main reason for the increase is the number of wells in operation, resulting in irregular exploitation of groundwater resources for agriculture in plain.