Dehgolān plain is one of the largest plains in Kurdistān Province, which provides high potentials for agriculture in terms of water and soil. The quality of the aquifer water has changed over time due to factors such as the variation in rainfall, the unbalanced pumping from the aquifer, and the return of the agricultural water. The aim of this study was to investigate the spatiotemporal variation in the quality of the Dehgolān plain groundwater. Therefore, water quality index (WQI) was used to simplify multiple parameters, and the fuzzy logic was used to analyze them over time. Accordingly, the quality index for each well was specified through definition of the standards of drinking water quality for the fuzzy logic. From the mean value for each sampling period, the quality index for the aquifer was then calculated using parameters including Na+, Ca2+, Mg2+, Cl−, SO42−, pH, TH, and TDS. In the second stage, the water quality index was modeled using fuzzy logic, where precipitation and water-table fluctuation were considered as the inputs of the model. The results indicated that the quality of the Dehgolān plain groundwater has fluctuated several times during the past thirty years (by 68–75%, averaging 71.66%.( The parameters of total dissolved solids and total hardness exhibited the greatest influence on the water quality index. The lowest quality was observed in the east of the aquifer, i.e., near the city of Qorve, and the highest occurred around Sample Points W17 and W32. The results of fuzzy logic modeling demonstrated that the model is capable of predicting the water quality index from the precipitation and groundwater levels with an error rate of less than 3%. This advantage could be very useful when quality data are not available for whatever reason.
