Assessment of groundwater salinity risk using kriging methods: A case study in northern Iran

The suitability of groundwater for paddy field irrigation in the alluvial plains of Guilan Province, northern Iran, was investigated using ordinary kriging and ordinary cokriging of continuous and indicator quality variables. The cross validation values of error measures showed that ordinary cokriging provides more accurate estimates of the quality variables of interest. Maps showing the spatial variability of electrical conductivity (EC) and sum of major cations and anions (SCA) were generated for the years 2010 through 2014, using ordinary cokriging. Based on the estimated values of EC and SCA, four groundwater salinity classes (excellent, good, risky, and unsuitable) were considered and the proportion of the study area covered by each class was obtained. Results showed that the portion of the study area covered by the risky class, in which the groundwater salinity is expected to reduce the rice yield, is located in the eastern part of the study area and has an average value of 25.4% in the period 2010-2014. The results also showed that the western part of the study area has excellent or good groundwater quality for rice irrigation. The probability maps of EC were also obtained using ordinary cokriging of EC indicator variable. Five probability classes were considered and the proportion of the study area covered by each class was obtained. It was observed that the probability that the rice yield is reduced more than 10% is above 0.4 in 6.2% of the study area. The maps generated in this study can be used to identify the regions in the province where groundwater could be allowed to be extracted and utilized by farmers to reduce the bad effects of the scarcity of surface water. Also, in the regions with a risk of rice yield reduction, conjunctive use of groundwater and surface water could be planned and advised to farmers.