Comparing of interpolation methods in rice cultivation vulnerability mapping due to groundwater quality in Guilan, north of Iran

Guilan is a famous province in growing rice in Iran. Recently, due to shortage of water, farmers´ desire to use groundwater in order to grow rice in northern part of Iran has been increasing rapidly. The fact that rice is sensitive to water quality, caused concern about rice cultivation sustainability in the area, especially in drought years. This study was done to investigate the interpolation methods and the spatial variability of some important ground water quality factors including EC, SAR and Na of samples taken from 135 wells over the region at September of 2007. First, the accuracy of ordinary kriging (Ok) and inverse distance weighting (IDW) with 3 different powers (1, 2 and 3) in mapping the studied parameters were evaluated. Then the final map was presented. The result showed that spherical model gave the best result to simulate the vriograms. Although negligible differences were observed between the methods, Ok and IDW-1 performed better in comparison to the IDW-2 and IDW-3. The final map showed that EC in central part of the region, where the Sephidroud meets the Caspian Sea is dramatically high which will threaten the sustainability of rice cultivation in the area. The other factors were in suitable level.