Because of limited surface water in reservoirs during a long term drought, this work aimed to locate clean and safe groundwater mixing with surface water for agricultural use in the Chianan plain, southwestern Taiwan subject to its hydrochemical parameters. Based on a standard of water quality for irrigation, multivariate indicator kriging (MVIK) was used to estimate integration of several hydrochemical parameters of groundwater mixing with surface water and to characterize spatial uncertainty. Different utilization ratios (UR) of groundwater regarded as multiple estimated thresholds significantly improved single-threshold drawbacks in previous studies. Recommended regions were determined under various groundwater utilization conditions according to probabilities estimated by MVIK The analyzed results illustrate that groundwater plays a subordinate role in development of agricultural water resources in this plain. After proper dilution with surface water (UR = 0.5 and 0.25), approximately 48.9% of this plain can supply groundwater for irrigation. The northern, northeastern and eastern aquifers are excellent locations to extract groundwater mixing with equal or triple amounts of surface water. The research results provide agricultural administrators and farmers with simple guidelines to obtain sufficient and reliable water for maintaining sustainable production of crops during a long term drought. (C) 2009 Elsevier B.V. All rights reserved.