A type-2 fuzzy interval programming approach for conjunctive use of surface water and groundwater under uncertainty.

•A type-2 fuzzy interval programming (T2FIP) method is developed.•T2FIP can tackle uncertainties expressed as type-2 fuzzy intervals.•Tradeoff between agricultural benefit and system reliability can be analyzed.•T2FIP is applied to Zhangweinan River Basin for conjunctive water management.•Results help decision markers identify optimal amount of groundwater utilization.