Drivers of spatial and temporal variation in soybean yield and irrigation requirements in the western US Corn Belt

•We assessed yield and irrigation amount in soybean fields using on-farm data.•Producers are achieving yields relatively close to the attainable yield.•Variation in rainfed yield was related to rainfall during the reproductive phase.•Sowing date explained most of the inter-annual variation in irrigated yield.•Efficiency in use of rainfall versus irrigation water to produce yield was similar.