Lettuce (Lactuca sativa L.) production in organic soils is important in Quebec, Canada. Lettuce is highly sensitive to tip burn, a physiological disorder that can lead to significant yield losses. Tip burn losses have been linked to various factors, such as root water uptake deficits. A precision irrigation approach using local applications of water based on lettuce requirements and soil water available capacity (SWAC) reduces the occurrence of tip burn but may need mapped spatial information of SWAC for proper irrigation management. The objectives of this study were (i) to determine a rapid, efficient, and reliable method for interpolating SWAC and (ii) to use this interpolation method in precision irrigation simulations in management zones to demonstrate the importance of using SWAC maps. The methods for SWAC interpolation used in this study were inverse distance weighting (IDW), thin plate splines (TPS) and kriging with external drift (KED). The simulation used a calculation procedure for mass balance that contained SWAC maps, evapotranspiration (ET) and precipitation. A comparison of each interpolation method and multiple statistical criteria revealed that IDW and KED were the most precise methods, depending on the study site. Simulations of precision irrigation showed that in many cases, local irrigation management in seven to eight zones must account for the spatial distribution of SWAC to attain an 80% irrigation adequacy for lettuce. Hence, using SWAC maps as a tool for managing irrigation would allow growers to save water and to apply an accurate amount of water in appropriate areas.