Irrigation research in the mid-south United States has not kept pace with a steady increase in irrigated area in recent years. This study used rainfall records from 1895 to 2016 to determine rainwater deficit and irrigation demand for soybean [Glycine max (L.) Merr.], corn (Zea mays L.), and cotton (Gossypium hirsutum L.) in the Blackland Prairie region of Mississippi with a soil water balance model developed in Structural Thinking and Experiential Learning Laboratory with Animation (STELLA) software. The longterm analysis showed annual rainfall exceeded 1125 mm in 3 out of 4 yr. Median growing season rainfall for soybean, corn, and cotton ranged from 226 to 412, 283 to 505, and 220 to 472 mm and was 30, 39, and 33% of annual rainfall in either normal or dry years, respectively. Total effective rainwater deficit (rainfall minus runoff, percolation, and evapotranspiration) during soybean, corn, and cotton growing seasons was 200, 217, and 184 mm, respectively. During the driest 25% years, the maximum effective rainwater deficit was 340, 324, and 327 mm for soybean, corn, and cotton, respectively. Across the 122-yr period, soybean, corn, and cotton did not appear to need irrigation for only 12, 17, and 14 yr; their average irrigation demand was 180, 167, and 175 mm yr–1, respectively. Soybean required irrigation from 29 June to 7 September, particularly in reproductive Growth Stage 3 (0.5-cm-long pods in the upper four nodes) to Stage 7 (pods mature in color anywhere), when the crop appeared to require at least five irrigations.