Simulating yield potential by irrigation and yield gap of rainfed soybean using APEX model in a humid region

Soybean is generally grown under rainfed conditions in a humid region, Mississippi, USA. In order to determine how much maximum yield could be increased by irrigation, it is crucial to investigate the yield potential (Yp) without any drought stress and yield gap (Yg, between Yp and rainfed yield (Yw)). Further, it is also important to determine the amount of irrigation water needed to alleviate any drought stress during the entire growing season, and conduct cost-return analysis for irrigated soybean. Therefore, the objectives of this study were to: (1) simulate Yp, analyze Yg and determine the irrigation timing and amount needed to achieve Yp for soybean using the Agricultural Policy/Environmental eXtender (APEX) model; and (2) compute water use efficiency (WUE), irrigation water use efficiency (IWUE) and conduct the cost-return analysis on irrigation events. Simulated Yp of soybean without water stress for nine soil types from 2002 to 2014 ranged from 4.47 to 6.51 Mg ha−1, and was strongly correlated with accumulative solar radiation during the growing season (R2 = 0.71, P ≤ 0.01). The Yg in dry years was much higher than that in normal and wet years, with average Yg of 1.58, 0.60 and 0.71 Mg ha−1 for dry, normal and wet years, respectively. Griffith, Sumter and Demopolis soils had the highest average Yg over 13 years, ranging from 1.37 to 1.60 Mg ha−1. The average irrigation amount was 308, 192 and 157 mm in dry, normal and wet years, respectively. The average irrigation amount was 75 mm from R1 to R8 stages. The WUE of nine soil types from 2002 to 2014, under non-limiting water conditions, ranged from 9.5 to 13.8 kg ha−1 mm−1. The magnitude of Yg was the principle factor affecting IWUE among nine soil types. The average IWUE over 13 years ranged from 1.8 to 7.8 kg ha−1 mm−1 for nine soil types. Compared with a rainfed condition, average net return of irrigated soybean increased by 93 $ ha−1 (dollar per hectare) among nine soil types from 2002 to 2014. The average net return increased by 195, 58 and 70 $ ha−1 for dry, normal and wet years, respectively.