Soybean Productivity Under Various Rainfed Conditions and Irrigation Levels Determined By Winapex Model in Blackland Prairie of East Central Mississippi State.

Abstract: As shortage of irrigation water resources and food demand continuously increase, it is of great significance to produce the most crop yield with the least irrigation water. Assessment of crop productivity and development of crop water production functions (CWPFs) could help guide irrigation scheduling for improving water use efficiency. However, such field experimentation is time consuming and expensive. Soybean is the dominant crop whose area was approximately 820,000 ha accounting for 44% of the total crop area in the state of Mississippi in 2014. Blackland Prairie located in the east and north central Mississippi is one of the major Soybean production regions. Therefore, the purpose of this simulation study was to assess soybean productivity under various rainfall and irrigation conditions and determine water production functions of soybean (SWPFs) using WinAPEX model and weather data from 2002 to 2014 in the region. Data measured from an field experiment in Noxubee county of Mississippi state during 2014 were used for calibration and validation of the model. The experiment was conducted on three soil types, Vaiden, Okolona and Demopolis soil. There were three treatments on each soil type which consisted of three irrigation levels of 25.4 mm, 12.7 mm and 0 mm during the entire soybean growing season. Most important parameters of soil, crop, management and weather required by the APEX model were periodically measured. Validation results indicated that the well calibrated APEX model is capable of simulating yield, total dry biomass (BIOM), leaf area index (LAI), evapotranspiration (ETc)  and soil water storage (SW) at 100 cm depth (coefficient of determination R²=0.55-0.89; Nash–Sutcliffe efficiencies EF= 0.53-0.89; index of agreement D=0.80-0.97, the root mean square error normalized to the mean of the observed values RRMSE=4.4 %-8.7 %). The calibrated and validated APEX model was applied to conduct a  simulation study in an effort to assess soybean potential production and determine SWPFs for different soil types under various climate conditions across the Blackland Prairie. The model estimated that soybean produced 4.88, 4.51 and 3.74 Mg ha^-1 yield in wet, normal and dry years of 2002 to 2014 in this region, respectively. Simulated soybean yield ranged broadly from 2.24 to 6.14 Mg ha^-1 on nine soil types under dry to wet rainfed conditions. It reveals that soybean yield probably could be increased by 3.90 Mg ha^-1 if irrigation is applied. In addition, the functions between soybean yield and the total amount of applied water and rainfall as well as total crop evapotranspiration were developed. The SWPFs can be used to estimate soybean yield for a given water supply level on a certain soil type. Such information is helpful to optimize given amount of irrigation water for achieving maximum yield and water use efficiency in the region.