Measurement and Modeling of Soybean Water Requirements.

Only 30% of annual rainfall occurs during crop season although there is as more as 1300 mm annual rainfall in the mid-south United States such as state of Mississippi. Row crops including soybean frequently experience seasonal drought in the subhumid region. Accurately quantifying crop water requirements is essential for supplemental irrigation with the right amount of water at the right time to optimize crop water productivity. Field experiments were conducted in Mississippi Noxubee county on Vaiden clay, Okolona silty clay, and Demopolis clay loam at a private Good Farm in 2014 and on the Brooksville silty clay at Mississippi State University Black Belt Branch experiment station in 2015 to 2017. Each component of soil water balance in root zone was measured in these fields. During soybean season in 2014, 2015 and 2016, rainfall were 365, 388 and 284 mm, soybean water requirements were 428, 455, and 504 mm. In 2014, 2015 and 2016, rainfed soybeans consumed 402, 417, and 347 mm water and produced 5672, 2736, and 1806 kg ha^-1 grain, in contrast, irrigated soybean consumed 440, 526, and 478 mm water and yielded 6264, 3109, and 3031 kg ha^-1 grain. RZWQM2 and APEX models were applied on nine soil types (Vaiden and Sharkey clay; Catalpa, Okolona, Griffith, Sumter, Kipling and Brooksville silty clay; Demopolis clay loam; and Leeper sandy loam) across Mississippi. Simulating the soybean growing seasons from 1895 to 2014, the average long-term reference evapotranspiration and crop water requirement (ETc) were 720 and 542 mm: whereas, the mean rainfall was 432 mm. Rainfall of wet, normal and dry category years was 597, 421 and 280 mm. Long-term simulation suggested that soybeans needed 187 mm of supplemental irrigation during reproductive stages. Average irrigation amount required to achieve potential yield ranged from 16 to 377 mm across the ten soil types. On the different soils, rainfed soybeans achieved yields ranging from 1806-6153 kg ha^-1, while irrigated soybeans produced yield ranging from 3078-7406 kg ha^-1. On the same soils, yield of rainfed soybeans varied greatly over years, irrigation can stabilize yield.