Overall, this work enhances the capability of APSIM in simulating production and environmental aspects of cropping systems, especially in regions with shallow water tables typical of the Corn Belt, USA. We also found that the inhibition of root growth in response to shallow WT substantially impacted the vertical distribution of the roots in both measurements and simulations. Validation of root depth simulations using independent field data from Iowa, USA (years 2016, 2017, 2018) confirmed the model. Inclusion of this factor into the model increased accuracy of root depth simulations from R2 of 0.65 to 0.97 and reduced root mean square error from 45 to 9 cm. The SOILWAT2 module in APSIM was used to model water infiltration and movement in the soil, and the crop modules (APSIM-Maize, APSIM-Wheat, APSIM-Soybean and APSIM-Lucerne) were used to simulate. Below that threshold, root growth linearly decreases to zero at saturation. Analysis indicated that unconstrained root growth occurs until volumetric soil moisture approaches 0.03 mm/mm below saturation. We used data from a controlled experiment (Rhizotron facilities) that included root depth observations for nine WT treatments to develop and calibrate the new model. We enhanced the APSIM-soybean model to accurately simulate root depth in fields with shallow water tables. Shallow water table (WT) influences crop growth and production in many major agricultural regions across the globe.
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