Testing and Improvement of Water Balance of the Daycent Ecosystem Model.

Measured evapotranspiration (ET) by Eddy covariance method and continuous daily soil water content (SWC) at 4 different depths from an experiment at eastern Nebraska were used to evaluate the DAYCENT ecosystem model on water balance. Measurement was made in three cropping systems: irrigated continuous maize, irrigated maize-soybean rotation, and rainfed maize-soybean rotation since 2001. The original model accurately simulates weekly cumulative ET in early and middle growing season but overestimates the ET in late growing season. We modified the DAYCENT model with a simple method to represent the decrease of crop green canopy coverage when leaves senesce. The modification significantly improved the simulated ET and SWC in the late growing season. Overall, the modified DAYCENT model could simulate the changes in SWC at different depths. However, our modification does not lead to better result of simulated final aboveground total biomass nor crop grain yield because the improved simulation of SWC is in late growing season, when both maize and soybean slows their growth rates. Additionally, our result suggests the original model might underestimate the green-house gas N₂O emission as a result of the underpredition of SWC.