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Title:   Author  item
                                            Malone, Robert

                                                         - *Rob*item
                                            KERSEBAUM, KURT - Leibniz Centreitem
                                            Kaspar, Thomasitem
                                            Ma, Liwangitem
                                            Jaynes, Danitem
                                            Gillette, Katrina
  Submitted to: Agricultural Water Management
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 1/31/2017
                                    Publication  Date: 3/1/2017
                                    Citation: Malone, R.W., Kersebaum, K.C., Kaspar, T.C., Ma, L., Jaynes, D.B., Gillette, K.L. 2017. Winter rye as a cover crop reduces nitrate loss to subsurface drainage as simulated by HERMES. Agricultural Water Management. 184:156-169. doi: 10.1016/j.agwat.2017.01.016.Interpretive Summary: HERMES is a widely used agricultural system model that has never been tested for simulating N loss to subsurface drainage. Here, we modified HERMES to simulate drain flow. We then compared the predictions to four years of field data (2002-2005) from central Iowa fields in corn-soybean with winter rye as a cover crop (CC) and without winter rye (NCC). The modified model accurately simulates N loss in subsurface drainage under both CC and NCC, and the simulations agree with field data that winter rye cover crop substantially reduces N loss to drainage. The simulations suggest that increased soil field capacity reduces HERMES simulated nitrate concentration in drain flow, increases simulated denitrification, and reduces simulated drain flow volume. Field capacity is the amount of soil water content held after excess water has drained away and the rate of downward movement has ceased or becomes very small. Our results suggest field capacity is an important variable associated with N concentration in subsurface drainage but previous research investigating the relationship is limited. Overall, the results suggest that: 1) HERMES is a promising tool to estimate annual N loss in drain flow under corn-soybean rotations with winter rye as a cover crop, and 2) soil field capacity is a critical variable to investigate to more thoroughly understand and predict denitrification and N loss to subsurface drainage. This research will help model developers, model users, and agricultural scientists more clearly understand N transport in subsurface drainage under different management including winter rye, which will help in the design of more effective management to reduce nitrate transport from tile drained agriculture to streams and rivers.
                                Technical Abstract: HERMES is a widely used agricultural system model; however, it has never been tested for simulating N loss to subsurface drainage. Here, we integrated a simple drain flow component into HERMES. We then compared the predictions to four years of data (2002-2005) from central Iowa fields in corn-soybean with winter rye as a cover crop (CC) and without winter rye (NCC). We also compared the HERMES predictions to the more complex Root Zone Water Quality Model (RZWQM) predictions for the same dataset. The average annual observed and simulated N loss in drain flow were 43.8 and 44.4 kg N/ha (NCC) and 17.6 and 18.9 kg N/ha (CC). The slightly over predicted N loss in drain flow for CC was because of over predicted nitrate concentration in drain flow, which may be partly associated with under predicted average annual rye shoot N by 1.8 kg N/ha (observed and simulated values were 47.8 and 46.0 kg N/ha). Also, recent research from the site suggests that the soil field capacity may be greater in CC while we used the same soil parameters for both treatments. A local sensitivity analysis suggests that increased field capacity reduces HERMES simulated nitrate concentration in drain flow, increases simulated denitrification, and reduces simulated drain flow volume. HERMES simulated cumulative monthly drain flow and simulated annual drain flow were reasonable compared to field data and HERMES performance was comparable to other published drainage model tests. Unlike the RZWQM simulations, however, the modified HERMES did not accurately simulate the year to year variability in nitrate concentration difference between NCC and CC, possibly due in part to the lack of partial mixing and displacement of the soil solution. The results suggest that 1) HERMES is a promising tool to estimate annual N loss in drain flow under corn-soybean rotations with winter rye as a cover crop and 2) soil field capacity is a critical parameter to investigate to more thoroughly understand and model denitrification and N loss to subsurface drainage.


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