Modelling subsurface drainage and nitrogen losses from artificially drained cropland using coupled DRAINMOD and WHCNS models

Abstract

For the simulation of soil water movement, nitrogen (N) dynamics, and crop growth under tile drainage, current interdisciplinary models or modules were integrated into an established model through a typical technique for solving specific soil-crop modeling issues. A DRAINMOD-based drainage component was developed for the soil Water Heat Carbon Nitrogen Simulator (WHCNS) model. The newly developed model was validated using two field experiment datasets from Iowa: (i) four year data of daily/monthly/annual subsurface drain flow and N losses, and seasonal crop growth data, and (ii) 16year data of subsurface drain flow. For the drainage-enhanced WHCNS model, the mean (n=4; calibration/validation×corn/soybean) normalized root mean square errors (nRMSE) for soil water storage, crop dry matter, yield, and crop N uptake simulation were 8.0%, 18.9%, 6.9% and 12.5%, respectively. The nRMSE values for annual subsurface drainage and N losses totals were 5.8% and 37.9%, respectively. Meanwhile, the monthly subsurface drainage was 12.1%, and the N losses was 63.0%. For the 16-year drainage plots, the annual subsurface drainage simulated during the validation phase closely matched measured values in most years (coefficient of determination, R2=0.92; RMSE=59mm, nRMSE=23.4%, index of agreement, IA=0.97, and Nash-Sutcliffe efficiency, NSE=0.9). Thus, indicating that the integrated model can be used to simulate water balance, N losses and crop growth in a subsurface drained crop production system. Based on scenario analyses, an optimal N application rate ranging from 100 to 120kgNha−1 was recommended to reduce subsurface drainage water nitrate concentrations within the federal drinking water standard (<10mgL−1) while maintaining corn yield. The newly developed model showed a great potential to evaluate the short-/long-term effects of subsurface drainage system operations on water balance, N losses and crop growth.