Calibration and validation of DRAINMOD to design subsurface drainage systems for Iowa's tile landscapes

The study presents the potential use of existing common data sources such as a soil database, and techniques such as a pedotransfer function and a non-linear parameter estimator to calibrate and validate a hydrologic model to design subsurface drainage systems for Iowa's tile landscapes. A field scale deterministic hydrological model, DRAINMOD has been calibrated and validated for two soils: Webster soil cultivated with continuous corn (WEBS_CC) and Canisteo soil cultivated with corn–soybean rotation (CANI_CS). The cumulative subsurface drainage over the calibration and validation years from 1990 to 2003 was predicted 2% higher than the observed subsurface drainage for WEBS_CC (coefficient of mass residual CRM = 0.02) and 10% lower for CANI_CS (CRM = −0.10). The overall values of index of agreement IoA and model efficiency EF were higher than 0.85 for both WEBS_CC and CANI_CS, and showed a close agreement between the predicted and observed subsurface drainage. The calibrated and validated DRAINMOD was further used to simulate impacts of varying designs of subsurface drainage system for WEBS_CC over the 14 (1990–2003) years of weather record in Iowa's tile landscapes. Simulation results suggest that a drainage system designed for a drainage intensity of 0.46 cm day−1 with a drain depth of 1.05 m and drain spacing of 25 m is sufficient enough to maximize crop production (average relative yield ≈98%) while minimizing subsurface drainage and its associated nitrate-nitrogen (NO3-N) loss. Though the results are based on the simulations with simple and approximate methods to represent the real system, they indicate that installing the drains at shallower depth (<1.05 m) might help to reduce subsurface drainage, but there might be negative impacts in terms of increased excess water stress on crop production and increased surface runoff. Field experiments are recommended to study the impact of shallow drainage on the complete water balance and crop production in Iowa's tile landscapes.