Regional Variability of Subsurface Drainage in the U.S. Corn Belt
Intensive agricultural drainage in the U.S. Corn Belt is the primary pathway for non-point source nutrient loading in the Upper Mississippi River Basin. Understanding the historic regional variability in hydroclimate drivers of subsurface drainage, as well as the influences of climate change, is important for implementing appropriate Drainage Water Management (DWM) practices to improve water quality. Observed drainflow data from Minnesota, Iowa, Illinois, Indiana, and Ohio were used to parameterize the Variable Infiltration Capacity (VIC) macro-scale hydrologic model, with a subsurface drainage algorithm, for use in the Corn Belt. Simulated soil frost depth, non-growing season precipitation, and USDA Hardiness Zone were found to be the strongest hydroclimate predictors of subsurface drainage regime during the historic period of 1981 – 2010. The northwestern portion of the region is drier as the southwestern portion of the region receives up to twice the amount of annual precipitation. In addition, northern colder hardiness zones (3a – 4b) historically experience deeper frost depths, resulting in shorter drainage seasons that start later in the spring and produce lower, more variable annual drainage compared to southern zones (5a – 6b). This generally aligns with field observations; however, the simulated drainage season was shorter and the ratio of annual drainflow to precipitation was on the lower end of the observed range. An ensemble of three Atmosphere-Ocean Global Climate Models (Parallel Climate Model, Geophysical Fluid Dynamics Laboratory Coupled Model, Hadley Centre Coupled Model Version 3) was used to calculate multi-model means across low (B1), moderate (A1B) and high (A2) carbon emissions scenarios to evaluate projected changes in hydroclimate and drainage metrics. By mid-century (2035 – 2064) for the A2 scenario, annual subsurface drainage is projected to increase between 2 and 64 mm across the region, with regional median increase of 38 mm. The projected increase in drainage coincides with shallower maximum frost depths and wetter non-growing seasons. Three main patterns of change in subsurface drainage regime were identified: the northwestern zones (3b – 4a), the mid-latitude transitional zones (4b – 5a), and the southeastern zones (5b – 6b). In general, projected subsurface drainage season is lengthening a few weeks in the northwestern zones and up to a full month in the mid-latitude transitional and southeastern zones. Compared to historical conditions, the northwestern zones are projected to have a 16 – 27 cm decrease in soil frost depth and a 12 – 13 % increase in annual precipitation occurring in the non-growing season. Subsurface drainage in the northwestern zone is projected to remain a cold season process dominated regime and increase the least (2 – 18 mm) across the region, while overland flow is projected to increase < 8 mm. The mid-latitude transitional zone is projected to experience a shift from a cold process to precipitation driven subsurface drainage regime as soil frost depths decrease 12 – 24 mm and the non-growing season precipitation increases 12 – 18 %. Across the region, the greatest percent increase in drainage (31 – 53%, 23 – 54 mm) and overland flow (9 – 24%, 6 – 24 mm) compared to historic conditions is projected for the mid-latitude transitional zone. The southeastern zones are projected to remain in a precipitation driven subsurface drainage regime, while already shallow soil frost depths decrease 4 – 18 cm and non-growing season precipitation increases 9 – 13%. The greatest increase in subsurface drainage (37 – 64 mm) and overland flow (4 – 48 mm) for the U.S. Corn Belt is projected for the southwestern zones.
Bowling, Purdue University.
Agronomy|Hydrologic sciences|Climate Change
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