The Long-term Field-scale Hydrology of Subsurface Drainage Systems in a Cold Climate

Subsurface drainage is a common practice in the agricultural regions of the northern Midwest. Concerns about the impact of subsurface drainage on surface water quality and hydrology have increased over the past decade, spawning continued research on artificial drainage. Annual and decadal climatic variability necessitates a long-term perspective to fully evaluate these impacts. In addition, less is known about the hydrology of artificially drained lands in cold regions, where soil freezing and snowmelt routinely occur. A field-scale hydrologic analysis of subsurface drainage was performed using DRAINMOD v5.1 for an 85-year climatic period (1915 to 1999) for south-central Minnesota. This version of DRAINMOD included modifications for soil freeze/thaw and snowmelt processes. The results give insight into the long-term variations of subsurface drainage and other precipitation abstractions in a cold climate. Seasonal hydrology was characterized by well defined drainage and evapotranspiration (ET) "seasons" from March to June and from July to October, respectively. On average, 74% of infiltrated water was removed by subsurface drainage during the drainage season (March to June), representing 27% of annual precipitation. Significant increases in the 28-year average precipitation and drainage depths were apparent over the 85-year simulation period. Frequency and extreme value analysis revealed that annual drainage represented approximately 40% of annual precipitation at the 2-year return interval. Simulated annual drainage volumes were distributed normally, wherein the predicted 100-year annual drainage depth was 532 mm. Decreased drain spacing was found to significantly affect infiltration and drainage over the 85-year period. The results have potential uses for drainage practitioners, water management decision makers, and environmental and agricultural scientists and may pose significant challenges for addressing water quality issues on artificially drained lands.