Agricultural Tile Drainage Influences the Thermal Regimes of Streams at Both Low and High Flow Conditions

ABSTRACT

Subsurface agricultural tile drainage is a widespread hydrologic modification, yet its influence on stream water temperature remains poorly understood. We evaluated relationships between watershed tile drainage extent and stream water temperature across 83 Midwestern United States watersheds using daily streamflow, air temperature, and water temperature observations combined with watershed attributes. We conducted three analyses: (1) random forest models to assess the importance of tile drainage relative to climatic, hydrologic, and landscape controls on seasonal water temperature, (2) analyses of water temperature anomalies under high‐ and low‐flow conditions, and (3) air‐water temperature sensitivity analyses to assess atmospheric coupling. Random forest models showed that air temperature, streamflow, and watershed characteristics were the dominant predictors of seasonal water temperature, whereas tile drainage was an important secondary predictor for several summer temperature metrics. High‐ and low‐flow analyses revealed stronger drainage‐related thermal responses, with greater tile drainage extent associated with increasingly negative summer high‐flow temperature anomalies ( ρ  = −0.51, p  < 0.001; approximately −0.32°C per 10% increase in tile‐drained area) and increasingly positive summer low‐flow anomalies ( ρ  = 0.43, p  < 0.001). These contrasting relationships are consistent with delivery of relatively cool shallow subsurface water during high‐flow conditions favourable for tile drainage, whereas warmer low‐flow conditions may reflect reduced groundwater buffering, altered watershed storage, or other characteristics of intensively drained agricultural landscapes. Tile drainage exhibited little relationship with air‐water temperature sensitivity (spring: ρ  = 0.003, p  = 0.981; summer: ρ  = 0.083, p  = 0.457), indicating little alteration of atmospheric coupling. Overall, tile drainage extent was associated with hydrologically dependent differences in stream thermal conditions, with the strongest relationships occurring under contrasting summer high‐ and low‐flow conditions.