The effects of tile-drain input on a low-gradient agricultural stream in central Illinois: Using a thermal end member mixing model and a statistical analysis approach

Tile-drains remove excess water from agricultural fields and channel it directly to the nearest surface water body decoupling the system from the natural flow paths. One way to measure the effects tile drains have on streams is to monitor the thermal energy both upstream and downstream of the tile input. In the temperate climate, a stream’s thermal signature experiences large fluctuations seasonally along with small diurnal changes. In contrast, groundwater temperature does not show these small-scale changes, and the seasonal changes are often muted and lagged in comparison to surface water. This project aimed to quantify any thermal change to the stream caused by the additional flow from a tile drain with a drainage basin 3% of the total watershed. Thermal signatures of the stream, streambed, tile-water and groundwater were measured using data loggers recording 15-minute intervals for a year. Temperature readings were collected throughout a 60-meter stretch of the streambed and within the hyporheic zone; the interactions were interpreted. These relationships were thus quantified and correlated using an ANOVA and unpaired t-tests. The tile has shown a more constant temperature (5-25°C) than streambed temperatures (nearly 0-30°C) over data collection from January to December of 2015 with a lack of statistically significant diurnal effects, however, seasonal cycles are visible. The streambed temperatures have no statistically significant relationship spatially, which represents uniform thermal conditions throughout the study site. Collectively, the data suggests the tile-drain has no thermal effect on the stream at the volume flux present at T3. Investigation into the hyporheic zone showed a consistent temperature regime from a depth of 0.1-0.4 m beneath the streambed; 5-10°C at the coldest and 16-26°C at their warmest. The hyporheic zone temperatures are not controlled by the groundwater (12-16°C) with colder temperatures throughout the winter and warmer ones during the summer. These data indicate that the hyporheic zone is controlled by surface water processes and less by upwelling from the groundwater with spatial heterogeneities of the streambed present