Numerical Investigation of the Spatiotemporal Contribution of Tile Drainage to Stream Flow

Abstract:

Tile drainage systems are pervasive in the Central U.S., significantly altering the hydrologic system. A physically based coupled hydrologic model was applied to a 45 km² agricultural Iowa watershed. The tile drainage contribution to stream flow (Q_T/Q) was derived from a tracer driven analysis of instream surface water. Q_T/Q varied instantaneously from 6% to 71 % at the basin outlet, with tile flow correlating linearly with total stream flow. In low precipitation periods 62 % of stream flow traveled through the tile system. In heavy precipitation periods a dilution effect shifted Q_T/Q to 27 %. Precipitation driven events produced a strong positive logarithmic correlation between Q_T/Q and drainage area. The addition of precipitation into the system saturated near surface soils, increased lateral soil water movement, and diluted the relatively stable instream tile flow. A negative logarithmic trend in Q_T/Q to drainage area persisted non-event durations. Larger groundwater (non-tile) contribution to stream flow at the outlet diluted instream tile flow at increased drainage areas. Logarithmic regression slopes were consistent for event and non-event periods, respectively. While, the intercept responded in a predicable manner to precipitation intensity. This study, indicates a strong systematic response of Q_T/Q to meteorological forcing, drainage area over a single year.