Hydrologic impacts of subsurface drainage from the field to watershed scale

Subsurface drainage systems are common in agricultural landscapes around the world, and yet there is insufficient understanding on how they modify the hydrologic flow regime, from the field to the watershed scale. Here, we study the effect of agricultural subsurface drainage (tiling) on the hydrologic response as a function of the watershed scale by using the field-scale model DRAINMOD in conjunction with a linearized routing equation for two watersheds in eastern Iowa. Note that we primarily used the stream network, dominant soil types, and climate forcings in these watersheds and focused on a synthetic analysis to uncover the effect of tiling, while keeping other factors constant. Tile drainage was observed to increase low flows, decrease intermediate flows, and have minimal effect on the largest floods. The reduction in peak flows was observed to be dependent on the event size, antecedent conditions, and spatial scale, such that the greatest flow reductions were apparent at the intermediate scale (100 to 1,000 km²), with lower reduction at the largest scale (>10,000 km²). The scale dependence can be attributed to the fact that peak flows at larger scales are typically not caused by a single rainfall event but by the accumulation of flows over a longer temporal window. We further demonstrate that peak flows can vary for a basin with the same percentage of tiling but a different spatial organization of the tiled hillslopes. We connect our results to the width function, a well-known geomorphological descriptor of river network topology, and demonstrate how tile placement in relation to the width function controls watershed response. Although we focus primarily on peak flows, our results indicate that the impact of tiling on the flow distribution is also scale dependent, and alterations to this distribution will, in turn, control sediment and nutrient transport in a basin.