Predicting drainage impacts
Predicting the drainage impacts on downstream peak events is complex because it involves studying many physical processes at different interdependent scales: the individual field, the drainage network, and the catchment.
This group of researchers from France and North Carolina performed an experiment performed in a small subsurface-drained catchment (130 ha), east of Paris, France. The main objective was to study the influence of the temporary pipe pressurization of the drainage network on the field drainage discharge during intense rainfall events.
The flow rates at one buried pipe collector within the drainage network and at the catchment outlet were monitored concurrently with the pressure head in the pipe. Water levels were recorded in rows of piezometers aligned between drains connected to the studied collector so that water storage in the water table could be investigated during pipe pressurization.
Two years of observations showed that when a pipe pressurization condition existed, the collector discharge was limited or reversed and the infiltrated water was temporarily stored within the field soil as the water table rose. Between the usual stages of normal subsurface drainage (peak flow and recession), two additional behaviors appeared to occur, similar to those resulting from implementing controlled drainage and subirrigation practices, and with noticeable consequences on the discharge hydrograph.
No strong relationship could be established between the collector discharge limitation and the catchment hydrology, probably because pressurization affected only a small part of the drainage network.