Modelling the Spatio-Temporal Effects of Water Level Control in an Agricultural Collector Ditch Network

The study focused on modelling open channel flow in an agricultural ditch network. The objectives were (1) to set up a computational tool to explore the hydraulic impacts of water level control in a collector ditch network and (2) to test the effects of damming height and ditch flow resistance on the water level and its relation to field drainage. The model platform FLUSH is a hydrological model that couples field hydrology with a description of open channel flow network hydraulics under subarctic climate conditions. A one-dimensional model setup described an existing collector ditch network in North Ostrobothnia, Finland, where fields under normal farming practices were instrumented to explore agricultural water management designs. In the model, drainage was fed to the ditch network as a spatially varied inflow. Four weir heights and flow resistances were used to run 16 simulations. Although flow resistance increased water levels in the ditch network, the absolute damming effect was the highest with low flow resistance. The water level control decreased the flow velocity within the control, which increased to 20%. According to the model simulations, a single weir downstream affected field drainage in up to 25% of the field area. The model provides a transparent means to investigate water management methods in fields and in collector ditches and to support the design of catchment-scale water level control structures.