Hydrologic Modeling of a Complex Wetland

A coupled model was used to simulate the hydrology of a 22 km² agricultural marsh characterized by a very dense channel network composed of 1,800 storage basins closely connected with 500 km of ditches. The model couples the field-scale drainage model SIDRA with the hydraulic model MAGE. The SIDRA-MAGE model simulates the hydrology of independent fields, either subsurface drained or undrained, and routes outflows from the fields through numerous basins, ditches, and canals to the outlet of the marsh. Evapotranspiration from surface water bodies colonized by aquatic plants, seepage from ditches, and management of water gates in the channel network are considered. Calibration and validation of the SIDRA, MAGE, and coupled SIDRA-MAGE models are not presented in the paper. The study focuses instead on the simulation of the hydrological effects of eight subsurface drainage schemes for a 72-h event. As the percentage of marsh drained increases from 0 to 88%, peak flow, mean flow, and total outflow volume at the marsh outlet are increased by 7, 27, and 27%, respectively. However, there is no significant changes in outflows at the marsh outlet when the percentage of marsh drained increases from 44 to 62%, even though flow from fields was predicted to increase by 8%. This is attributed, first, to a small change of the channel network efficiency, which is less than 8%, and, second, to the distribution of drained areas within the marsh. Drainage effects on the hydrology of the marsh were also simulated for summer conditions. Reduction of the channel network length and the area of basins by 72 and 84%, respectively, would reduce evapotranspiration losses from surface water bodies by 30% and save up to 11,000 m³/day of freshwater.