Investigating Evapotranspiration from a Two-Stage Ditch in Southern Minnesota

Drainage has drastically altered hydrology and water quality of the Upper Midwest. Studies show that baseflows have increased in agriculturally dominated watersheds which can increase stream power and sediment loading. Tile discharge to ditches and streams also contain nitrates that have contributed to hypoxia in the Gulf of Mexico. Two-stage ditches are being investigated for their potential to mitigate these effects. These designs incorporate a low flow channel flanked by riparian benches into a widened ditch. The benches provide a foundation for riparian plant establishment close to the water table. In this study, we examined evapotranspiration (ET) from the Mullenbach Two-Stage Ditch in Mower County, Minnesota. Streamflows were gauged and diurnal fluctuations in the hydrograph, indicative of ET, were observed. Using a water budget approach, ET from the two-stage ditch was found to range from 1.1 to 3.6 mm/day, averaging 60% of potential evapotranspiration during the study period in August. Channel flows were recorded downstream of both the two-stage ditch and a standard ditch and the fraction of groundwater flows lost due to ET were compared. The fraction of groundwater loss in the twostage reach (M=10.11%, SD=2.59) was approximately 3.8% greater than from the standard ditch reach (M=6.31%, SD=2.24). This difference was significant as determined by a paired t-test (p=0.021), but may be conservative due to sluggish groundwater seepage rates and lower overall flows in the standard ditch. Nitrate levels available to transpiring plants sampled in the ditch ranged from 0.2 to 44.7mg/L. In October, following plant senescence, the fraction of groundwater loss due to ET did not vary between the two reaches. These findings suggest that during the growing season, two-stage ditches evapotranspire larger volumes of water than standard ditches and therefore have the potential to reduce baseflows and promote plant sequestration of nitrates from drainage waters.