| Summary: | Intensive agricultural activities have altered the hydrology of many Midwestern USA landscapes. Drain tiles (subsurface corrugated and perforated flexible tubing) has changed how and when water is discharged into the streams. Stable isotopes of oxygen (oxygen-18) and hydrogen (deuterium) were used to investigate hydrologic characteristics of three intensively managed agricultural landscapes in southern Minnesota (MN) and South Dakota (SD). Monthly δD and δ<sup>18</sup>O samples were collected across a climatic gradient from March 2016 to March 2018. Local meteoric water lines were established for the comparison of precipitation and evaporation magnitude from different sources at each location. These included vadose zone, phreatic zone, deep groundwater, tile drain, and river source waters. Two end-member hydrograph separation was performed at each site on selected dates to partition the shallow groundwater tile drainage contribution to streamflow. A lumped parameter modeling approach was applied to each dataset to investigate the mean transit time of water through different zones. Local meteoric water lines demonstrated differences in isotopic signatures due to the climate gradient to show the impact of low humidity and less rainfall. The hydrograph separation results showed that, from west South Dakota to eastern Minnesota, tile drains contributed about 49%, 64%, and 50% of the watershed streamflow. Precipitation took an average of 9 months to move through different pathways to end up in groundwater and an average of 4 months to end up in tile drains. This study confirms the important role tile drains play in the intensively managed fields and watersheds of Midwestern agriculture.
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