The impact of drain tiles was not very well documented. The purpose of tile installation was to reduce soil moisture and increase ventilation in the vadose zone to promote crop growth. Since the extensive tile installations in the 1900s, Minnesota has lost large amount of wetland as surface water storage. Reduction of the surface storage reduces water holding time and release water downstream at a higher rate. Drain tiles also altered subsurface hydrology. Prior to tile installation, subsurface water seeps into the streambank by capillary flow and preferential flow. Both are a lot slower than pipe flow. Once water enters the drain tile, it will move through and end up in the stream quicker than usual. Drain tiles are known for their potential to cause stream peak flow increases. However, drain tiles don’t allow control over soil moisture during drier season. Lack of soil moisture also negatively affect the yield. One of the agricultural best management practices, controlled drainage, let water to be held in the field by controlling the outlet of the tile. This practice also affects local hydrology by increasing residence time of water in the soil profile. Water quality is closely tied to hydrology as residence time is one of the controlling factors of biogeochemical processes in the soil. This study investigated the hydrological impact of agricultural drain tiles and controlled drainage by: • Estimating field water budget with field measured data including soil moisture and tile flow; • Investigating tile drained landscape water characteristics by using stable water isotopes to perform hydrograph separation and estimate water mean transit time through different depths in the field. Three tile drained fields were analyzed in this study: Beresford, SD, Tracy, MN, and Waseca, MN. All three sites had plots of the field functioning as controlled drainage and tile drainage for comparison purposes. Waseca site also had part of the field as perennial vegetation. This study found that although under a controlled drainage condition, water was kept in the field rather than let out through the tile, measured soil moisture content was lower than that of the drained condition, causing a decrease in the evapotranspiration. Controlled drainage could behave differently under various field conditions. Therefore, timing of release is critical to controlled drainage system and field monitoring data should be used to support decision making. Stable isotopes of oxygen (oxygen-18) and hydrogen (deuterium) were used to investigate hydrologic characteristics for the three sites. Monthly hydrogen and oxygen stable isotope samples were collected for tile flow, well water, stream flow, precipitation, and soil water. Local meteoric water lines were established for the comparison of magnitude of evaporation from different sources at each location. Two end-member hydrograph separation was performed at each site on selected dates to partition tile drainage contribution to streamflow. The separation results showed significant contribution of tile flow to the streamflow. Same isotopes were used to estimate the mean transit time of water through different depths in the fields. Lumped parameter modeling approach was applied to each data set to investigate the mean transit time of water through different depths of the field, such as groundwater and tile. This study found that precipitation water took an average of 9 months to move through different pathways and gain groundwater isotopic signature and an average of 4 months to gain tile water signature. In summary, vadose zone is a complicated system. The information provided by the study helps gain understanding of average holding time of water in the soil profile before discharging out via tiles, the magnitude of tile water contribution to stream flow, and impact of controlled drainage on evapotranspiration. However, due to the limitation of sampling and monitoring, questions remain that how surface water and vadose zone water affects water budget. Also, the variation in the soil vertical structure can add difficulty to understanding the behavior of the system.