Accurate quantification of evapotranspiration (ET) is crucial for surface water resources and best agricultural management practices in watersheds. The aim of this study was to better understand ET changes caused by the rapid expansion of C4 (corn) cover and rapid changes in stomatal conductance, which may be amplified in the future due to environmental and human-contributing factors, such as climate change and agricultural practices. Linking the enlargement of agricultural land with the physiological properties of crops, such as photosynthetic adaptations and stomatal conductance, is necessary to explore the magnitude of these impacts. This study examined the effects of increased C4 (corn) crop cover and stomatal conductance on evapotranspiration (ET) rates in the Lower Maumee River Watershed, Ohio, USA, during the 2018 growing season. Simulation results using a modified-for-crops version of the Boreal Ecosystem Productivity Simulator (BEPS) showed that a hypothetical increase of corn cover by as much as 100% would not significantly impact the watershed ET rate, with a 5.05% overall increase in ET in July and a 3.96% increase in August. Changes in the stomatal conductance of crops, however, impacted ET more. The results showed a significant increase in the ET rate (up to 24.04% for corn and 5.10% for soybean) for the modeling scenario that integrated high stomatal conductance, which agreed with the thermal-based ECOSTRESS ET product derived over the study area (+/−0.9 mm day−1) for the same period. We suggest that the alteration of the crop stomata mechanism, caused largely by rapid climate change and intensive farming practices, should be carefully quantified, and its impact on hydrology at the ecosystem level further explored.