Evaluating headwater hydrology: Tracer dye studies reveal prolonged residence times and potential modeling limitations during severe low-flow periods.

While models are commonly used to predict the fate and transport of agricultural chemicals in the environment, they often struggle to accurately estimate key stream characteristics, such as discharge and residence time, in headwater systems. To improve our understanding of these systems and assess model performance, tracer studies were conducted under diverse flow conditions in two headwater watersheds in the Midwestern United States. These studies revealed exceptionally long residence times (4-10 days) during low-flow conditions, explaining prolonged pesticide concentrations in the study system. Furthermore, residence times were found to decrease exponentially with small increases in stream discharge. Comparisons with model predictions (NHDPlus and PRZM-SWAT) highlighted significant limitations, particularly during low-flow conditions, where models underpredicted residence time by up to nine days. These results suggest that under extreme conditions (e.g., low flow), traditional models may underestimate exposure and persistence in headwater watersheds, potentially affecting risk assessment. Ultimately, these findings underscore the value of incorporating tracer study insights into future model assumptions to more accurately characterize potential agricultural chemical exposure in headwater systems under extreme conditions.