THE INFLUENCE OF CLIMATE ON WATERSHED TRANSIT TIMES, WITH IMPLICATIONS FOR NITRATE TRANSPORT IN THE CHESAPEAKE BAY WATERSHED

The ability to accurately model the timing and quantity of contaminant transport from landscapes to surface waters under different climate conditions is vital to the development of climate-resilient watershed management tools and strategies. Although hydrologic transport cannot be directly measured at the full range of relevant scales, a measurable proxy at catchment scale is the integrated transit time distribution (TTD). The TTD is the time-varying, probabilistic distribution of water travel times or, equivalently, water ages in catchment outflow. This dissertation presents advances in hydrologic theory and catchment-scale modeling and uses them to learn about the influence of climate on TTD behavior at multiple sites. The specific contributions of this work include (1) the first benchmarking of the sensitivity of catchment transit times to present and projected climate conditions, which shows that climate change could significantly shift the phenology of stream age; (2) the introduction of a computationally efficient approach to calibrating integrated surface-subsurface hydrology models (ISSHMs) under realistic climate forcing to both discharge and stream age, and subsequent virtual experiments suggesting that the age of baseflow is significantly influenced by upper soil properties due to dynamic hydrologic partitioning, which is not captured in steady-state simulations; (3) a novel analysis using flowpath decomposition in an ISSHM to understand the influence of climate on catchment TTD dynamics, which reveals a complex relationship between flowpath and water age that belies suggestions in the literature of a one-to-one mapping; and (4) proof-of-concept of an enhancement to the popular Soil and Water Assessment Tool (SWAT) that allows users to calculate groundwater TTDs, calibrate TTDs to available data, and more realistically simulate groundwater nitrate transport with variable recharge rates. The dissertation concludes with a brief discussion of its implications on our understanding of groundwater nitrate transport in the Chesapeake Bay watershed under present and future climates