MODELING THE LINKAGES BETWEEN TRANSIT TIME DISTRIBUTIONS, NITRATE TRANSPORT, AND CLIMATE VARIABILITY: OPPORTUNITIES AND CHALLENGES IN THE CHESAPEAKE BAY WATERSHED

The effect of climate change on Chesapeake Bay water quality will likely depend in part on the interplay between watershed transit time distributions (TTDs), nitrate transport, and climate variability. The TTD represents the distribution of travel times that water takes from entrance (e.g., rainfall) to exit (e.g., discharge) of a watershed system. The TTD varies spatially and temporally according to local watershed characteristics including catchment geometry, topography, soil type, and climatic conditions. Higher precipitation, for example, can be linked with faster transit times, which in turn have been associated with faster and more effective transport of nitrate from the land surface to nearby surface waters. The transport of nitrate from land sources is a major cause of nutrient pollution to the Chesapeake Bay. In this work, we review scientific literature on the following topics with an eye towards application in the Chesapeake Bay Watershed: (1) recent theoretical advances in TTD modeling, (2) applications of TTDs in nitrate transport models, (3) the influence of climate variability on TTDs, and (4) the effect of climate-induced changes in TTDs on nitrate transport. We also present relevant, preliminary modeling results using the rank StorAge Selection (rSAS) function method for time-varying TTD estimation (Harman 2014, in press). We conclude with a synthesis of important knowledge gaps and recommendations for future modeling experiments.