Understanding the impact of climate change, land use, and land management on riverine nitrogen loading

Anthropogenic actions have more than doubled the amount of reactive nitrogen in the terrestrial ecosystem. This over-enrichment of nitrogen has contributed to excessive nitrogen runoff to water bodies, also known as eutrophication. Eutrophication threatens water quality worldwide by increasing the occurrence, frequency, and intensity of impaired water quality events such as harmful algal blooms and hypoxia. Development of efficient management strategies for reducing the occurrence of harmful water quality events are dependent on understanding the spatial and temporal variability in historical nitrogen loading and associated drivers and anticipated changes in future loading due to climate change and changes in societal choices. Model-based nitrogen loading estimates used in the absence of sufficient observational water quality data, however, are available only for long-term hydrologically average conditions or for select regions. Also lacking are robust estimates of the long-term and large-scale impact of climate change and societal choices, individually and combined, on nitrogen loading. This dissertation fills these gaps and improves the understanding of nitrogen loading under current and future climatic conditions through four research studies. First, an empirical model is developed for predicting riverine nitrogen loading for watersheds within the continental United States. The developed model is used to estimate spatial and temporal variability in nitrogen loading and their drivers at both watershed and large aggregated scales. Second, robust estimates of the impact of changes in precipitation patterns on riverine nitrogen loading within the continental United States are developed. Third, the long-term impact of six different socioeconomic pathways on riverine nitrogen loading within the continental United States is examined. Fourth, estimates of riverine nitrogen loading are developed for river basins in southern India, along with identifying the primary drivers of variability in loading in this region. Overall, this work shows that considerable spatial and temporal variability exists in nitrogen loading in both the continental United States and India, and in both countries precipitation is the primary driver of year-to-year variability in loading. These studies also reveal that riverine total nitrogen loading within the continental United States will substantially increase (19 +/-14%) due to changes in precipitation patterns projected for the "business-as-usual" emission scenario and will also be substantially impacted, by up to an increase of over 50%, by future societal choices. This work has advanced the state of knowledge on current and future riverine nitrogen loading by developing nitrogen loading estimates for large spatial regions over a range of years as well as generating robust estimates of anticipated change in loading across a range of emission and socioeconomic scenarios. The findings from this work provide key guidance for developing effective management strategies to reduce the occurrence of impaired water quality events. Management strategies must account for the compounding roles of nitrogen inputs and meteorological conditions on year-to-year variability in loading and for the impact of future trends in both climate change and societal development on nitrogen loading.

Keywords: Environmental Earth System Science
Advisor: Field Christopher B. 
Michalak Anna M.
Boehm Alexandria 
Matson P. A. (Pamela A.)