Advancing a Watershed Hydro-Biogeochemical Theory: Linking Water Travel Time and Reaction Rates Under Changing Climate

Mountain watersheds are often regarded as the 'water towers for humanity' (Viviroli et al. 2007). In the western USA, mountain snow packs are estimated to feed about three fourth of freshwater and more than 60 million people (Bales et al. 2006). They also represent the dominant-yet-vulnerable control on water availability. Here we choose Coal Creek, Colorado, a high-elevation (~ 2,700 – 3,700 m) mountainous catchment that has already seen 4 decades of continued temperature increases (> 5?C based on SNOTEL measurements) and declines in snow fractions ( ~ 70% in 1980 to ~50% in 2018). We ask the overarching question of how and how much do hydrological and biogeochemical functioning co-evolve in a warming climate? In particular, how do hydrological characteristics (water storage, flow paths, and travel time) change in response to a warming climate? What are the corresponding alterations in biogeochemical and chemical weathering rates at the watershed scale? What is the general rate law that causally links metrics of hydrological and biogeochemical rates to measures of external hydroclimatic conditions and internal watershed structure characteristics? Using a combination of field measurements, transit time and age modeling, and process-based watershed hydro-biogeochemical modeling, we aim to develop a general rate law at the watershed scale, establishing a largely-unknown linkage between the extensively studied hydrology travel time theory and the recently developed theory on old and new water fraction characteristics affecting biogeochemical and weathering rates. Reaction rate laws are generally derived and parameterized in small- scale and often well-mixed systems that have often been found non-applicable at the watershed scale. The Coal Creek watershed will serve as a model watershed to develop the general watershed-scale rate law, which will be further tested in a constellation of catchments within the USA and across the globe where the PIs have been engaged. These additional sites differ in the levels of snow dominance and geology, topography, and land cover conditions. The general law may be modified and expanded to ultimately encompass a variety of environmental conditions. The proposed work will offer mechanistic understanding of how a warming climate alters water flow, storage, and travel time characteristics; and how and how much such alterations speed up reactions that drive the evolution of catchments and ultimately Earth surfaces. Although these aspects have been extensively studied within disciplinary boundaries, the inter-linkage is not clear and the hydro- biogeochemistry community is often puzzled by watershed process coupling, feedbacks, and emergent behaviors, all of which are important for projecting to the future. This work aims to unravel such linkages by integrating existing knowledge and approaches, crossing hydrology and geochemistry boundaries, and advancing new fronts regarding climate change influences on water quantity and quality. The outcome of this work will advance our forecasting capabilities for water, mass, and energy in the rapidly changing planet. Notably, the developed general rate law will illustrate the first-order principles of hydro-biogeochemical process coupling. Ultimately, it will help develop forward projections to provide insight for adaptation, management, and prevention of water quantity and quality-related catastrophes. The proposed work continues our exploratory project on concentration discharge relationships at Coal Creek, CO. It will extend comprehensive data collection of soil, geologic and hydrologic characteristics, isotopic analyses and modeling of hydro-biogeochemical processes at the watershed scale. More importantly, this work will extend the insights gained from Coal Creek to other sites that are impacted by decreasing snow fractions, and advance the theory that hydrological and biogeochemical coupling are both influenced by Earth system changes.