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.