Biogeochemical Regimes in Intensively Managed Catchments: Confounding Complexity or Emerging Simplicity?

Abstract:

Watersheds have been shown to demonstrate a range of both chemostatic and chemodynamic responses to changes in discharge based on land use, solute species, and hydrologic regime as well as the presence or absence of large solute mass stores within the watershed. The dynamics of solute export, as captured by concentration-discharge patterns, can be described mathematically based on a power-law concentration-discharge relationship of the form , where a purely chemostatic system is characterized by a b value equal to 0. In practice, catchments across a range of land uses and geologic conditions may exhibit a distribution of solute export behaviors, and such dynamics can be grouped into different regimes as a function of differences in b values. It is our hypothesis that different solute export regimes are created primarily as a function of solute mass stores in different compartments of the landscape, e.g. the vadose zone or groundwater, and that these regimes may vary temporally based on the triggering of these compartments as a function of stochastic forcing within and between years.

In the present work, we synthesize both high and low temporal resolution concentration data for a range of geogenic and anthropogenic solutes to better our understanding of the variations in concentration-discharge relationships as a function of land use, climate and solute type. We then develop a parsimonious model to explain these patterns as a function not only of solute mass stores, but also of the triggering of fast and slow transport pathways across a range of precipitation dynamics. Specifically, we have developed a modeling framework to address the following questions: (1) How do C-Q relationships vary across climate and land-use gradients? (2) Is there any seasonality to C-Q patterns? (3) Can C-Q patterns be explained as a function of the activation of fast and slow hydrologic pathways in response to stochastic, hydro-climatic forcing? (4) How do these interactions lead to the development of hot spots and hot moments in catchment biogeochemical processing?