Hypothesis-Based Model-Experiment (ModEx) Reveals Effects of Drivers and Disturbances on Basin-Scale Ecosystem Respiration
Authors
Vanessa Garayburu-Caruso1* (vanessa.garayburu-caruso@pnnl.gov), Etienne Fluet-Chouinard1*, Bob Hall2, Matt Kaufman3, Stephanie Fulton1, Morgan Barnes1, Jake Cavaiani1, Allison Myers-Pigg1, Xingyuan Chen1, James Stegen1, Tim Scheibe1
Institutions
1Pacific Northwest National Laboratory, Richland, WA; 2Univeristy of Montana, Missoula, MT; 3Worcester University, Worcester, MA
URLs
Abstract
River corridors emit significant amounts of carbon dioxide CO2 to the atmosphere through both abiotic and biotic processes. Biotic CO2 contributions (i.e., ecosystem respiration) from different riverine components (e.g., benthic, hyporheic, water column) are highly variable and poorly understood at basin scales. Researchers use hypothesis-based model-experiment (ModEx) to increase mechanistic understanding of ecosystem respiration within and between river corridor components and extend these mechanisms into conceptual models including disturbance effects at the basin-scale. The first ModEx cycle used River Corridor model outputs to hypothesize basin scale drivers of ecosystem respiration and design field experiments to test those hypotheses. Researchers evaluated whether the spatial variation in model-predicted deep hyporheic respiration explained spatial variation of field observed ecosystem respiration rates. The team found no correlative relationship between the predicted and observed rates; thus, researchers explored additional mechanisms studying the contributions from water column and sediment-associated components. This ModEx cycle revealed that sediment-associated respiration, excluding deep hyporheic zone respiration, are the primary drivers of spatial variation in ecosystem respiration in the testbed basin. A second ModEx cycle investigated the effects of wildfire disturbance on ecosystem respiration. Researchers first coupled field observations with a recently developed fire module of the SWAT model to evaluate how watershed function may be impacted by changing burned area and severity. Model simulations suggested that high burn severity led to the greatest increase in nitrate while moderate burn severity led to the greatest increase in dissolved organic carbon concentrations.
Researchers then designed a field campaign to test how these post-fire water quality changes could impact ecosystem respiration in fire impacted systems across different periods of hydrologic connectivity in the testbed basin. Together, the hypothesis-based ModEx cycles uncovered landscape predictors and mechanisms modulating ecosystem respiration at the basin scales, enabling hypothesis testing and model improvements not otherwise possible.