Search for: All records

High elevation mountain watersheds are undergoing rapid warming and declining snow fractions worldwide, causing earlier and quicker snowmelt. Understanding how this hydrologic shift affects subsurface flow paths, biogeochemical reactions, and solute export has been challenging due to the entanglement of hydrological and biogeochemical processes. Coal Creek, a high-elevation catchment (2,700 3,700 m, 53 km2) in Colorado, is experiencing a higher rate of warming than surrounding low-lying areas. This warming corresponds with dynamic and increased responses from biogenic solutes and dissolved organic carbon (DOC), whereas the behavior of geogenic solutes and dissolved inorganic carbon (DIC) has remained relatively unchanged. DOC has experienced the largest concentration increase (>3x), with annual average flow weighted concentrations positively correlated to average annual temperature. This suggests temperature is the main driver of increasing DOC levels. Although DOC and DIC response to warming is influenced by many drivers, the relative contribution of each remains unknown. DOC and DIC were analyzed to incorporate both carbon component products of soil respiration (DOC and CO2) and to represent high solute concentrations transported by shallow (DOC) versus deep (DIC) subsurface flow. The contrasting behavior of these carbon solutes indicates climate change and warming are driving changes in organic matter decomposition and soil respiration. Modeling results from the process-based model HBV-BioRT show increased temperatures cause earlier snowmelt and streamflow generation and lower peak discharge. As stream flow generation occurs earlier, so do DOC flushing and DIC dilution events. Additionally, post-snowmelt periods show greater DOC production and concentrations under warming scenarios. Results indicated increased production of DOC in post-snowmelt periods. DOC is then flushed out by earlier snowmelt partitioned through the shallow soil zone. Most process-based studies lack a watershed-scale understanding of carbon transformation and flow path alterations. This work demonstrates complex hydrologic and biogeochemical coupling at the watershed scale to illustrate how water flow paths and chemistry are responding to a changing climate in highelevation mountain watersheds. 

How does the physical and chemical structure of the Critical Zone (CZ), defined as the zone from treetops to the bottom of groundwater, govern its hydro-biogeochemical functioning? Multiple lines of evidence from past and newly emerging research have prompted the shallow and deep partitioning concentration-discharge (C-Q) hypothesis. The hypothesis states that in-stream C-Q relationships are shaped by distinct source waters from flow paths at different depths. Base flows are often dominated by deep groundwater and mostly reflect groundwater chemistry, whereas high flows are often dominated by shallow soil water and thus mostly reflect soil water chemistry. The contrasts between shallow soil water versus deeper groundwater chemistry shape stream solute export patterns. In this context, the vertical connectivity that regulates the shallow and deep flow partitioning is essential in determining chemical contrasts, biogeochemical reaction rates in soils and parent rocks, and ultimately solute export patterns. This talk will highlight insights gleaned from multiple lines of recent studies that include collation of water chemistry data from soils, rocks, and streams in intensively monitored watersheds, meta-analysis of stream chemistry data at the continental scale, and integrated reactive transport modeling at the hillslope and watershed scales. The hypothesis underscores the importance of subsurface vertical structure and connectivity relative to the extensively studied horizontal connectivity. It also alludes to the potential of using streams as mirrors for subsurface water chemistry, and the potential of using C-Q relationships to infer flow paths and biogeochemical reaction rates and the response of earth’s subsurface to climate and human perturbations. Broadly, this simple conceptual framework links CZ subsurface structure to its functioning under diverse climate, geology, and land cover conditions. 

Soil biota generate CO2 that can vertically export to the atmosphere, and dissolved organic and inorganic carbon (DOC and DIC) that can laterally export to streams and accelerate weathering. These processes are regulated by external hydroclimate forcing and internal structures (permeability distribution), the relative influences of which are rarely studied. Understanding these interactions is essential a hydrological extremes intensify in the future. Here we explore the question: How and to what extent do hydrological and permeability distribution conditions regulate soil carbon transformations and chemical weathering? We address the questions using a hillslope reactive transport model constrained by data from the Fitch Forest (Kansas, United States). Numerical experiments were used to mimic hydrological extremes and variable shallow-versus-deep permeability contrasts. Results demonstrate that under dry conditions (0.08 mm/day), long water transit times led to more mineralization of organic carbon (OC) into inorganic carbon (IC) form (>98\%). Of the IC produced, ~ 75\% was emitted upward as CO2 gas and ~ 25\% was exported laterally as DIC into the stream. Wet conditions (8.0 mm/day) resulted in less mineralization (~88\%), more DOC production (~12\%), and more lateral fluxes of IC (~50\% of produced IC). Carbonate precipitated under dry conditions and dissolved under wet conditions as the fast flow rapidly droves the reaction to disequilibrium. The results depict a conceptual hillslope model that prompts four hypotheses for our community to test. H1: Droughts enhance carbon mineralization and vertical upward carbon fluxes, whereas large hydrological events such as storms and flooding enhance subsurface vertical connectivity, reduce transit times, and promote lateral export. H2: The role of weathering as a net carbon sink or source to the atmosphere depends on the interaction between hydrologic flows and lithology: transition from droughts to storms can shift carbonate from a carbon sink (mineral precipitation) to carbon source (dissolution). H3: Permeability contrasts regulate the lateral flow partitioning via shallow flow paths versus deeper groundwater though this alter reaction rates negligibly. H4: Stream chemistry reflect flow paths and can potentially quantify water transit times: solutes enriched in shallow soils have a younger water signature; solutes abundant at depth carry older water signature. 

Woody encroachment is a widespread phenomenon in grassland ecosystems, driven by overgrazing, fire suppression, nitrogen deposition and climate change, among other environmental changes. The influence of woody encroachment on processes such as chemical weathering however is poorly understood. In particular, for fast reactions such as carbonate weathering, root traits associated with woody encroachment (e.g., coarser, deeper, and longer residence times) can potentially change fluxes of inorganic carbon into streams and back to the atmosphere, providing CO2-climate feedbacks. Here we examine the influence of deepening roots arising from woody encroachment on catchment water balance and carbonate weathering rates at Konza a tallgrass prairie within a carbonate terrain where woody encroachment is suspected to drive the groundwater alkalinity upwards. We use a watershed reactive transport model BioRT-Flux-PIHM to understand the ramifications of deepening roots. Stream discharge and evapotranspiration (ET) measurements were used to calibrate the hydrology model. The subsurface CO2 concentration, water quality data for groundwater, stream, soil water and precipitation were used to constrain the soil respiration and carbonate dissolution reaction rates. The hydrology model has a Nash-Sutcliffe Efficiency value of 0.88. Modelling results from numerical experiments indicate that woody encroachment results in overall lower stream flow due to higher ET, yet the groundwater recharge is higher due to deep macropore development from deepening roots. The deeper macropores enhance carbonate weathering rate as more acidic, CO2-rich water recharges the deeper calcite bedrock. Accounting for the change in inorganic carbon fluxes caused by such land use changes gives a better estimate of carbon fluxes in the biosphere. Such knowledge is essential for effective planning of climate change mitigation strategies. 

Clearcutting and other land-use changes are known to release terrestrial carbon and mobilize organic carbon into streamwater, significantly augmenting aquatic carbon levels in the short-term. However, little is known about the lasting impacts of forest management decisions on the riverine concentration levels of Dissolved Organic Carbon (DOC). Here we compare data from HJ Andrews Experimental Forest, a long-term ecological research (LTER) site located in the Oregon Cascades. We paired stream chemistry and discharge measurements spanning 15-30 years. Two watersheds that were 100\% clear-cut 40-50 years ago (WS01 and WS10) were compared with their unharvested and controlled counterparts (WS02 and WS09). Temporal analysis showed that, on average, DOC concentrations in the old-growth watersheds are notably higher than their harvested analogs to this day. This suggests even though clearcutting can release DOC from soil and vegetation to water, the terrestrial organic carbon stock is ultimately depleted post-clearcutting resulting in lower DOC concentrations. Concentration-discharge (CQ) analysis also revealed a sharp difference in behaviors between watersheds 1 and 2, with WS01 exhibiting a slight flushing pattern bordering on hysteresis while WS02 displayed a pronounced dilution pattern. Based on the shallow-deep hypothesis (Zhi et al. 2019; Zhi and Li, 2020) this indicates that the old-growth watershed has a pronounced groundwater DOC source, and clearcutting could have altered this source within WS01 and significantly lowered baseflow organic carbon concentrations. However, it should be noted that WS09 and WS10 displayed DOC behavior similar to that of WS01, which could also signify that the previously mentioned opposing CQ behaviors are a result of some underlying geological or lithological contribution unique to WS02. These competing hypotheses will be further tested using a watershed scale reactive transport model HBV-BioRT.",