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- All Subjects: Sierra Nevada
- Creators: Heimsath, Arjun M.
Description
Ponderosa pine forests are a dominant land cover type in semiarid montane areas. Water supplies in major rivers of the southwestern United States depend on ponderosa pine forests since these ecosystems: (1) receive a significant amount of rainfall and snowfall, (2) intercept precipitation and transpire water, and (3) indirectly influence runoff by impacting the infiltration rate. However, the hydrologic patterns in these ecosystems with strong seasonality are poorly understood. In this study, we used a distributed hydrologic model evaluated against field observations to improve our understandings on spatial controls of hydrologic patterns, appropriate model resolution to simulate ponderosa pine ecosystems and hydrologic responses in the context of contrasting winter to summer transitions. Our modeling effort is focused on the hydrologic responses during the North American Monsoon (NAM), winter and spring periods. In Chapter 2, we utilized a distributed model explore the spatial controls on simulated soil moisture and temporal evolution of these spatial controls as a function of seasonal wetness. Our findings indicate that vegetation and topographic curvature are spatial controls. Vegetation controlled patterns during dry summer period switch to fine-scale terrain curvature controlled patterns during persistently wet NAM period. Thus, a climatic threshold involving rainfall and weather conditions during the NAM is identified when high rainfall amount (such as 146 mm rain in August, 1997) activates lateral flux of soil moisture and frequent cloudy cover (such as 42% cloud cover during daytime of August, 1997) lowers evapotranspiration. In Chapter 3, we investigate the impacts of model coarsening on simulated soil moisture patterns during the NAM. Results indicate that model aggregation quickly eradicates curvature features and its spatial control on hydrologic patterns. A threshold resolution of ~10% of the original terrain is identified through analyses of homogeneity indices, correlation coefficients and spatial errors beyond which the fidelity of simulated soil moisture is no longer reliable. Based on spatial error analyses, we detected that the concave areas (~28% of hillslope) are very sensitive to model coarsening and root mean square error (RMSE) is higher than residual soil moisture content (~0.07 m3/m3 soil moisture) for concave areas. Thus, concave areas need to be sampled for capturing appropriate hillslope response for this hillslope. In Chapter 4, we investigate the impacts of contrasting winter to summer transitions on hillslope hydrologic responses. We use a distributed hydrologic model to generate a consistent set of high-resolution hydrologic estimates. Our model is evaluated against the snow depth, soil moisture and runoff observations over two water years yielding reliable spatial distributions during the winter to summer transitions. We find that a wet winter followed by a dry summer promotes evapotranspiration losses (spatial averaged ~193 mm spring ET and ~ 600 mm summer ET) that dry the soil and disconnect lateral fluxes in the forested hillslope, leading to soil moisture patterns resembling vegetation patches. Conversely, a dry winter prior to a wet summer results in soil moisture increases due to high rainfall and low ET during the spring (spatially averaged 78 mm ET and 232 mm rainfall) and summer period (spatially averaged 147 mm ET and 247 mm rainfall) which promote lateral connectivity and soil moisture patterns with the signature of terrain curvature. An opposing temporal switch between infiltration and saturation excess runoff is also identified. These contrasting responses indicate that the inverse relation has significant consequences on hillslope water availability and its spatial distribution with implications on other ecohydrological processes including vegetation phenology, groundwater recharge and geomorphic development. Results from this work have implications on the design of hillslope experiments, the resolution of hillslope scale models, and the prediction of hydrologic conditions in ponderosa pine ecosystems. In addition, our findings can be used to select future hillslope sites for detailed ecohydrological investigations. Further, the proposed methodology can be useful for predicting responses to climate and land cover changes that are anticipated for the southwestern United States.
ContributorsMahmood, Taufique Hasan (Author) / Vivoni, Enrique R. (Thesis advisor) / Whipple, Kelin X. (Committee member) / Shock, Everett (Committee member) / Heimsath, Arjun M. (Committee member) / Ruddell, Benjamin (Committee member) / Arizona State University (Publisher)
Created2012
Description
Geochronology and thermochronology are valuable tools for investigating the synergy between the deformational and erosional processes that shape mountainous terrains. Though numerous techniques have been developed to probe the rate and timing of events within these settings, the research presented here explores how scientists can use fewer samples to produce richer data products with broader contextual importance.
The beginning of this compilation focuses on establishing laboratory techniques to facilitate this goal. I developed a novel laser ablation ‘double dating’ (LADD) technique that rapidly yields paired U/Pb and (U-Th)/He dates for the accessory minerals zircon, titanite, and apatite. The technique obviates the need for geometric corrections typically applied during (U-Th)/He data reduction, enables the analysis of a broader spectrum of detrital crystals, and provides the opportunity for additional mapping and isotopic analyses that are traditionally challenging to procure and/or fraught with assumptions. Despite the technique’s promise, I also found it essential to weigh several considerations of relevance when attempting to date young (≤ Miocene) accessory minerals with low concentrations of U + Th. Consequently, I discuss the impact that such variables have on the magnitude of analytical imprecision and the data’s flexibility for geologic interpretation.
Beyond the lab, I collected a suite of bedrock and detrital samples from small catchments draining the southeastern Sierra Nevada mountains of California. Using the techniques described above as well as conventional methods for (U-Th)/He zircon dating, I compared the utility of both bedrock and detrital approaches for extrapolating local exhumation histories. I additionally tested the ability to employ detrital datasets to extrapolate cooling histories that span from mineral crystallization to rock exhumation through the upper crust. Employing principal mode dates from a combination of zircon and apatite LADD dates and detrital hornblende 40Ar/39Ar dates, I was able to derive thermal models that demonstrate the existence of significant variability in the cooling histories of various intrusive units along the eastern Sierra Nevada. While these results only scratch the surface of what’s possible within the realm of detrital-based research, this contribution demonstrates the utility of expanding the temporal and spatial scope of traditional detrital methodologies.
The beginning of this compilation focuses on establishing laboratory techniques to facilitate this goal. I developed a novel laser ablation ‘double dating’ (LADD) technique that rapidly yields paired U/Pb and (U-Th)/He dates for the accessory minerals zircon, titanite, and apatite. The technique obviates the need for geometric corrections typically applied during (U-Th)/He data reduction, enables the analysis of a broader spectrum of detrital crystals, and provides the opportunity for additional mapping and isotopic analyses that are traditionally challenging to procure and/or fraught with assumptions. Despite the technique’s promise, I also found it essential to weigh several considerations of relevance when attempting to date young (≤ Miocene) accessory minerals with low concentrations of U + Th. Consequently, I discuss the impact that such variables have on the magnitude of analytical imprecision and the data’s flexibility for geologic interpretation.
Beyond the lab, I collected a suite of bedrock and detrital samples from small catchments draining the southeastern Sierra Nevada mountains of California. Using the techniques described above as well as conventional methods for (U-Th)/He zircon dating, I compared the utility of both bedrock and detrital approaches for extrapolating local exhumation histories. I additionally tested the ability to employ detrital datasets to extrapolate cooling histories that span from mineral crystallization to rock exhumation through the upper crust. Employing principal mode dates from a combination of zircon and apatite LADD dates and detrital hornblende 40Ar/39Ar dates, I was able to derive thermal models that demonstrate the existence of significant variability in the cooling histories of various intrusive units along the eastern Sierra Nevada. While these results only scratch the surface of what’s possible within the realm of detrital-based research, this contribution demonstrates the utility of expanding the temporal and spatial scope of traditional detrital methodologies.
ContributorsHorne, Alexandra Michelle (Author) / Hodges, Kip V. (Thesis advisor) / van Soest, Matthijs C. (Committee member) / Whipple, Kelin X (Committee member) / Heimsath, Arjun M. (Committee member) / Reynolds, Stephen J. (Committee member) / Arizona State University (Publisher)
Created2019