Understanding controls of hydrologic and geochemical processes using model-data integration at the watershed scale
Open Access
- Author:
- Xiao, Dacheng
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 23, 2020
- Committee Members:
- Li Li, Dissertation Advisor/Co-Advisor
Li Li, Committee Chair/Co-Chair
Zuleima T Karpyn, Committee Member
Shimin Liu, Committee Member
Susan Louise Brantley, Outside Member
Mort D Webster, Program Head/Chair - Keywords:
- Stream generation
Chemical weathering
Critical measurement
Watershed - Abstract:
- The watershed is the basic unit for the coexistence of the atmosphere, hydrosphere, lithosphere and biosphere, connecting minerals, plants, living things and water closely through many physical and chemical processes. It provides water for biological survival and food growth, and carries many physical and chemical processes in the evolution of landforms. The streamflow generation and chemical weathering are important functions of watersheds. These processes are often influenced by a multitude of competing factors including external forcings (e.g., climate) and internal structure characteristics (e.g., topography, lithology, subsurface property distribution). Among these factors, it remains poorly understand how different aspects of internal structure characteristics, in particular geomorphological properties such as topography and subsurface characteristics such as permeability distribution, affect these processes. Field observations have been used to understand the mechanisms of streamflow generation and chemical weathering. However, field observations are expensive, demanding labor, time, and money, which implies observations cannot be made every time everywhere. The question is then what are the critical measurements that are most important to understand processes? Answering these two questions could largely improve the efficiency of field observation and enhance the fundamental understanding of stream generation and chemical weathering. This dissertation will answer these two questions. For stream generation, we conducted watershed comparison and critical measurement identification to evaluate different types of soil moisture measurements and controlling factors of streamflow generation. We combined data and model to understand the relative influence of soil properties, topography, and catchment size on streamflow generation in two first-order, monolithological catchments in central Pennsylvania experiencing the same climate (temperate) and land use (forests). We conducted the model transferability test, model calibration, and sensitivity analysis using the physically-based, spatially distributed land surface hydrologic model Flux-PIHM. The swap experiments were carried out where catchment characteristics were swapped one at a time, to tease apart the relative influence of topography, soil properties, and size. We found that soil and macropore properties predominantly control storage-discharge relationships. In addition, dynamic water storage increases with catchment size due to enhanced hillslope-stream connectivity in larger catchments. To identify critical measurements for soil moisture and streamflow generation, we assessed three hydrologic measurements: discharge, local soil moisture (SM) measured by frequency domain reflectometry (FDR at centimeter scale), and area-averaged SM by cosmic-ray soil moisture system (COSMOS, at 300-meter scale), in representing hydrologic responses and in constraining the hydrologic model Flux-PIHM at the Garner Run catchment. The Flux-PIHM model was used to reproduce observations and to generate catchment scale understanding of total water storage, unsaturated water storage, hydrological connectivity, and soil moisture at different depth. The Hornberger-Spear-Young algorithm was adopted to evaluate the performance of constraining sensitive model parameters using different observation combinations and locations. We found the COSMOS data can largely represent the dynamics of the discharge with a strong correlation with the total water storage and hydrological connectivity. Observation combination - discharge and COSMOS - were identified as critical measurements in representing the streamflow generation. For chemical weathering, we conducted hillslope numerical experiments of reactive transport to evaluate the relative strength between lithology (subsurface permeability) and topography (slope shape) characteristics in influencing water travel time, weathering rates, and solute export patterns. We defined vertical connectivity, a quantitative measure of how much infiltrated rainfall coming out of the subsurface zones deeper than soils for hillslopes of different characteristics. This work shows that permeability distribution has the most decisive influence on the fraction of water with older ages and longer mean transit time (MTT), and therefore on weathering rates. Results from this work support the shallow and deep hypothesis that the vertical chemical contrasts shape solute export patterns. For solutes produced during weathering increase with depth (Na, Ca, Si), stream concentrations decrease with discharge (dilution pattern); for solutes that re-precipitate during weathering, its concentrations decrease as flow paths deepen (Al) such that stream concentration increases with discharge (flushing pattern). Non-reactive solutes show chemostatic patterns, with negligible concentration variation as discharge changes by orders of magnitude. Changing the permeability distribution and hillslope shape did not alter the type of C-Q behavior but influenced the slope b value of the C-Q pattern. Collectively, the lithology and soil properties dominate both stream generation and chemical weathering.