Browsing Theses (Civil and Environmental Engineering) by Subject "Hydrologic sciences"
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Dynamic Modeling Of The Hydrologic Processes In Areas Of Discontinuous PermafrostThe overarching hypothesis of this dissertation is "in the sub-arctic environment, the presence or absence of permafrost is dominant influence on hydrologic processes." The presence or absence of permafrost is the defining hydrologic characteristic in the sub-arctic environment. Discontinuous permafrost introduces very distinct changes in soil hydraulic properties, which introduce sharp discontinuities in hydrologic processes and ecosystem characteristics. Hydraulic properties vary over short and long time scales as the active layer thaws over the course of a summer or with changes in permafrost extent. The influence of permafrost distribution, active layer thaw depth, and wildfire on the soil moisture regime and stream flow were explored through a combination of field-based observations and computer simulations. Ice-rich conditions at the permafrost table do not allow significant percolation of surface waters, which result in saturated soils near the ground surface and limited subsurface storage capacity, compared to well-drained non-permafrost sites. The removal of vegetation by wildfire results in short-term (<10 years) increases in moisture content through reduced evapotranspiration. Long-term (>10 years) drying of soils in moderate to severe wildfire sites is the result of an increased active layer depth and storage capacity. A spatially-distributed, process-based hydrologic model, TopoFlow, was modified to allow spatial and temporal variation in the hydraulic conductivity and porosity of soils. By continual variation of the hydraulic conductivity (proxy for permafrost distribution and active layer thaw depth) and porosity (proxy for storage capacity), the dynamic soil properties found in the sub-arctic environment are adequately represented. The sensitivity of TopoFlow to changes in permafrost condition, vegetation regime, and evapotranspiration is analyzed. The net result of the field observations and computer simulations conducted in this research suggest the presence or absence of permafrost is the dominant influence on soil moisture dynamics and has an important, but secondary role in the stream flow processes.
The Hydrologic Regime At Sub-Arctic And Arctic Watersheds: Present And ProjectedThe wetlands in the Arctic Coastal Plain, Northern Alaska, support a multitude of wildlife and natural resources that depend upon the abundance of water. Observations and climate model simulations show that surface air temperature over the Alaskan arctic coast has risen in recent history. Thus a growing need exists to assess how the hydrology of these arctic wetlands will respond to the warming climate. A synthesis study was conducted combining the analysis of an extensive field campaign, which includes direct measurements of all components of the water balance, with a physically-based hydrologic model forced by downscaled climate projections. Currently, these wetlands exist despite a desert-like annual precipitation and a negative net summer water balance. Although evapotranspiration is the major pathway of water loss, there are multiple non-linear controls that moderate the evapotranspiration rates. At the primary study site within the Barrow Environmental Observatory, shallow ponding of snowmelt water occurs for nearly a month at the vegetated drained thaw lake basin. Modeling studies revealed that the duration and depth of the ponding are only replicated faithfully if the rims of low-centered polygons are represented. Simple model experiments suggest that the polygon type (low- or high-centered) controls watershed-scale runoff, evapotranspiration, and near-surface soil moisture. High-centered polygons increase runoff, while reducing near-surface soil moisture and evapotranspiration. Soil drying was not projected by the end-of-the century but differential ground subsidence could potentially dominate the direct effects of climate warming resulting in a drying of the Arctic Coastal Plain wetlands. A drier surface would increase the susceptibility to fire, which currently is a major part of the Alaskan sub-arctic but not the arctic landscape. High quality pre- and postfire data were collected in the same location in central Seward Peninsula, uniquely documenting short-term soil warming and wettening following a severe tundra fire. Overall, this research concludes that arctic and sub-arctic watershed-scale hydrology is affected by changes in climate, surface cover, and microtopographic structures. It is therefore crucial to merge hydrology, permafrost, vegetation, and geomorphology models and measurements at the appropriate scales to further refine the response of the Arctic Coastal Plain wetlands to climate warming.
The Physical Dynamics Of Patterned Ground In The Northern Foothills Of The Brooks Range, AlaskaPeriglacial landforms, called patterned ground, change the vegetation, microtopography and organic content of the surface soil horizons. Because they are uniquely products of the periglacial environment, changes in that environment affect their distribution and activity. As surface features, they mitigate heat and mass transfer processes between the land and atmosphere. For environmental change detection, the state of the soil and active layer must be monitored across temporal and spatial scales that include these features. It is suggested here that changes in the state of the active layer due to the abrupt spatial changes in surface soil character lead to changes in the distribution of soil components, soil bulk thermal properties and the thermal and hydrological fluxes result. The determination of soil volumetric moisture content using the relative dielectric permittivity of the soil is extended to include live and dead low-density feathermoss. High temporal resolution monitoring of the thermal conductivity of mineral and organic soil horizons over multiple annual cycles is introduced, along with a new method for analyzing the results of transient heat pulse sensor measurements. These results are applied to studies of frost boils and soil stripes in the northern foothills of the Brooks Range in Alaska. Active layer ice dynamics determine the thermal properties of the frozen soil in the frost boil pedon. Annual heaving and subsiding of the ground surface reflects these changes in ice content and can be used to estimate active layer ice content as a function of depth. These estimates correlate with bulk soil thermal diffusivity, inferred as a function of depth from temperature data. Differences in soil thermal diffusivity determine thaw depth differences between frost boil and tundra, and between wet and dry soil stripes. For the latter, deeper subsurface flow through the high organic content wet stripes is delayed until mid-summer; when it does occur, it has a large component normal to the hillslope as a consequence of differential heave. Dynamics in these periglacial landforms can be identified from surface features, highlighting the potential for scaling up their net effect using remote sensing techniques.
Winter Precipitation Depths Across The North Slope Of Alaska Simulated From The Weather Research And Forcasting Model And Snowtran-3DAccurately predicting snow distribution and blowing snow conditions in the Arctic is critical to the design of ice road construction and maintenance as well as for predicting water supplies and runoff during snowmelt, estimating the cost of snow removal, and forecasting tundra travel conditions. A current atmospheric model used by both the operational weather prediction and research communities is the Weather Research and Forecasting model. However, the built-in snow schemes in the model neglect redistribution of snow via wind, one of the key processes in snow pack evolution. This study will involve three parts: (1) diagnostic of the differences in the current snow schemes of the model, (2) evaluation of the model's snow schemes as compared to observational data, and (3) asynchronous coupling of the SnowTran-3D to model predictions using a simple algorithm. The approach provides a simple method for the prediction of snow distribution, improving the realism of current snow distribution models, and will be easily employable for both operational and research applications.