Surface water and groundwater in a changing climate: applications of hydrogeophysics and trend analysis to understand hydrologic systems

Water resources are being impacted by anthropogenic climate change worldwide, the nature and severity of that impact is determined by geographic position, local topography, underlying geology, and human influences such as land use, water availability, and water regulation. As global temperatures continue to rise due to greenhouse gas emissions, it is imperative that we better understand how water availability fluctuates with changes in air temperature, rainfall, snowpack, and glacial ice. This dissertation provides an analysis of the hydrologic response to both short-term and long-term climatic changes in both humid-temperate and semi-arid tropical regions. Within the tropical alpine zone, shifts in water availability have turned investigators’ attention to groundwater systems that may function to buffer water losses due to climate change, and these systems are characterized and reviewed herein. In the arid outer tropics of Peru, alpine proglacial valleys are storing groundwater that supplies a significant quantity of base flow to the regional surface water system. This is especially evident during the region’s dry season from May to October, during the Austral winter. The Cordillera Blanca contains the world’s highest coverage of tropical glacial ice, and this ice is melting at an unprecedented rate due to warming regional temperatures. As glacial meltwater and dry season streamflow decline, we focus on the groundwater storage potential of high-altitude wetlands and grasslands that are located in Cordillera Blanca’s proglacial valleys. The valley aquifers, known as pampas, are composed of alpine gravitational slope deposits, such as talus cones and debris fans. These slope deposits are buried beneath thick (5 to 10 m) fine lacustrine sediment, indicating that they were first deposited in proglacial lakes, where their large pore space was infilled slowly with lake sediments over time. Multiple geophysical surveys were carried out to study the internal structure of a buried talus aquifer in the Quilcayhuanca valley of the Cordillera Blanca, which lies in the center of the range, upstream from the city of Huaraz. Seismic and electrical methods were interpreted with borehole data from five piezometers that were installed in a transect that extends outward from the exposed portion of talus. The boreholes were drilled through fine-grained lacustrine sediments until the depth of auger refusal, which deepens with proximity to the stream and consists of large (.1 to 1 m in diameter) talus boulders. Geophysical evidence suggests that these boulders are part of a clast-supported deposit of talus at depth, where the shallower portions of the deposit are infilled with low-permeability sediment. The resulting confined aquifer discharges to surface springs via preferential flow pathways, providing year-round supply to tributaries and eventually the Quilcay stream. Total sediment thickness in this area is likely between 20 and 85 m, which likely includes glacial deposits beneath the talus aquifer. This research highlights the importance of future aquifer studies in other valleys of the Cordillera Blanca in order to assess their spatial distribution, as well as differences in structure and thickness. For residents and industries in the Callejon de Huaylas watershed, which receives a high proportion of glacial runoff, future water availability is highly uncertain. Although hydropower operations along the Callejon de Huaylas utilize reservoirs to offset water losses, large-scale water diversions for agricultural operations are at risk for reduced dry season streamflow of the principal river, the Rio Santa. As the glaciers continue to recede, dependence on groundwater will increase both at the basin-wide scale and across individual proglacial catchments. Small-scale farmers, villages and cities that lie directly downstream of larger pampa aquifer systems will benefit from the buffering effects of groundwater to water losses caused by glacial melt. To compliment the aforementioned investigations into tropical alpine hydrogeology, hydrology, and water resources, this dissertation also focuses on the effects of anthropogenic climate change in the Northern Hemisphere in the mid-latitude region of New York State. Because the state is so diverse in topography and land cover, we conduct a state-wide statistical analysis of streamflow from 1961 to 2016 that incorporates consideration of soil thickness, land cover, and water regulation across six spatial clusters that span New York State and adjacent areas. By incorporating the full spectrum of low, median, and high flows in the analysis, changes in specific flow regimes can be associated with seasonal precipitation and temperature changes. Change points, or temporal shifts in the long term mean of the series, are identified for both streamflow and precipitation, and the timing of those change points shows coincidence between summer precipitation and low flows, as well as post-drought increases in summer and winter precipitation in the early 1970s and 2000s. The early 1970s also saw an increase in peak flow frequency throughout the state of NY, which marked the beginning of an extended wet period that continues to the present day. Long-term trends were detected in January streamflow and the winter-spring center of volume (WSCOV), which is a measure of volumetric streamflow timing in the winter and spring months. These trends will likely continue into the future as winters are projected to have more precipitation as rain and warmer temperatures, although winter temperatures may fluctuate more widely due to a shift in the Arctic polar vortex that cause colder temperatures in parts of North America. This study provides insight into the regional responses to changes in precipitation and temperature across New York State, providing a base study for future hydrologic predictions that include a warmer and wetter Northeastern US. Surface water and groundwater in a changing climate: applications of hydrogeophysics and trend analysis to understand hydrologic systems