In Pacific North America, a substantial amount of the streamflow available during the dry summer months originates from melting mountain snowpacks. Since the start of the twenty-first century, these mountain snowpacks have been declining due to the impacts of global climate change and could have severe implications for future water availability in many regions. To develop robust predictive models of future water availability derived from mountainous snowpacks, the longest possible data record is required. However, instrumental data for snow measurements, when available, are limited to a length of only five or six decades in most regions of Pacific North America. In this study, tree-rings from snow-depth sensitive tree species (mountain hemlock (Tsuga mertensiana (Bong.) Carrière) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.)) were used as a proxy to develop a 258-year record of precipitation as snow (PAS) for the southern Coast Mountains of British Columbia. Four snow models were evaluated based on a suite of dendroclimatological model diagnostics. From these, one PAS reconstruction was carried out. The reconstruction was unable to properly validate using the leave-one-out cross validation method. This result is attributed to the combination of a short calibration period, a potentially weak climate signal, and the absence of signal enhancement. Despite this outcome the research resulted in number of inferences and recommendations useful for future research. / Graduate

The Ob is the third largest Eurasian river supplying heat and freshwater to the Arctic Ocean. These inputs influence water salinity, ice coverage, ocean temperatures and ocean circulation, and ultimately the global climate system. Variability of Ob River flow on long time scales is poorly understood, however, because gaged flow records are short. Eleven tree-ring width chronologies of Pinus sibirica and Larix sibirica are developed from the floodplain of the Lower Ob River, analyzed for hydroclimatic signal and applied as predictors in a regression model to reconstruct 8-month average (December-July) discharge of the Ob River at Salekhard over the interval 1705-2012 (308 yrs). Correlation analysis suggests the signal for discharge comes through air temperature: high discharge and floodplain water levels favor cool growing-season air temperature, which limits tree growth for the sampled species at these high latitudes. The reconstruction model (R-2 = 0.31, 1937-2009 calibration period) is strongly supported by cross validation and analysis of residuals. Correlation of observed with reconstructed discharge improves with smoothing. The long-term reconstruction correlates significantly with a previous Ob River reconstruction from ring widths of trees outside the Ob River floodplain and extends that record by another century. Results suggest that large multi-decadal swings in discharge have occurred at irregular intervals, that variations in the 20th and 21st centuries have been within the envelope of natural variability of the past 3 centuries, and that discharge data for 1937-2009 underestimate both the variability and persistence of discharge in the last 3 centuries. The reconstruction gives ecologists, climatologists and water resource planners a long-term context for assessment of climate change impacts.

Little research has been conducted on the sensitivity of earlywood vessel in ring-porous tree species in response to flooding. The impact of flooding and climate on vessel characteristics in black ash (Fraxinus nigra Marsh.) was studied in the boreal region of Lake Duparquet, northwestern Quebec. In addition to standard tree-ring measurements, numbers and cross-sectional area of earlywood vessels were examined and measured using an image analyzing program. Interestingly, among all Spearman rank correlations between chronologies and hydrologic/climatic variables, the strongest associations were found between earlywood vessel chronologies from floodplain trees and spring river discharge data. High water discharge in the spring was negatively correlated to earlywood vessel area and inversely correlated to number of vessels. The mean earlywood vessel area chronology developed from floodplain trees was found to be the best proxy for high magnitude flood events and it has potential to be used for flood reconstructions.

Little research has been conducted on the sensitivity of earlywood vessel in ring-porous tree species in response to flooding. The impact of flooding and climate on vessel characteristics in black ash (Fraxinus nigra Marsh.) was studied in the boreal region of Lake Duparquet, northwestern Quebec. In addition to standard tree-ring measurements, numbers and cross-sectional area of earlywood vessels were examined and measured using an image analyzing program. Interestingly, among all Spearman rank correlations between chronologies and hydrologic/climatic variables, the strongest associations were found between earlywood vessel chronologies from floodplain trees and spring river discharge data. High water discharge in the spring was negatively correlated to earlywood vessel area and inversely correlated to number of vessels. The mean earlywood vessel area chronology developed from floodplain trees was found to be the best proxy for high magnitude flood events and it has potential to be used for flood reconstructions.

Anthropogenic climate change has triggered widespread shifts in the global hydrological cycle. In south coastal British Columbia, these changes have led to more winter precipitation falling as rain rather than snow, more rain on snow events, and generally reduced snowpacks. Since snowmelt is a primary source of summer surface runoff and groundwater, snowpack declines have caused severe seasonal streamflow droughts in recent decades. For accurate water supply forecasting under future climate change, it is crucial to know if snowpack and runoff declines are unprecedented in the last several hundred years. This research focused on developing multi-century, annually-resolved records of snow water equivalent and streamflow drought to determine if recent conditions deviate from long-term norms. The research targeted small temperate watersheds that are not usually conducive to application of dendrohydrological methodologies. Traditional dendrohydrology relies on moisture-limited tree-ring records from arid settings. This dissertation presents a new method for developing tree-ring based reconstructions from energy-limited trees. Tree-ring records from high-elevation mountain hemlock (Tsuga mertensiana (Bong.) Carrière) and amabilis fir (Amabilis (Dougl.) Forbes) stands were collected at sites in south coastal British Columbia. Ring-width measurements were used to develop multi-century dendrohydrological models of snow water equivalent and streamflow drought. A 322-year reconstruction of May 1 snow water equivalent for Vancouver Island explains 56% of the instrumental SWE data variance and suggests snowpacks in 2015 were lower than in any year since 1675. A 477-year reconstruction of summer streamflow for Tsable River explains 63% of gauged streamflow variance and indicates that since 1520 twenty-one droughts occurred that were more extreme than recent “severe” droughts. Finally, a reconstruction of regionally synchronous streamflow among four south coastal rivers explains 64% of the regionalized streamflow variance. In addition to snow-sensitive tree-ring data, the latter model incorporated a paleorecord of the Palmer Drought Severity Index as a summer temperature and aridity proxy. The reconstruction suggests that since the mid-1600s sixteen regional-scale droughts occurred that were more extreme than any within the instrumental period. All three models were particularly accurate at estimating lowest snow and runoff years, and reflected the long-term influence of cool phases of the Pacific Decadal Oscillation on regional snowmelt and summer discharge trends and patterns. The reconstructions suggest: 1) snowpack declines in 2015 were unmatched in the past ~340 years; and, 2) existing water management strategies based on hydrometric data records underestimate potential magnitudes of natural droughts. Worst-case scenario droughts compounded by land use change and climate change could result in droughts more severe than any in the past several hundred years. Energy-limited tree-ring records have strong potential as paleohydrological proxies and for expanding applications of dendrohydrology to arid settings. For some of the tree-ring chronologies examined in this study, the correlation with snow water equivalent became non-significant after the mid-1990s, possibly due to warming spring temperatures. Future studies using this type of tree-ring data must carefully evaluate the recent stability of climate-growth relationships. / Graduate / 0368 / 0388 / coulthard.bethany@gmail.com

In Louisiana, the beds of natural lakes are owned by the state. The locations of property boundary lines separating state property from private lands have usually been set by determining the levels of natural monuments known as ordinary high water marks. The term is confusing and subjective, leading to controversy in reference to its determination. Catahoula Lake in central Louisiana was chosen as a study site because of its large size, its 20-foot variation in water levels, and its low-relief perimeter. Geology, geomorphology, hydrology, archaeology, vegetation, dendrochronology and dendrohydrology of the ancient cypress fringe, nineteenth century land survey records, historical records, and time-series statistics were applied to determine the elevation range of ordinary high water. Research suggests that the level of the ordinary high water natural monument used universally in Louisiana to define lake boundaries is not the correct natural monument. This research suggests that, for example, the natural monument that defines the Catahoula Lake boundary is not a vertical monument, but rather an areal monument that was originally recognized by nineteenth century U.S. government surveyors and embedded in the evidence extracted from their original field notes. The solution coincides with the boundaries of regional land patents and offers a consistent solution to determining the boundaries of thousands of acres of disputed lands. The procedure is applicable to other lake and swamp boundaries in Louisiana and other states.