Reconstructing groundwater levels from tree ring widths in the North American Cordillera

Water resources around the world are periodically impacted by droughts, which can reduce water available for human use and put stress on the environment. The effects of long-term changes in climate on groundwater resources are difficult to determine, as most observed groundwater level records are short (< 60 years). However, observational records can be extended using proxy records such as tree ring widths. This research uses two methods to study past groundwater changes (1850-2002) in the North American Cordillera: a nested principal component regression technique to reconstruct past groundwater levels from tree ring widths, and a nonparametric approach which reconstructs daily temperature and precipitation based on annual tree ring widths, to be used as input into a hydrological model for simulating groundwater levels. The past spatial and temporal occurrence of droughts in this region is examined, as are the associations between past groundwater levels and climate teleconnections. The groundwater level reconstructions demonstrate that the relationships between groundwater levels and snowpack, and tree ring widths and snowpack, are important for creating groundwater level reconstructions in mountain regions. In addition, separating groundwater observation wells by dominant aquifer-stream system strengthens the reconstructions. Analysis of past groundwater drought (between 1850 and 2004) shows that drought events were not synchronous across the North American Cordillera, with variations in timing occurring between the north and south cordillera, as well as between different aquifer-stream systems. Teleconnections patterns that affect observed groundwater levels in this region were also found to influence past groundwater levels. Hydrologic modeling results show that the nonparametric approach was useful for creating simulations of past groundwater levels, and that in the past time periods analyzed (1920-1950 and 1950-1980), peak recharge occurred earlier in the year than in the observed climate; this earlier peak in recharge is likely due to warmer spring temperatures and earlier snowmelt during the paleo-periods. These results show that the relationship between groundwater levels and tree rings can be used to successfully reconstruct and simulate past groundwater levels in mountain regions, providing a long-term perspective into groundwater variability throughout the past.