Evidence based on sparse tree-ring data suggests a severe sustained drought occurred in the 2nd century CE that could have rivaled medieval period droughts in the Colorado River basin (Gangopadhyay et al. 2022). Most of these tree-ring data have been used in gridded drought reconstructions (Cook et al., 2010) which extend back to 1 CE over an area that includes the intermountain western US. However, the 2nd century drought has not been highlighted in prior studies given the sparseness of the data available for this time period. A new reconstruction of Colorado River flow based on these data documents a notably severe and sustained drought over much of the 2nd century (Gangopadhyay et al. 2022). While this reconstruction suggests that the drought exceeds the severity and duration of any drought in the past 2000 years, a complete assessment of the 2nd century drought is challenging due to the sparseness of data. In this poster presentation, we describe the tree-ring data available, along with other proxy data that provide evidence for the 2nd century drought and support its severity. In our conclusions, we discuss outstanding questions and thoughts for further work.

Reconstructions of river discharge and other hydrologic variables often exploit large available networks of tree-ring chronologies from multiple species and hydrologic settings. A common early step in such studies is screening to reduce the predictor data set and focus on chronologies with a strong hydrologic signal. A stepwise regression approach to screening is proposed and illustrated for reconstruction of April 1 snow-water equivalent (SWE) at three snow courses in the northern Sierra Nevada and Lake Tahoe region from a multi-species tree-ring network. SWE is regressed separately on each chronology lagged t-2 to t+2 years from the year of SWE. A chronology is accepted based on specified criteria for temporal stability of signal and skill of the lagged model in predicting SWE outside the calibration space. A cross-validation stepwise cutoff rule is applied to guard against over-fitting the lagged model. Illustration for a network of 23 chronologies of five snow-adapted species (Juniperus occidentalis, Pinus jeffreyi, Pinus ponderosa, Abies magnifica, and Tsuga mertensiana) underscores the critical importance of lags in the tree-ring response to SWE. For Abies and Tsuga, in particular, chronologies passing screening are characterized by lagged models with a positive coefficient on the year following the hydrologic anomaly (deep snowpack this year, wide ring the following year) and no dependence or a negative coefficient on the current year (deep snowpack, narrow current ring). The SWE signal is strongest for one particular Juniperus chronology whose regression explains 39% of the SWE variance at two snow courses. The strength of SWE signal varies greatly over sites within species. More than half of the Juniperus and Pinus chronologies were rejected by screening because of either weak or temporally unstable signal. A repeat of the regression screening using water-year precipitation (Global Historical Climate Network) instead of SWE suggests that different subsets of chronologies are optimal depending on the target hydologic variable. Few chronologies have a significant signal for the residual of SWE regressed on water-year precipitation. This suggests little snow-specific information on the moisture signal in annual ring width. Sub-annual ring measurements and quantitative wood anatomy are suggested as possible ways to help discriminate the rain and snow signals in tree rings from the region.

Polar regions are highly sensitive to climate change. In recent decades, the Arctic has warmed twice as fast as the world average, which has led to a significant loss of ice cover in the Arctic Ocean. The positive feedback from continental hydrology to Arctic warming amplifies perturbations in the climate system in response to changes in heat and freshwater fluxes. As the surface warms and storm paths change, precipitation and glacier melt have increased pan-Arctic runoff. Melting permafrost adds even more water to river systems. Our research aims to understand 1) whether this feedback impacts the river flow only in the warm season, 2) whether the runoff change in the cold (winter) season can amplify the initial warming, and 3) what is the spatiotemporal pattern of seasonal variations in streamflow across pan-Arctic watersheds. Tree rings serve as critical proxies for quantifying hydrological responses to climate change. We are exploring the potential of modeling seasonal flows, especially winter versus spring and summer flows, and water temperature using intra-seasonal aggregation of climate signals in subsets of ring chronologies derived from quantified tracheal cell parameters: cell area, tangential cell diameter, radial cell diameter, cell wall thickness. The preliminary results demonstrate significant correlation of summer water temperature with cell area and radial cell diameter, whereas the discharge correlates stronger with cell wall thickness and tangential cell diameter. We emphasize the importance of modeling intra-seasonal hydrological parameters versa the hydrological year average to analyze the contribution of continental hydrology to Arctic warming.