Date of Award

Spring 5-2018

Degree Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geography and Geology

Committee Chair

George Raber

Committee Chair Department

Geography and Geology

Committee Member 2

Grant Harley

Committee Member 2 Department

Geography and Geology

Committee Member 3

Franklin Heitmuller

Committee Member 3 Department

Geography and Geology

Abstract

New Mexico is heavily dependent on hydrologic inputs along high elevation sites where much of the cool-season precipitation accumulates as snowpack in the lower Southern Rocky Mountains. Snowpack runoff from the Sangre de Cristo (SDC) range provides critical headwater resources for the two major rivers that run through New Mexico and by extension the greater population. Yet, over the past four decades snowpack data from high and mid-elevation sites exhibit a linear trend of declining snowpack in conjunction with earlier seasonal melting. Due to the importance of these cool-season inputs for the region, a decline in montane runoff availability is alarming. Observing hydro-climatic trends over a much broader paleo-historic range is necessary to understand the implications and historical significance of recent snowpack decline in New Mexico and the broader region.

A nested principle components regression was used to reconstruct 1 April snow water equivalent (SWE) along the SDC range from a tree-ring derived dataset. The reconstruction extends from 630–2014 CE and instrumental data has been appended to 2017 for event analysis. The reconstruction is composed of 25 individually nested regression models. All models share a common period of 1955–85 CE with forward (1985–2014 CE) and backward (1955–630 CE) nests extending out temporally. The optimal calibration period (1955–85 CE) explains 64% of the variance in the instrumental record. The instrumental period is statistically representative of the overall range and variability exhibited over the entire historic period; yet, the historic record depicts extended periods of drought which eclipse, in terms of both persistence and intensity, those revealed over the instrumental period.

Based on one event classification scheme, an ongoing drought event (2011–17) ranks third in overall intensity behind the well-known mid-12th century “great-drought” and the late-16th century “mega-drought” and ranks in the 60th percentile regarding overall duration of events. In terms of drought persistence, only two 15-year periods of historic drought extend beyond a 12-year dry period in the 1950s. This 12-year dry period is in the 50th percentile regarding overall intensity of events. An alternative method of classification, targeting extended event periods, reveals five multi-decadal drought periods, all prior to the 17th century.

Numerous pluvial events revealed over the historic record eclipse wet periods observed over the instrumental record in terms of persistence and intensity. The longest and most intense pluvial event over the instrumental record (1983–89 CE) was found to be in the 50th – 60th percentile regarding overall persistence of events, while ranking 7th in overall intensity (85th percentile).

There are more individual anomalous dry years [< -0.5 standard deviations from the mean (SD)] over the instrumental period than wet, a characteristic shared only with the 8th century, most of these occurring during the 21st century. The 20th century is by far the wettest century of the 1385-year record. Although the 1950s drought was severe in terms of persistence it is eclipsed numerous times in terms of intensity. Also, while the current 21st century drought is severe in terms of intensity it is surpassed in terms of duration numerous times. Under predicted climate change scenarios, intense and prolonged droughts, like those revealed over the historic period (e.g., the late-16th century, mid-12th century, and the extended dry period of the early 8th century), would become far more frequent.

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