20TH CENTURY DUNE MOVEMENTS AT THE GREAT SAND DUNES NATIONAL PARK AND PRESERVE, COLORADO AND RELATION TO DROUGHT VARIABILITY

Active and stabilized dune landforms in the western USA are abundant and serve as a proxy for climate change reflecting a complex response to periods of extreme aridity in the past 10,000 years. The Great Sand Dunes National Park and Preserve (GRSA), located in the San Luis Valley (southern Colorado), contains one of the largest and climatically sensitive, but least studied dune systems in North America. A large dune mass is banked against the Sangre de Cristo Mountains and is bounded for 100’s of km by a vegetated low relief sand sheet. The focus of this study is the scattered active parabolic and barchanoid dunes located respectively on the south and west areas of this sand sheet. The clear morphology of these dunes, the formation by unidirectional wind from the southwest and the unambiguous identification of these forms in remotely sensed-images provide straightforward targets to assess changes in dune position. A digital database of georeferenced remote-sensed images from 1936 to 1999 is developed to understand dune activity with evolving drought conditions at GRSA in the 20th Century. Quantitative data on dune movements and surface reflectance and the interactions with droughts is generated by Geographic Information System based strategies. The extent of dune movement is evaluated in respect to climatic parameters like precipitation and Palmer Drought Severity Index (PDSI), a valuable measure to assess drought status. The total net movement of parabolic dunes in 63 years is 0.4 to 0.6 km; barchan dunes moved less, with an average net movement of 0.2 to 0.3 km. Results show a 3-fold increase in parabolic dune movement, compared to intervening wet years during well-documented droughts in the 1930’s and 1950’s, with concomitant reduction in vegetation cover and surface water resources. The landscape response to the most recent drought in the late 20th to early 21st centuries is documented by an 8% increase in the rate of parabolic dune movement between 1998 and 1999 compared to prior wet years. This analysis indicates non-linear threshold for parabolic dune extension with accelerated movements with lower quartile PDSI’s of < -2.