Publication Date

12-2018

Date of Final Oral Examination (Defense)

10-26-2018

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Major Advisor

Jennifer Pierce, Ph.D.

Advisor

Elowyn Yager, Ph.D.

Advisor

Frederick B. Pierson, Ph.D.

Advisor

Nancy Glenn, Ph.D.

Abstract

Aeolian processes play a significant role in the redistribution of sediment and nutrients in sparsely vegetated sagebrush-steppe ecosystems. When fire is introduced to the landscape, decreased surface roughness and associated threshold friction velocities allow for the increased mobility of surface sediments and burnt organic material, mobilizing previously stable material. Once material is entrained, interactions between a dynamic atmosphere and complex topography control the spatial distribution of aeolian deposition over a landscape. Given the significant impact of fire on aeolian processes in semi-arid deserts, we posit that postfire aeolian redistribution of material is an important control on the spatial variability of soil depth and characteristics in semi-arid deserts with complex topography. Our study uses over two years of continuous passive dust trap data collected following the Soda Fire of August 2015 in the northern Great Basin. We analyze the mass flux, organic material content, grain size distribution, and geochemistry of the collected samples to trace the fingerprint of the Soda Fire through space and time. As such, the results of this study will inform research on postfire sediment and carbon redistribution, the spatial variability of soil characteristics, and landform evolution in western rangelands. Our results indicate that seasonal variation in aeolian mass flux is pronounced, with the fall months generating the highest rates of dust flux. Immediately following the Soda Fire of August 2015, the mass flux of both sediment and organic material increased by two to three-fold within and proximal to the burned area. Increases in flux lasted on the landscape until the revegetation of the burned area in the spring of 2016, leaving roughly 8 months of disturbed soil surface conditions. Samples impacted by fire contained 88% fine silt and clay-sized material while undisturbed samples averaged 94%, indicating a temporary increase in the particle size distribution within the burned area. A geochemical comparison of regional and local dust and its sources also indicates a pulse of local sediment mobility following the fire through an increase in the relative concentrations of Titanium (found in local soil) and a decrease in the relative concentrations of Barium and Strontium (found in regional soluble salts). We interpret the cessation in local mobility after revegetation to adequate surface roughness provided from spring “green up” (grasses and forbes) to return vertical fluxes of organic matter and sediment to within pre-disturbance fluctuations.

Recent studies in the northern Great Basin have found aspect-controlled differences in soil depth and volumetric water content in mid-elevation sagebrush-steppe ecosystems. North-facing aspects tend to have deeper soils, with higher organic content and greater volumetric water contents throughout the water year than south-facing slopes. Our results indicate that local material is suspended and deposited over small scales (0-10 km) to spatially controlled locations within the watershed following wildfire, while background rates of dust flux and aerosol characteristics are spatially homogenous.

The preferential redistribution of locally derived material onto sheltered, leeward slopes and topographically low positions via aeolian processes following fire adds a layer of complexity to our understanding of the spatial variability of soil characteristics, typically explained with an aspect-controlled water-energy balance within the Great Basin. Future studies should focus on quantifying and modeling the extent, magnitude, and character of postfire horizontal flux of sediment and organic material from exposed windward slopes to sheltered leeward hollows in the sagebrush steppe ecosystem using tracers, saltation sensors, and ground-based dust traps to better understand the implications of aeolian processes in semi-arid deserts.

DOI

10.18122/td/1506/boisestate

Available for download on Saturday, December 19, 2020

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