Constraining Erosion Timescales and Magnitudes with Multiple Fallout Radionuclide Tracers in Owens Valley, CA
Abstract
In central Owens Valley, California, groundwater withdrawals during periods of drought have been linked to reduced vegetation cover and a shift from grass-dominated to shrub-dominated landscapes over the past few decades. While there is field evidence of significant soil deflation in areas where native grass cover has declined, the timing and magnitude of wind erosion in this part of the valley is not well understood. An understanding of the spatiotemporal relationship between groundwater pumping, vegetation change, and erosion is necessary for effective water and land use management in this ecologically sensitive area. Here we use measurements of fallout radionuclides 7Be, 137Cs, and 210Pb in soils and dust to quantify the timing and extent of soil deflation in areas of Owens Valley where climate, land use history, and ecological changes are well constrained. We measure cosmogenic 7Be in soils to constrain the heterogeneity of wet deposition and to scale reference inventories for radioactive fallout across an arid precipitation gradient in the valley. Using a mass balance advection-diffusion-decay model, we show that in arid environments, soil 137Cs inventories are sensitive to erosion magnitude, but not so much the timing of the erosion, because vertical advection rates are slow and fallout halted in the 1960s. In contrast, soil 210Pb inventories are sensitive to both the timing and the magnitude of erosion because of near-constant meteoric inputs that can replenish the soil in years following a deflation event. Using a coupled model, we can constrain both the timing and magnitude of soil erosion in Owens Valley over the past half century, and assess how water management and abrupt climate fluctuations can lead to soil loss. Our model indicates a higher likelihood of erosion in central Owens Valley in the past three decades, particularly during and following the drought period of 1986-1992. However, we observed significant spatial variability in total erosion, from 0 to 5 cm over the past 50 years on short length scales of just 50 m. Fallout radionuclides are very useful tracers of soil loss in arid environments because of limited advection rates, and can be used to quantify connections between surface erosion, land use, and climate change.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMEP41B0603K
- Keywords:
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- 1120 GEOCHRONOLOGY / Isotopic disequilibrium dating;
- 1130 GEOCHRONOLOGY / Geomorphological geochronology;
- 1809 HYDROLOGY / Desertification;
- 1862 HYDROLOGY / Sediment transport