Location: Range Management ResearchTitle: Evaluation of a new model of aeolian transport in the presence of vegetation) Author
|Herrick, Jeffrey - Jeff|
Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2013
Publication Date: 4/1/2013
Publication URL: http://handle.nal.usda.gov/10113/57170
Citation: Li, J., Okin, G.S., Herrick, J.E., Belnap, J., Miller, M.E., Vest, K., Draut, A.E. 2013. Evaluation of a new model of aeolian transport in the presence of vegetation. Journal of Geophysical Research. 118:288-306. Interpretive Summary: Wind erosion affects climate, ecosystem dynamics, soil biogeochemical cycling, snow accumulation and 56 melt, precipitation runoff, and public safety and health. Models are required to predict wind erosion because it is expensive to measure. A recently developed model for rangelands (WEMO) has the potential to provide accurate estimates of dust flux (amount of soil moving through a vertical per unit time) using relatively few measurements. This paper describes the results of tests of this model. The results show that the model can predict horizontal dust flux with a level of error is within what would be expected given uncertainties in the input variables. The model outperforms alternative approaches that it was compared with.
Technical Abstract: Aeolian transport is an important characteristic of many arid and semiarid regions worldwide that affects dust emission and ecosystem processes. The purpose of this paper is to evaluate a recent model of aeolian transport in the presence of vegetation [Okin, 2008]. This approach differs from previous models by accounting for how vegetation affects the distribution of shear velocity on the surface rather than merely calculating the average effect of vegetation on surface shear velocity or simply using empirical relationships. Vegetation, soil, and meteorological data at 65 field sites with measurements of horizontal aeolian flux were collected from the Western United States. Measured fluxes were tested against modeled values to evaluate model performance, to obtain a set of optimum model parameters, and to estimate the uncertainty in these parameters. The same field data were used to model horizontal aeolian flux using three other schemes. Our results show that the Okin  model can predict horizontal aeolian flux with an approximate relative error of 2.1 and that further empirical corrections can reduce the approximate relative error to 1.0. The level of error is within what would be expected given uncertainties in threshold shear velocity and windspeed at our sites. The model outperforms the alternative schemes both in terms of approximate relative error as well as the number of sites at which threshold shear velocity was exceeded. These results lend support to an understanding of the physics of aeolian transport in which 1) vegetation’s impact on transport is dependent upon the distribution of vegetation rather than merely its average lateral cover, and 2) vegetation impacts surface shear stress locally by depressing it in the immediate lee of plants rather than by changing the bulk surface’s threshold shear velocity. Our results also suggest that threshold shear velocity is exceeded more than might be estimated by single measurements of threshold shear stress and roughness length commonly associated with vegetated surfaces, highlighting the variation of threshold shear velocity with space and time in real landscapes.