Skip to main content
ARS Home » Plains Area » Las Cruces, New Mexico » Range Management Research » Research » Publications at this Location » Publication #367635
Research Project: Science and Technologies for the Sustainable Management of Western Rangeland Systems

Location: Range Management Research

Title: A note on the use of drag partition in aeolian transport models

Author
item WEBB, NICHOLAS - New Mexico State University
item CHAPPELL, ADRIAN - Cardiff University
item LEGRAND, SANDRA - Us Army Corp Of Engineers (USACE)
item ZIEGLER, NANCY - Us Army Corp Of Engineers (USACE)
item EDWARDS, BRANDON - New Mexico State University

Submitted to: Aeolian Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2019
Publication Date: 1/1/2020
Citation: Webb, N., Chappell, A., LeGrand, S., Ziegler, N., Edwards, B. 2020. A note on the use of drag partition in aeolian transport models. Aeolian Research. 42:100560. https://doi.org/10.1016/j.aeolia.2019.100560.
DOI: https://doi.org/10.1016/j.aeolia.2019.100560

Interpretive Summary: Many of the world’s active aeolian environments are covered with rocks and vegetation that reduce wind flow over the land surface and influence where and how much wind erosion and dust emission occur. Representing these interactions presents a challenge for accurate wind erosion modelling. Drag partition schemes (functions that describe the effect of surface roughness on the wind energy) have been used in models to adjust the threshold at which wind erosion occurs. In this paper, we identify an error in the application of some drag partition schemes and show the impact of this error for model outcomes. We show why, for approximately the last three decades, wind erosion and dust emisssion models have been substantially overestimating sediment fluxes. To address model inaccuracy, it is common to adjust model estimates by the vegetation cover fraction - reducing the land surface area that can erode. We show that for some models, this adjustment constitutes double accounting of physical surface protection (e.g., by vegetation) and is unnecessary if the drag partition is implemented fully.

Technical Abstract: Sediment transport equations used in wind erosion and dust emission models generally incorporate a threshold for particle motion (u*t) with a correction function to account for roughness-induced momentum reduction and aerodynamic sheltering. The prevailing approach is to adjust u*t by the drag partition R, estimated as the ratio of the bare soil threshold (u*ts) to that of the surface in the presence of roughness elements (u*tR). Here, we show that application of R to adjust only the entrainment threshold (u*t = u*ts/R) is physically inconsistent with the effect of roughness on the momentum partition as represented in models and produces overestimates of the sediment flux density (Q). Equations for Q typically include a wind friction velocity scaling term (u*n). As Q scales with wind friction velocity at the soil surface (uS*), rather than total wind friction velocity uT* (implicitly u* in models), u*n must be also adjusted for roughness effects. We further note that the practice of reducing Q by the vegetation cover fraction to account for the physically-protected surface area constitutes double accounting of the surface protection when R is represented through the basal-to-frontal area ratio of roughness elements (s) and roughness density ('). If the drag partition is implemented fully, additional tuning for surface protection is unnecessary to produce more accurate aeolian transport estimates. These findings apply equally to models of the vertical dust flux.