Abstract. Simulations of the dust cycle and its interactions with the changing Earth system are hindered by the empirical nature of dust emission parameterizations in weather and climate models. Here we take a step towards improving dust cycle simulations by using a combination of theory and numerical simulations to derive a physically based dust emission parameterization. Our parameterization is straightforward to implement into large-scale models, as it depends only on the wind friction velocity and the soil's threshold friction velocity. Moreover, it accounts for two processes missing from most existing parameterizations: a soil's increased ability to produce dust under saltation bombardment as it becomes more erodible, and the increased scaling of the dust flux with wind speed as a soil becomes less erodible. Our treatment of both these processes is supported by a compilation of quality-controlled vertical dust flux measurements. Furthermore, our scheme reproduces this measurement compilation with substantially less error than the existing dust flux parameterizations we were able to compare against. A critical insight from both our theory and the measurement compilation is that dust fluxes are substantially more sensitive to the soil's threshold friction velocity than most current schemes account for.

Accurate and reliable methods of measuring windblown sediment are needed to confirm, validate, and improve erosion models, assess the intensity of aeolian processes and related damage, determine the source of pollutants, and for other applications. This paper outlines important principles to consider in conducting field-scale wind erosion studies and proposes strategies of field data collection for use in model validation and development. Detailed discussions include consideration of field characteristics, sediment sampling, and meteorological stations. The field shape used in field-scale wind erosion research is generally a matter of preference and in many studies may not have practical significance. Maintaining a clear nonerodible boundary is necessary to accurately determine erosion fetch distance. A field length of about 300 m may be needed in many situations to approach transport capacity for saltation flux in bare agricultural fields. Field surface conditions affect the wind profile and other processes such as sediment emission, transport, and deposition and soil erodibility. Knowledge of the temporal variation in surface conditions is necessary to understand aeolian processes. Temporal soil properties that impact aeolian processes include surface roughness, dry aggregate size distribution, dry aggregate stability, and crust characteristics. Use of a portable 2 tall anemometer tower should be considered to quantify variability of friction velocity and aerodynamic roughness caused by surface conditions in field-scale studies. The types of samplers used for sampling aeolian sediment will vary depending upon the type of sediment to be measured. The Big Spring Number Eight (BSNE) and Modified Wilson and Cooke (MWAC) samplers appear to be the most popular for field studies of saltation. Suspension flux may be measured with commercially available instruments after modifications are made to ensure isokinetic conditions at high wind speeds. Meteorological measurements should include wind speed and direction, air temperature, solar radiation, relative humidity, rain amount, soil temperature and moisture. Careful consideration of the climatic, sediment, and soil surface characteristics observed in future field-scale wind erosion studies will ensure maximum use of the data collected.

tration studies also examined total soil N (Bauer and

Grasslands, which provide fundamental ecosystem services in many arid and semiarid regions of the world, are undergoing rapid increases in fire activity and are highly susceptible to postfire‐accelerated soil erosion by wind. A quantitative assessment of physical processes that integrates fire‐wind erosion feedbacks is therefore needed relative to vegetation change, soil biogeochemical cycling, air quality, and landscape evolution. We investigated the applicability of a novel tracer technique—the use of multiple rare earth elements (REE)—to quantify soil transport by wind and to identify sources and sinks of wind‐blown sediments in both burned and unburned shrub‐grass transition zone in the Chihuahuan Desert, NM, USA. Results indicate that the horizontal mass flux of wind‐borne sediment increased approximately threefold following the fire. The REE tracer analysis of wind‐borne sediments shows that the source of the horizontal mass flux in the unburned site was derived from bare microsites (88.5%), while in the burned site it was primarily sourced from shrub (42.3%) and bare (39.1%) microsites. Vegetated microsites which were predominantly sinks of aeolian sediments in the unburned areas became sediment sources following the fire. The burned areas showed a spatial homogenization of sediment tracers, highlighting a potential negative feedback on landscape heterogeneity induced by shrub encroachment into grasslands. Though fires are known to increase aeolian sediment transport, accompanying changes in the sources and sinks of wind‐borne sediments may influence biogeochemical cycling and land degradation dynamics. Furthermore, our experiment demonstrated that REEs can be used as reliable tracers for field‐scale aeolian studies.

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those fields. Both of these techniques, however, require more observations than are practical for most field man-Estimation of mean water status in a field is crucial to effective agers. Efforts have been made to characterize fields irrigation water management. Problems encountered with the estimation of mean field soil water status may be attributed to spatial variabil-from fewer observations. The application of bootstrapity of soil physical properties. Several investigators have shown tempo-ping techniques (Dane et al., 1986) has been used to ral stability of spatial patterns of field measured soil water content, estimate the minimum number of observations necesbut temporal stability of field measured soil matric potential ( m ), a sary for the reliable estimation of soil parameters in a measure of soil water status more appropriate for irrigation schedulvariable field. ing, has not previously been reported to last for more than a few A number of studies (Ottoni, 1984;Vachaud et al., days within one irrigation cycle. This study investigated the temporal 1985; Kachanoski and de Jong, 1988; van Wesenbeeck stability of spatial patterns of m both within and between sequential and Kachanoski, 1988; Jaynes and Hunsaker, 1989; Gooirrigation cycles. Sixty locations in a 1-ha field were outfitted with a vaerts and Chiang, 1993; Chen et al., 1995) have shown 1-m neutron probe access tube and three tensiometers placed at that, although soil water content varies with time and 0.15-, 0.3-, and 0.5-m depths. The observations obtained from 14 d with location in the field, the pattern of spatial variabilof soil water content measurements and 46 d of m measurements within eight irrigation cycles were analyzed with Spearman's rank ity does not change with time when the observations correlation coefficients and a relative differencing technique. Theare ranked according to the magnitude of soil water results showed temporally stable soil water content spatial patterns content or scaled against the field mean soil water conand also indicated temporally stable m spatial patterns if assumptions tent. This phenomenon has been termed temporal stabilof full soil wetting at the beginning of the cycle and uniform evapoity. The covariants of significance in these cases were transpiration among locations were satisfied. Several locations in the determined to be primarily soil texture and topography. field estimated the field mean m to within 10% within a given rangeThe dependence of water content upon soil texture has of potentials, and a few estimated the field mean to within 20% across also been used to locate textural boundaries in a field the entire range of potentials tested. Other locations estimated the from measurements of soil water content along a tranlower and higher percentiles of m with similar accuracy. sect in an irrigated field soil (Hendrickx et al., 1986).Although temporal stability has been demonstrated for soil water contents, it has not been shown for m . R.S. Van Pelt, USDA-ARS Plant Stress and Water Conservation eters...

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