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A multi-tracer method employing rare earth elements (REE) was used to determine sediment yield and to track sediment movement in a small semiarid watershed. A 0.33-ha watershed near Tombstone, AZ was divided into five morphological units, each tagged with one of five REE oxides. Relative contribution of each unit to the total sediment yield was determined by collecting runoffand sediment, and the spatial redistribution of sediment was determined from sampling the soil surface. Average sediment yield was 1.0 Mg[ha.sup.-1] [yr.sup.-1] from the entire watershed, but varied between 0.1 Mg [ha.sup.-1] [yr.sup.-1] from the upper slope to 5.0 Mg [ha.sup.-1] [yr.sup.-1] from the lower channel. Little re-deposition occurred in the channels indicating an effective transport system. The erosion pattern and rates were in agreement with the current morphology of the watershed, which has a well-developed channel network. Abbreviations: DEM, digital elevation model; ICP-MS, Inductively Coupled Plasma Mass Spectrometer; LC, lower channel; LS, lower slope; MS, middle slope; REE, rare earth element; RHEM, Rangeland Hydrology and Erosion Model; RTK GPS, real time kinematic global positioning system; UC, upper channel; US, upper slope; USLE, Universal Soil Loss Equation; USDA-ARS, United States Department of Agriculture-Agricultural Research Service; WEPP, water erosion prediction project; WGEW, Walnut Gulch Experimental Watershed.

Soil loss rates predicted from erosion models for rangelands have the potential to be important quantitative indicators for rangeland health and for assessing conservation practices. Splash and sheet erosion processes on rangelands differ from croplands, where the process is conceptualized in part as interrill erosion. Previous interrill equations were developed from cropland soils where interrill erosion was conceptualized and modeled for small plots, which are not generally large enough to encompass the relative high spatial heterogeneity of rangelands. Also, interrill erosion is usually modeled as a function of rainfall intensity (I) and runoff rate (q) such that I and q are independent of each other. Splash and sheet erosion is the dominant type of erosion on most undisturbed rangeland hillslopes where there is adequate vegetation, and these important erosion processes need to be addressed to develop an appropriate rangeland erosion model. In this study, we developed a new equation for calculating the combined rate of splash and sheet erosion (D) using a large set of rainfall simulation data from the western United States. The equation we propose: [D.sub.ss] = [K.sub.ss][I.sup.1052][q.sup.0.592], where [K.sub.ss] is a splash and sheet erosion coefficient, takes into account a key interrelationship between I and q revealed in the data. This equation was successfully evaluated using independent sets of multiple-intensity experimental data. The new equation should enable improved estimation of water erosion on rangelands in the western United States and in other parts of the world. Abbreviations: IRWET, lnteragency Rangeland Water Erosion Team; WEPP, Water Erosion Prediction Project.

Identification of primary sediment source areas in watersheds is necessary to ensure that best management practices are installed in areas that maximize reductions in sediment and chemical loadings of receiving waters. Our objectives were to use a soil geomorphology-erodibility approach to locate sediment sources in the Walnut Gulch Experimental Watershed (WGEW). Major soil mapping units were sampled along transects in six subwatersheds (SWs). At each sampling point, latitude-longitude, slope gradient, slope aspect, and hillslope position were recorded. Samples collected from the surface 5.0 cm were characterized for a range of basic soil characterization properties. Additionally, [sup.137]Cs, [sup.40]K, [sup.226]Ra, and stable C isotope distributions were quantified as potential source area indicators. Suspended sediment samples collected from WGEW and SW flumes were characterized for the same properties. Relative to the SW soils, the suspended sediments were generally enriched in silt, clay, organic C, inorganic C, total N, extractable cations, extractable Fe and Mn, [sup.13]C from C3 plants, [sup.40]K, and [sup.226]Ra. The suspended sediment from three SWs was enriched in [sup.137]Cs. Eleven characterization parameters were used in a multivariate mixing model to identify the SWs contributing the greatest sediment loads in the WGEW. The mixing model results indicated that three SWs were contributing approximately 86% of the sediment, and that the greatest amount originated in the three SWs with the lowest soil aggregation index (highest erodibility). These results were supported by the [[delta]sup.13]C data, which indicated that approximately 65% of the stable C isotopes leaving the WGEW during this period were derived from C3 plants (shrubs), the dominant vegetation on the three SWs. Abbreviations: AI, aggregation index; ER, enrichment ratio; [Fe.sub.d] and [Mn.sub.d], citrate-bicarbonate-dithionite extractable Fe and Mn; [Fe.sub.0] and [Mn.sub.0], acid ammonium oxalate extractable Fe and Mn; [Fe.sub.p] and [Mn.sub.p], sodium pyrophosphate extractable Fe and Mn; OC, organic carbon; SW, subwatershed; WDC, waterdispersible clay; WGEW, Walnut Gulch Experimental Watershed.

Soil erosion models are indispensable tools for conservation planning, erosion inventory, risk assessment, and policy development. The most successful rain-fed soil erosion models have been the statistically-based Universal Soil Loss [...]

Data on soil erosion at the slope scale is essentially limited to experiments on uniform slopes. The objective of this research was to measure the rates and patterns of erosion on complex shaped slope elements under controlled laboratory conditions where surface morphology changes could be carefully quantified. Artificial rainfall was applied for 90 min to a silt loam soil in a 4 by 4 m box. Five slope shapes were formed: uniform, concave-linear, convex-linear, nose slope, and head slope. Digital elevations models (DEMs) of the surface were measured using photogrammetry after 0, 10, 20, 40, 60, and 90 min. Slope shape had a significant impact on rill patterns, sediment yield, and runoff production. The uniform, nose, and convex-linear slopes yielded more sediment than the concave-linear and head slopes, where sediment deposited on toeslopes. Soil topography led to flow convergence and divergence, resulting in a nonuniform distribution of rill spacing and efficiency. The degree of rill incision was related to slope steepness and length, and rill success was related to the contribution area of the rill. Drainage density approached a similar value for all networks during the experiments. Development of the drainage system was similar to the development of optimum channel networks, in that during the evolution of the rill network energy expenditure was reduced. This indicated that energy expenditure was a quantifiable measure of network development and self-organization.

Climate change can affect agricultural production and soil and water conservation. The potential for global climate changes to increase the risk of soil erosion is clear, but the actual damage is not. The objectives of this study were to develop a method for downscaling monthly climate forecasts to daily weather series using a climate generator (CLIGEN), and to determine the potential impacts of projected mean and variance changes in precipitation and temperature on soil erosion and wheat (Triticum Aestivum L.) yield. Monthly forecasts for the periods of 1950-1999 and 2056-2085 for the Oklahoma region, projected by a general circulation model (HadCM3), were used. Projected mean and variance changes in precipitation and temperatures between the two periods were satisfactorily incorporated into CLIGEN input parameters derived for the El Reno station, Oklahoma, and future transitional probabilities of precipitation occurrence were estimated as a linear function of historical monthly precipitation. Five climate change scenarios were constructed, and the Water Erosion Prediction Project (WEPP) model was run for each combination of five climate scenarios and three tillage systems. A 50% increase in C[O.sub.2] resulted in some 26% increase in wheat yield. At that elevated C[O.sub.2] level, projected decrease in total precipitation decreased surface runoff, soil loss, and wheat yield. However, predicted changes in precipitation variance increased runoff by 15 to 17%, and increased soil loss by 10 and 19% under conservation and conventional tillage, respectively. Predicted increase in mean temperature reduced wheat yield by 31%, and increased soil loss by 40 and 19% under conservation and conventional tillage, respectively. Under the assumed climate change, predicted average soil loss under conventional tillage was about 2.6 times that under conservation tillage and 29 times that under no-till. With all changes considered, predicted average wheat yield during 2056-2085, compared with the present climate at the present C[O.sub.2] level, would decrease by 12%; runoff would increase by

Sediment has been identified as one of the most important nonpoint source pollutants of streams, lakes, and estuaries. Sediment is recognized as a pollutant that has an impact on aquatic [...]

The consensus of atmospheric scientists is that climate change is occurring, both in terms of global air temperature and precipitation patterns. For instance, the year 1998 was likely the warmest [...]

Spatially averaged soil erosion data provide little information on soil erosion dynamics. Dynamically varied, spatially distributed erosion data are needed to better understand erosion processes and thoroughly evaluate process-based erosion prediction models. The objectives of this study are to examine the feasibility of direct mixing rare-earth element (REE) oxides with soil materials to trace soil erosion at a plot scale and to explore further the potential of using this technique to study soil erosion dynamics. Five REE oxide powders were mixed with a Camden silt soil (fine, silty, mixed, mesic Typic Hapludalt) at five slope positions. Six rainstorms were applied to a 4 by 4 m soil bed at a 10% slope. Runoff was collected during the rains and surface soil was sampled after each rain. All samples were extracted for REE with a quick acid-leaching method and extracts were analyzed by inductively coupled plasma mass spectrometry (ICPMS). Mean error of REE-derived sediment discharge relative to measured total across six rainstorms was 14.5%. Our results demonstrate that the REE-tracing technique is feasible and sound, being capable of producing both spatially and temporally distributed erosion data. Results indicate that the most severe erosion occurred in the upper middle section of a uniform slope, and total sediment delivery from each segment was positively related to amounts of sediment deposition on the entire slope from that segment. This technique has the potential to bring new perspectives to soil erosion research.

********** Climate change resulting from greenhouse gas-induced global warming is expected to affect the extent, frequency, and magnitude of soil erosion by water in several ways. Erosional response will occur [...]

********** ABSTRACT. The erosive power of rainfall can be expected to change as climate changes. Such erosive changes are likely to have significant impacts on local and national soil conservation [...]

Shallow surface runoff is a primary transport agent for interrill sediment delivery. Runoff, rainfall intensity, and slope interactively affect interrill erosion. We hypothesized that the inclusion of a runoff factor in an interrill erosion model can reduce the dependence of the interrill soil erodibility ([K.sub.i]) on soil infiltration characteristics as well as improve model predictability. A complete factorial rainfall simulation experiment with two soils (Cecil sandy loam, a clayey, kaolinitic, thermic Typic Kanhapludult, and Dyke clay, a clayey, mixed, mesic Typic Rhodudult), four rainfall intensities, four slopes, and two replicates was conducted under prewetted conditions to measure runoff and sediment delivery rates. Tap water with electrical conductivity

In 1940, A. W. Zingg published an equation expressing the effect of slope steepness and length on soil erosion, to be followed a year later by D. D. Smith's paper [...]

A study constructed a set of statistics that would assess the effectiveness of the Universal Soil Loss Equation (USLE). Researchers compared assessments of soil loss using USLE with assessed values on 208 natural runoff plots, for greater than 1700 plot years of data, to evaluate the error related to the USLE predictions. The USLE underpredicted soil loss on plots with high erosion rates, and overpredicted soil loss on those with lower rates.

Recently proposed relationships for the effect of slope steepness on soil loss by water are linear functions of the sine of the slope angle. The Revised Universal Soil Loss Equation (RUSLE) uses two such functions: one for slopes 9%. Recent research indicates that yet a different linear function is necessary for slopes greater than approximately 22%. The objective of this study was to develop a single slope steepness function that is representative of the data for all slopes. The resultant equation takes the form of a logistic function. It closely follows the RUSLE relationships for the slope steepness factor for slopes up to 22%, and also fits existing data for slopes greater than those from which the RUSLE relationships were derived.

The investigation of the hypothesis that the removal of soil by flowing water at a certain shear stress value is influenced by turbulence reveals that the detachment of soil is not only due to the localized turbulence but also due to the pitting associated with turbulence. The investigation is performed by the measurement of the detachment rates of three different soils in a hydraulic flume at seven flow depth and bed slope combinations. The measurements are obtained by subjecting the soil to laminar and turbulent flow conditions.