Robert R. Wells

Coupled effects of solution chemistry and hydrodynamics on the mobility and transport of quantum dot nanomaterials in the vadose zone

To investigate the coupled effects of solution chemistry and hydrodynamics on the mobility of quantum dot (QD) nanoparticles in the vadose zone, laboratory scale transport experiments involving single and/or sequential infiltrations of QDs in unsaturated and saturated porous media, and computations of total interaction and capillary potential energies were performed. As ionic strength increased, QD retention in the unsaturated porous media increased; however, this retention was significantly suppressed in the presence of a non-ionic surfactant in the infiltration suspensions as indicated by surfactant enhanced transport of QDs. In the vadose zone, the non-ionic surfactant limited the formation of QD aggregates, enhanced QD mobility and transport, and lowered the solution surface tension, which resulted in a decrease in capillary forces that not only led to a reduction in the removal of QDs, but also impacted the vadose zone flow processes. When chemical transport conditions were favorable (ionic strength of 5 × 10(-4)M and 5 × 10(-3)M, or ionic strengths of 5 × 10(-2)M and 0.5M with surfactant), the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were meso-scale processes, where infiltration by preferential flow results in the rapid transport of QDs. When chemical transport conditions were unfavorable (ionic strength of 5 × 10(-2)M and 0.5M) the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were pore-scale processes governed by gas-water interfaces (GWI) that impact the mobility of QDs. The addition of surfactant enhanced the transport of QDs both in favorable and unfavorable chemical transport conditions. The mobility and retention of QDs was controlled by interaction and capillary forces, with the latter being the most influential. GWI were found to be the dominant mechanism and site for QD removal compared with solid-water interfaces (SWI) and pore straining. Additionally, ripening phenomena were demonstrated to enhance QDs removal or retention in porous media and to be attenuated by the presence of surfactant.

Modeling the Evolution of Incised Streams. III: Model Application

Incision and the ensuing widening of alluvial stream channels represent important forms of channel adjustment. Two accom- panying papers have presented a robust computational model for simulating the long-term evolution of incised and restored or rehabili- tated stream corridors. This work reports on applications of the model to two incised streams in northern Mississippi, James Creek, and the Yalobusha River, to assess: 1 its capability to simulate the temporal progression of incised streams through the different stages of channel evolution; and 2 model performance when available input data regarding channel geometry and physical properties of channel boundary materials are limited in the case of James Creek. Model results show that temporal changes in channel geometry are satisfactorily simulated. The mean absolute deviation MAD between observed and simulated changes in thalweg elevations is 0.16 m for the Yalobusha River and 0.57 m for James Creek, which is approximately 8.1 and 23% of the average degradation of the respective streams. The MAD between observed and simulated changes in channel top width is 5.7% of the channel top width along the Yalobusha River and 31% of the channel top width along James Creek. The larger discrepancies for James Creek are mainly due to unknown initial channel geometry along its upper part. The model applications also emphasize the importance of accurate characterization of channel boundary materials and geometry.

Effect of Topographic Characteristics on Compound Topographic Index for Identification of Gully Channel Initiation Locations

Abstract. Sediment loads from gully erosion contribute to water quality problems, reduction in crop productivity by removal of nutrient-rich topsoil, and damage to downstream ecosystems. The identification of areas with high potential for gully channel development is often performed using spatially derived stream power estimates from second-order topographic indices, such as the compound topographic index (CTI). The utilization of CTI to identify where gullies develop is affected by field and local topographic characteristics and DEM resolution. In this study, the effect of overall terrain slope, local relief variance, and raster grid cell size on CTI cumulative distribution values was investigated using theoretical and observed catchment methodology. In the theoretical analysis, stochastic methods were used to generate simulated catchments to quantify the influence of overall terrain slope, local relief variance, and raster grid cell size (each considered individually). The observed methodology used three sites with distinct topographic characteristics, measured gully channels, and high-resolution topographic information. Raster grids for the three observed study sites were generated at varying raster grid cell sizes. Critical CTI values were determined through comparison of measured gully thalwegs with threshold CTI raster grids of the observed watersheds at different resolutions. Results from the theoretical investigation indicate that CTI values were linearly influenced by changes in relief variance and overall slope, while variations in raster grid cell size caused an inverse power variation in CTI values. In addition, variations in raster grid cell size, produced changes in cumulative distributions of the top 0.1% CTI values. The use of normalized CTI values (CTI n ) produced merged cumulative distribution curves when varying overall slope, terrain relief variance, and to a lesser degree DEM resolution. Similar findings were obtained from the analysis of observed catchments. When DEM resolution varied, the differences in critical CTI n values in the same field were significantly reduced when compared to original critical CTI values, although differences were not fully eliminated. Normalization of the CTI cumulative distributions improved comparisons between different sites with distinct drainage area sizes and topographic characteristics, providing a possible alternative for investigations of large watersheds with more than one topographic characteristic. Results suggest that a normalized critical CTI between 1 and 2 could be used for the identification of areas with high potential for gully development. Knowing where gullies develop is important in understanding the effect of conservation practices on soil erosion through the use of field-scale and watershed-scale simulation models. Effective watershed management plans depend on this information to target the placement of conservation practices for the efficient use of available resources.

Spatial Characterization of Riparian Buffer Effects on Sediment Loads from Watershed Systems

Understanding all watershed systems and their interactions is a complex, but critical, undertaking when developing practices designed to reduce topsoil loss and chemical/nutrient transport from agricultural fields. The presence of riparian buffer vegetation in agricultural landscapes can modify the characteristics of overland flow, promoting sediment deposition and nutrient filtering. Watershed simulation tools, such as the USDA-Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model, typically require detailed information for each riparian buffer zone throughout the watershed describing the location, width, vegetation type, topography, and possible presence of concentrated flow paths through the riparian buffer zone. Research was conducted to develop GIS-based technology designed to spatially characterize riparian buffers and to estimate buffer efficiency in reducing sediment loads in a semiautomated fashion at watershed scale. The methodology combines modeling technology at different scales, at individual concentrated flow paths passing through the riparian zone, and at watershed scales. At the concentrated flow path scale, vegetative filter strip models are applied to estimate the sediment-trapping efficiency for each individual flow path, which are aggregated based on the watershed subdivision and used in the determination of the overall impact of the riparian vegetation at the watershed scale. This GIS-based technology is combined with AnnAGNPS to demonstrate the effect of riparian vegetation on sediment loadings from sheet and rill and ephemeral gully sources. The effects of variability in basic input parameters used to characterize riparian buffers, onto generated outputs at field scale (sediment trapping efficiency) and at watershed scale (sediment loadings from different sources) were evaluated and quantified. The AnnAGNPS riparian buffer component represents an important step in understanding and accounting for the effect of riparian vegetation, existing and/or managed, in reducing sediment loads at the watershed scale.

Methods for Gully Characterization in Agricultural Croplands Using Ground-Based Light Detection and Ranging

Soil erosion has long been recognized as the primary cause of soil degradation in agricultural fields (Wells et al., 2010). Overland flow in agricultural fields is the main process associated with soil erosion, which is often grouped into categories of: sheet erosion, rill erosion, and gully erosion (Smith, 1993). Traditionally, research has focussed on understanding and modelling sheet and rill erosion processes (Poesen et al., 1996). Recent research has begun to focus on addressing gully issues such as the understanding of the formation of gullies, their contribution to overall soil loss, development of tools to locate channel initiation, and appropriate measuring techniques (Poesen et al., 2003). The increased focus on gully research can be partially attributed to recent studies demonstrating that gully formation is very common on cropland, especially in conventional tillage systems (Gordon et al., 2008) and can be as significant as sheet and rill erosion in terms of sediment yield (Bingner et al., 2010). Without a good understanding of gully processes, technology cannot be developed that can provide information needed by watershed managers when evaluating and implementing effective conservation practice plans. Gullies can be generally classified as ephemeral, classical, or edge-of-field. The Soil Society of America (2001) defines ephemeral gully as “small channels eroded by concentrated flow that can be easily filled by normal tillage, only to reform again in the same location by additional runoff events”. As the headcut migrates upstream and the channel gets wider, faster than the interval between farming tilling operations, farming equipment is forced to operate around the gully and as result the gully becomes permanent (classical gully). Finally, as the name suggests, edge-of-field gullies are defined by channels where concentrated flow crosses earth bank (Poesen et al., 2003). New methodologies are being researched to understand gully formation and estimate sediment yield (Souchere et al., 2003, Cerdan et al., 2002, and Woodward, 1999). Studies use Digital Elevation Models (DEMs) as the basis to formulate theories explaining the relationship between field topography and gully occurrence (Woodward, 1999, Parker et al., 2007, and Cerdan et al., 2002). These efforts greatly benefit from accurate and detailed topographic information which can aid in the understanding of where and when gullies form and how these features evolve over time (headcut migration). Despite the availability of DEMs at regional and local scales (spatial resolution ranging from 1 to 30 meters), these datasets often do not offer the necessary spatial and/or temporal

Emergence, persistence, and organization of rill networks on a soil-mantled experimental landscape

Soil erosion remains a critical concern worldwide, and predicting the occurrence, location, and evolution of rills on hillslopes and agricultural landscapes remains a fundamental challenge in resource management. To address these questions, a relatively large soil-mantled experimental landscape was subjected to continuous rainfall and episodes of base-level lowering to force the development of a rill network system, and high-resolution digital technologies were used to quantify its evolution over time and space. These results show that waves of degradation and landscape incision occurred in response to base-level lowering, where headcut development and its upstream migration produced a fourth-order rill network. Stream order indices derived for this incised rill network confirm that this pattern emerges relatively early in time, and it remains relatively unchanged despite continued application of rainfall and additional base-level lowering. Using the same digital technologies, a surface drainage system was defined and mapped on the landscape prior to any soil erosion and rill development, and similar network indices also were derived. These results show that network characteristics and organization of this surface drainage system, as well as its location in space, were in very close agreement with the subsequent incised rill network following base-level lowering. It is demonstrated here that rill networks formed in this experiment are strongly conditioned by surface drainage patterns prior to any significant soil erosion and that the location of rill networks can be accurately delineated through analysis of the high-resolution digital terrain.

Numerical Simulation of Post Dam Removal Sediment Dynamics Along the Kalamazoo River between Otsego and Plainwell, Michigan

The state of Michigan is interested in removing two low-head dams in an 8.8 km reach of the Kalamazoo River between Plainwell and Otsego, Michigan while minimizing impacts to the study reach and downstream reaches. The study was designed to evaluate the erosion, transport, and deposition of sediments over a 17.7year period using the channel evolution model CONCEPTS for three simulation scenarios: Dams In, Dams Out, and Design. The total mass of sediment emanating from the channel boundary, for the Dams In case, shows net erosion of 3,350 T/y for the study reach, with net transport (suspended and bed load) of 5,010 T/y passing the downstream boundary. For the Dams Out case, net erosion jumps to 41,600 T/y with net transport of 59,200 T/y passing the downstream boundary. For the Design case, net erosion was 3,870 T/y with transport of 20,100 T/y passing the downstream boundary. The most significant findings were: (1) removal of the low-head dams will cause erosion in the study reach and increased sediment loads passing the downstream boundary, (2) bed erosion is the major source of eroded sediment, and (3) the Plainwell reach is the greatest contributor of total sediment and fine-grained

Sediment detachment and transport processes associated with internal erosion of soil pipes

Subsurface flow can be an important process in gully erosion through its impact on decreasing soil cohesion and erosion resistance as soil water content or pressure increases and more directly by the effects of seepage forces on particle detachment and piping. The development of perched water tables in duplex soils fosters lateral flow that can result in seepage at the surface of hillslopes and/or formation of soil pipes by internal erosion of lateral preferential flow paths. Continued internal erosion of soil pipes can lead to gullies, dam and levee failures. However, the processes involved in particle and aggregate detachment from soil pipe walls and transport processes within soil pipes have not been well studied or documented. This paper reviews the limited research on sediment detachment and transport in macropores and soil pipes and applies the knowledge learned from the much more extensive studies conducted on streams and industrial pipes to hydrogeologic conditions of soil pipes. Knowledge gaps are identified and recommendations are made for future research on sediment detachment and transport in soil pipes.

Spatially Distributed Sheet, Rill, and Ephemeral Gully Erosion

AbstractEphemeral gully erosion seriously degrades agricultural soils, but few conservation planning tools adequately account for this form of erosion. To address this deficiency, this paper describes a spatially distributed adaptation of version 2 of the Revised Universal Soil Loss Equation and a new ephemeral gully erosion estimator. The modeled results were compared to runoff and sediment yield measured from 1975 to 1991 on a 6.3-ha instrumented watershed near Treynor, Iowa, managed with conventional tillage corn and containing a grassed waterway. Using a 3-m rectangular grid, this investigation determined surface drainage patterns and delineated concentrated flow channels where contributing areas exceeded 600  m2. Computed gully evolution based on soil properties, runoff, and sediment transport contributed approximately one-fourth of the total erosion, with the rest contributed by sheet and rill erosion. More than half of the eroded sediment was deposited within the grassed waterway. Without local cal...

Combined Geomorphic and Numerical-Modeling Analyses of Sediment Loads for Developing Water-Quality Targets for Sediment

The principle objective of the study was to determine sediment loads for James Creek, Mississippi and for similar, but stable “reference” streams to develop water-quality targets for sediment. “Reference” sediment-transport loads were determined from stable streams with historical flow and sediment-transport data in the Southeastern Plains Ecoregion. Using the discharge that occurs, on average every 1.5 years (Q1.5) as the “effective discharge,” an initial “general reference” of 0.31 T/d/km was obtained. This value, however, is skewed towards streams with sand beds and does not accurately reflect conditions along James Creek. A refined “reference” condition was developed for stable silt/clay-bed streams in the Southeastern Plains resulting in a “reference” suspended-sediment yield of 3.23 T/d/km at the Q1.5. A weighted-reference condition based on the percentage of the drainage area encompassed by the various bed-material types results in a reference yield at the Q1.5 of 2.2 T/d/km. Similarly, a weightedreference concentration of 160 mg/l was obtained. “Actual” sediment-transport loads were obtained by: simulations of flow and sediment transport using the Simon is a Research Geologist at the USDAARS, National Sedimentation Laboratory, Oxford, MS 38655. E-mail: asimon@ars.usda.gov. Bingner is an Agricultural Engineer, Langendoen is a Research Hydraulic Engineer, Wells is a Postdoctoral Researcher , and Alonso is Research Leader, all at the USDAARS, National Sedimentation Laboratory, Oxford, MS 38655. . model AnnAGNPS and by simulations of channel flow and sediment transport by the channel-evolution model CONCEPTS. Average sediment loads at the mouth of James Creek over the 35-year period are about 250,000 T/y with 88% emanating from channels and 12% from upland sources. This loading value, however, is somewhat misleading in that severe channel erosion occurred between 1967-1968 following channel clearing and snagging over the lower 17 km. Since this time, sediment loads attenuated and the contribution from channels and uplands over the period 1970-2002 shifted to 70% and 30%, respectively. “Actual” simulated suspended-sediment loads at the Q1.5 show a 35-year average of 675 T/D/km; 155 T/D/km over the past 10 years. Following the installation of low-water crossings in 1999 loads decreased to about 39 T/D/km. This value is more than an order of magnitude greater than the “reference” yield.