Christopher G. Wilson

From soilscapes to landscapes: A landscape‐oriented approach to simulate soil organic carbon dynamics in intensively managed landscapes

Most available biogeochemical models focus within a soil profile and cannot adequately resolve contributions of the lighter size fractions of organic rich soils for enrichment ratio (ER) estimates, thereby causing unintended errors in soil organic carbon (SOC) storage predictions. These models set ER as constant, usually equal to unity. The goal of this study is to provide spatiotemporal predictions of SOC stocks at the hillslope scale that account for the selective entrainment and deposition of lighter size fractions. It is hypothesized herein that ER values may vary depending on hillslope location, Land Use/Land Cover (LULC) conditions, and magnitude of the hydrologic event. An ER module interlinked with two established models, CENTURY and Watershed Erosion Prediction Project, is developed that considers the effects of changing runoff coefficients, bare soil coverage, tillage depth, fertilization, and soil roughness on SOC redistribution and storage. In this study, a representative hillslope is partitioned into two control volumes (CVs): a net erosional upslope zone and a net depositional downslope zone. We first estimate ER values for both CVs I and II for different hydrologic and LULC conditions. Second, using the improved ER estimates for the two CVs, we evaluate the effects that management practices have on SOC redistribution during different crop rotations. Overall, LULC promoting less runoff generally yielded higher ER values, which ranged between 0.97 and 3.25. Eroded soils in the upland CV were up to 4% more enriched in SOC than eroded soils in the downslope CV due to larger interrill contributions, which were found to be of equal importance to rill contributions. The chronosequence in SOC storage for the erosional zone revealed that conservation tillage and enhanced crop yields begun in the 1980s reversed the downward trend in SOC losses, causing nearly 26% of the lost SOC to be regained.

Implementing streambank erosion control measures in meandering streams: design procedure enhanced with numerical modelling

ABSTRACT Dike structures have been used in many streams to control streambank erosion by redirecting the flow away from the banks towards the centre of the channel. In this study, the 2D finite-element surface water modelling system (FESWMS) was used to enhance the design of dike structures in two meandering stream reaches along the Raccoon River, Iowa, USA. FESWMS was used to find the optimal number and spacing between the dikes and to access their overall performance in controlling river-bank erosion in the study reaches. The study included also field measurements for model calibration and verification. The model results showed that the proposed dike structures, consisting of alternate five bendway weirs and four spurs at each site with a spacing of about 50 m, successfully reduced the flow velocity along the outside bank and increased the conveyance of the flow to the stream centre. The average lengths of the spurs and bendway weirs were about 16 and 32 m, respectively. The estimated velocity and Froude number values along the outside riverbank where the dikes were constructed were less than 1.0 m/s and 0.3, respectively, which are within the recommended values for erodible channel stability design. Site visits one year after the dike construction showed that the dikes were successful in controlling streambank erosion by allowing incoming sediment to deposit between the dikes and along the bank line. Sediment transport measurements at the study sites showed that the maximum deposition was about 0.8 m, which is much higher than a typical averaged deposition value of about 0.3 m/year. Further, recent Google Earth pictures for the sites showed that the bank slope lines almost recovered back in their original profiles and that the dikes became part of the restored banks, a strong indication of the success of the proposed dikes design.

Improved streambank countermeasures: the Des Moines River (USA) case study

Abstract In the Midwestern USA, bank erosion is a common hazard due to the high erodibility of the bank soils. In this paper, an improved methodology aimed at identifying the optimal countermeasures to control bank erosion was developed and applied in two sites of the Des Moines River (USA). In situ flow measurements, bed bathymetry and soil properties were collected for providing boundary conditions and parameters of the two-dimensional, depth-averaged hydrodynamic finite element surface water modeling system (FESWMS) model. The model was used to compare the hydraulic performances of four streambank countermeasures: riprap lining (referred to as S1); a series of barbs (S2); alternating barbs and spurs (S3); and the combination of barbs with lining (S4). A key feature of FESWMS was its ability to simulate the wetting/drying conditions of mesh elements, which allowed the simulations of unsubmerged or partially submerged structures for different hydraulic conditions. This research showed that the combination of alternating barbs and spurs (S3) was the only design which provided protection during overbank flows at a competitive cost compared to the other designs analysed. The uniqueness of this methodology is found in the coupling of field measurements and theoretical approaches for depth-averaged velocity profiles to calibrate and validate a hydrodynamic model; and in the proposed design to protect streambanks from severe erosion.

Understanding mass fluvial erosion along a bank profile: using PEEP technology for quantifying retreat lengths and identifying event timing

This study provides fundamental examination of mass fluvial erosion along a stream bank by identifying event timing, quantifying retreat lengths, and providing ranges of incipient shear stress for hydraulically driven erosion. Mass fluvial erosion is defined here as the detachment of thin soil layers or conglomerates from the bank face under higher hydraulic shear stresses relative to surface fluvial erosion, or the entrainment of individual grains or aggregates under lower hydraulic shear stresses. We explore the relationship between the two regimes in a representative, US Midwestern stream with semi-cohesive bank soils, namely Clear Creek, IA. Photo-Electronic Erosion Pins (PEEPs) provide, for the first time, in situ measurements of mass fluvial erosion retreat lengths during a season. The PEEPs were installed at identical locations where surface fluvial erosion measurements exist for identifying the transition point between the two regimes. This transition is postulated to occur when the applied shear stress surpasses a second threshold, namely the critical shear stress for mass fluvial erosion. We hypothesize that the regimes are intricately related and surface fluvial erosion can facilitate mass fluvial erosion. Selective entrainment of unbound/exposed, mostly silt-sized particles at low shear stresses over sand-sized sediment can armor the bank surface, limiting the removal of the underlying soil. The armoring here is enhanced by cementation from the presence of optimal levels of sand and clay. Select studies show that fluvial erosion strength can increase several-fold when appropriate amounts of sand and clay are mixed and cement together. Hence, soil layers or conglomerates are entrained with higher flows. The critical shear stress for mass fluvial erosion was found to be an order of magnitude higher than that of surface fluvial erosion, and proceeded with higher (approximately 2–4 times) erodibility. The results were well represented by a mechanistic detachment model that captures the two regimes. Copyright

Redistribution Effects on Changes in Soil Carbon Storage Potential in Intensely Managed Landscapes

Currently, biogeochemical models lack the ability to simulate accurately soil organic carbon (SOC) dynamics, especially in intensely managed landscapes (IMLs) located throughout much of the U.S. Midwest, as they do not account for lateral and downslope redistribution of soil and SOC. This limitation can increase the uncertainty in predicting SOC sequestration potential (SOC-SP) when quantifying carbon budgets for a landscape. In this study, the limitation was addressed by complementing event-based and seasonal SOC observations for a hillslope within the Clear Creek, IA watershed with the development of a coupled modeling framework focused on SOC redistribution by vertical mixing and downslope/lateral mobilization. The framework links an off-the-shelf, spatially distributed, hillslope erosion model (Water Erosion Prediction Project, WEPP) with a biogeochemical model CENTURY. Specifically, key physical and biogeochemical parameters were monitored throughout several growing seasons, while soil samples were collected along various hillslope positions and measured for SOC. Results show heterogeneous stocks of SOC across the hillslope, with eroding zones having lower SOC concentrations than depositional zones. Accounting for the spatial heterogeneity and temporal variability of SOC within a landscape will lead toward improved SOC-SP predictions as well as the development of more sustainable agricultural practices.

Groundwater monitoring at the watershed scale: An evaluation of recharge and nonpoint source pollutant loading in the Clear Creek Watershed, Iowa

1 Iowa Geological Survey, University of Iowa, Iowa City, IA, USA Department of Earth and Environmental Sciences, University of Iowa, Iowa City, IA, USA Department of Civil and Environmental Engineering, Hydraulics and Sedimentation Laboratory University of Tennessee, Knoxville, TN, USA Correspondence Keith E. Schilling, Iowa Geological Survey, University of Iowa, Iowa City, IA, USA. Email: keith‐schilling@uiowa.edu