Location: Watershed Physical Processes Research2011 Annual Report
1a. Objectives (from AD-416)
Assisting agricultural landowners to produce food and fiber in an economically and environmentally sustainable manner requires an integrated approach to land management practices, and protection of streams and impounded waters. This project contributes to those goals by developing and testing practices based on a scientific understanding of hydrologic, erosion, and sedimentation processes. This project also contributes to the Conservation Effects Assessment Project (CEAP) with the goal of quantifying effects of conservation management in selected CEAP watersheds, two of them managed within this project. To meet these challenges, the focus of all the proposed research activities has been chosen to evaluate innovative practices and to fill knowledge gaps in watershed models currently in use. This is realized by: (1) developing databases of weather, soil, land use, soil conservation practices, runoff, sediment yield, and nutrient data for assessing the impacts of conservation practices on the Goodwin Creek and Topashaw Creek CEAP watersheds; (2) evaluating relative magnitudes of sources and fates of sediment in CEAP-benchmark and other watersheds, and develop methodologies to establish criteria for identifying agricultural watersheds impaired by clean-sediment loadings; (3) quantifying and validating the uncertainties of model predictions at field, farm, and watershed scales for Yazoo River Basin CEAP sub-watersheds; (4) conducting field and laboratory studies to quantify the surface and subsurface flow processes governing the initiation, development and migration of ephemeral gullies and the effect of conservation management practices on infiltration, erosion, and transport; (5) conducting field and laboratory studies to improve the understanding of stream channel processes including channel evolution, sediment transport, protection of erodible embankments, edge-of-field gullies, and sediment deposition in impounded waters for CEAP and other watersheds; and (6) improving models to identify sources of sediment, determine their fate and transport within watersheds with complex channel drainage networks, and evaluate watershed water quality impacts in terms of implementation of land conservation and stream rehabilitation practices.
1b. Approach (from AD-416)
An extensive body of literature exists that describes plot or field-scale conservation practices aimed at reducing soil erosion or enhancing water conservation. However, results from plot- and field-scale studies are limited in that they cannot capture the complexities and interactions of conservation practices at the whole-farm level or at the watershed scale. Soil erosion and sediment movement processes involve the interactions of land management practices with climate, weather, soil, and landscape properties. Concentrated runoff and subsurface flow results in rill and gully erosion thus increasing soil losses and downstream sediment loads leading to increased costs of crop production, ecological degradation, and impairment of water supplies. This research focuses on developing tools and techniques to quantify the impact of implementing conservation practices within a watershed in the most efficient manner to achieve sustainable and targeted reductions of sediment loadings to the nation’s stream waters to help establish total maximum daily load requirements. New methods to measure and characterize changes in runoff, gully and stream channel erosion, and sediment deposition rates utilizing hydrological, geomorphic, and hydraulic engineering principles, and remote-sensing techniques will be tested in CEAP watersheds within the Yazoo River Basin, and in other watersheds when appropriate. Improved computer models and assessment tools will be provided to evaluate the impact of land conservation and stream rehabilitation practices in the most efficient manner to assist watershed managers achieve sustainable crop production systems and targeted reductions of sediment loadings.
3. Progress Report
In FY 2011, research was performed by scientists working in collaboration with many research institutions to assist agricultural landowners in producing food and fiber in an economically and environmentally sustainable manner using an integrated watershed approach to improve land management practices for controlling sedimentation, and for protection of streams and impounded waters. This required the development and testing of practices based on a scientific understanding of hydrologic, erosion, and sedimentation processes. This project contributed to the Conservation Effects Assessment Project (CEAP) by quantifying the effects of conservation management in two watersheds managed within this project. As a result of the research activities, enhancements to field, watershed, channel, and bank models have been implemented that provides evaluations of innovative practices needed by action agencies for effective watershed management needed to control erosion. Both inland and coastal shorelines are susceptible to erosion by waves and storm surges, but mechanical shore protection can be prohibitively expensive. Vegetative bank protection is self-sustaining and is a much more ecologically sound alternative. Adequate protection depends on both the type of vegetation and the amount available. It is essential for planning to be able to predict the performance of vegetation based on measureable quantities such as stem diameter and the number stems in a given area. The project is focused on measuring the performance of marsh grasses for attenuating waves in relatively shallow water. Phase I of work supported by the Department of Homeland Security was completed with setup and run measurements on a bare and vegetated (scale model) floating beach. Phase II of the work was begun with further improvements to the wave flume, including a walkway, instrument rails on top of the flume, and additional stroke for the piston wave generator. During site visits to coastal marshlands, video information on waves impacting marsh edges and samples of marsh-edge material were obtained. Additional data regarding the distribution of stem mass and frontal area with elevation was collected for both Spartina alterniflora and Juncus romerianus. Experimental facilities were constructed at Mississippi Agriculture and Forestry Experiment Station at Holly Springs, MS, that consist of six above-ground plots, 5.5 m wide by 18.3 m long (0.1 ha), at 10% slope with a water restrictive layer that surfaces at the toeslope. Plots were equipped with F-1 stage recorders and Coshocton wheel flow dividers diverted to storage tanks for flow and sediment sampling. The back side of each plot contains a water reservoir for inducing subsurface flow under controlled conditions. The field-scale plots will enable experiments to be conducted under conditions of induced seepage above water restricting horizons. During FY 2010, the plot walls were erected, sealed, and partially filled with loess soil from the station. After 6 months of soil settling, a compacted clay layer was emplaced above the loess material. In addition, the water reservoirs were fabricated and installed into the plots.
1. Fraction of sediment derived from channel sources determined for watersheds. ARS scientists at the National Sedimentation Laboratory at Oxford, MS, found that the dominant sources of sediment in agricultural watersheds are important for designing practices that will reduce sediment loads to streams. Measurements using naturally occurring radionuclides to track sediment on Conservation Effects Assessment Project (CEAP) Watersheds have revealed that most of the sediment in the studied watersheds was derived from channel sources. This indicates that if erosion control practices reduce sediment concentration without reducing runoff volumes from fields, sediment loading in the channels may not be significantly reduced. Information on the sources of sediment is essential for watershed managers to design effective sediment control measures on agricultural watersheds.
2. Reductions in wave runup and setup due to model vegetation were measured. ARS scientists at the National Sedimentation Laboratory at Oxford, MS, found that both inland and coastal shorelines are susceptible to erosion by waves and storm surges, but mechanical shore protection can be prohibitively expensive. Vegetative bank protection is self-sustaining and is a much more ecologically sound alternative. Adequate protection depends on both the type of vegetation and the amount available. It is essential, for planning purposes, to be able to predict the performance of vegetation based on measureable quantities such as stem diameter and the number stems in a given area. Video analysis was used to measure wave runup and wave setup, and it was found that wave setup and runup were significantly reduced by the presence of scaled vegetation elements. These results are valuable for predicting the effect of vegetation on water level for slopes that are subjected to wave impact.
3. Tillage berms alter runoff flow patterns. ARS scientists at the National Sedimentation Laboratory at Oxford, MS, found that the effect of field edges and buffer strips on runoff flow patterns has been recognized but our ability to quantify these effects remains uncertain. They quantified the progressive development of small earthen berms at the margins of tilled fields, determined their influence on runoff flow patterns and sediment delivery, and developed computer modeling technology to predict these effects. Without a berm most runoff passed into and through near-contour vegetative buffers. However, tillage with a tandem disk immediately adjacent to buffers quickly built a berm that diverted most runoff from small storms. When a berm had formed, even for the largest storms, more than half of the runoff flowed along the buffer rather than through it. Runoff flowing upslope of and parallel to contour buffers had a longer flow path with increased opportunity time for water infiltration. This resulted in decreased runoff volume but had no significant impact on sediment delivery compared to runoff passing through buffers. Runoff diverted by tillage berms may become concentrated at areas of flow convergence within agricultural fields where a stable outlet must be provided to avoid gully erosion. These studies support the current design criteria in the USDA-Natural Resources Conservation Service (NRCS) National Practice Standards: Vegetative Barrier, code 601; Filter Strip, code 393; and Contour Buffer Strip, code 332.
4. Acoustic techniques used to analyze dam safety. Seepage through dams can result in erosion through soil pipes and cavernous areas leading dam failure. ARS scientists at the National Sedimentation Laboratory at Oxford, MS, worked in collaboration with the Natural Resources Conservation Service (NRCS), to conduct geophysical surveys at the Big Nance Site 4 earthen dam located in Lawrence County, Alabama. This dam has known problems associated with piping and cavernous areas. A survey line on the crest of the dam starting from the east abutment shows no indication of seepage through the body of the dam. However, anomalous subsurface zones of low P-wave velocity and low ray coverage were observed in areas of piping or cavity formation. A survey conducted across the auxiliary spillway suggests that excessive seepage is occurring in the direction of the auxiliary spillway and could result in additional sinkhole formation upslope of the currently observed sinkhole. An additional large anomalous area of lower P-wave velocity and low ray coverage was observed to the left of the principal drainage pipe. This information is critical to NRCS in developing dam safety and rehabilitation plans.
5. Sediment contributions from streambanks a significant source of sediment in several Conservation Effects Assessment Project (CEAP) watersheds. ARS scientists at the National Sedimentation Laboratory at Oxford, MS, found that conservation efforts to reduce sediment loadings to receiving streams and other water bodies may only be successful if mitigation measures target the major sources of sediment. Channel sources are largely ignored. Results of research show that CEAP watersheds such as Fort Cobb and Little Washita, OK, Goodwin Creek, MS, South Fork Iowa River, IA, and Town Branch, NY, produce substantially more sediment than those from stable channels in their respective regions. In some cases, these streams produce orders of magnitude more sediment than their stable counterparts. Reconnaissance of these channels showed that they are dominated by streambank erosion while in comparison, the Little River, GA, a stream in dynamic equilibrium without accelerated rates of bank erosion, displayed transport rates similar to calculated background rates. Knowledge of the dominant sources of sediment is necessary for the designing and implementation of measures to stabilize the channels in watersheds and reduce the impacts to aquatic communities and surface-water supplies.
6. Residue removal for biofuel production or animal consumption can lead to loss of soil fertility and soil degradation. ARS scientists at the National Sedimentation Laboratory at Oxford, MS, (NSL) found that a suitable tillage-residue management system is needed that sustains soil fertility and agronomic productivity, while preventing soil degradation. A study, conducted in the North China Plain (NCP) of China in collaboration with the NSL, measured the effects of different tillage-residue managements for a winter wheat and summer corn double-crop system on soil organic matter and total nitrogen contents. When dry manure was applied to substitute for the nitrogen (N.) lost by residue removal, crop residue could be removed for biofuel use under the no-tillage system without degrading the soil organic matter and nitrogen pools. In addition, this study showed that while removal of residue from conventional tillage system resulted in significant loss of organic matter and nitrogen in the upper 2 inches, when the deeper soil profile (0 to 2 ft) was accounted for, the conventional tillage system stored more organic matter than the no-tillage systems. The result was that agronomic productivity of the conventional tillage system with residue removed was comparable to the no-tillage systems.
Wells, R.R., Bennett, S.J., Alonso, C.V. 2010. Modulation of headcut soil erosion in rills due to upstream sediment loads. Water Resources Research. 46:W12531, doi:10.1029/2010WR009433.