Location: Watershed Physical Processes Research2010 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 2010, 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. To understand the critical processes that influence ephemeral gully erosion, soil physical, chemical, and biological indicators of soil quality were measured along transects perpendicular to the gullies, at background locations representing natural conditions in Mississippi, Kansas, China, and in laboratory experiments. This dataset is being used to determine the best indicators of soil quality and the impact of ephemeral gully erosion on soil quality. The effect of soil texture and subsurface pore-water pressures were found to greatly affect soil erodibility, headcut migration rates, scour depths, and sediment delivery. To understand the growth, development and evolution of rill erosion networks, studies were completed to quantify the formation of rills subjected to various rainfall and downstream flow controls. We determined that rill incision was formed by headcuts by water flow levels lowering downstream. Rill networks migrated upstream by headcut erosion where peaks in sediment transport occurred periodically and were linked to downstream water levels. The rill sediment discharge and drainage density attained constant levels following downstream water level adjustments despite continuous rainfall. To develop enhanced technology for evaluating the affect of conservation practices on ephemeral gully erosion at the field scale, the Revised Universal Soil Loss Equation, (RUSLE2 2.0) was completed and at to provide capabilities that link hillslope runoff and erosion to ephemeral gully erosion. The enhancements also include a perennial plant growth model that predicts the management effects on forage availability and biomass resulting in affects on runoff and erosion. In cooperation with the Natural Resources Conservation Service (NRCS), we have distributed RUSLE2 2.0 to NRCS offices for evaluating the effects of grazing systems on soil erosion.
1. Tillage Significantly Effects Ephemeral Gully Erosion. Ephemeral gully erosion has been shown to be a significant source of sediment from many agricultural fields as a result of soil disturbing tillage operations. The important processes that influence ephemeral gully erosion have been incorporated into the USDA Annualized Agricultural Non-Point Source pollution model (AnnAGNPS) by utilizing procedures developed from basic laboratory research studies. Evaluations of agricultural practices have been performed using AnnAGNPS in watersheds throughout the United States as part of the USDA Conservation Evaluation and Assessment Project (CEAP), as well as watersheds in Spain, Italy and China for their effect on sediment delivered from sheet and rill erosion, as well as from ephemeral gully erosion. This provides an important tool for action agencies, such as the USDA-Natural Resources Conservation Service, to evaluate the effect of agricultural conservation practices at field or watershed scales and implement cost effective conservation programs targeted to the most appropriate sources of sediment.
2. Rill and Gully Erosion in Upland and Agricultural Areas can Result in Significant Soil Degradation. Worldwide, headcuts are the primary mechanism that gully erosion landscape dissection occurs. Experiments were conducted to further examine the morphodynamic behavior of actively migrating headcuts in upland concentrated flows with varying boundary conditions. The effect of soil texture was found to greatly modify the erodibility of select soils, which affects headcut migration rates, scour depths, and sediment discharges. Increasing tailwater height downstream from gullies can greatly reduce the flow angle at the brink of the headcut scour hole, completely stopping soil erosion processes. Altering subsurface pore-water pressures markedly changed the erodibility of the select soil. The presence of a subsurface water table caused greater headcut migration rates and sediment discharges, but shallower gully scour holes. Current equations based on scour theory were successfully used to predict these experimental observations, further demonstrating the capability of using such equations in improving watershed management technology when evaluating the effect of conservation practices by action agencies.
3. Release of RUSLE2 2.0. The first major public release since 2005 of the revised universal soil loss equation for conservation practice evaluations, RUSLE2, provides new capabilities in (1) prediction hillslope runoff and erosion estimates suitable for linkage to an ephemeral gully model and (2) a perennial plant growth model that predicting management effects on forage availability and amounts of biomass that affect runoff and erosion. Included in the release are core databases and documentation that illustrate the capabilities of these new features. The new version of RUSLE2 has been adopted by NRCS to consider soil erosion when planning grazing systems. The enhanced runoff generation capabilities developed provide a critical step toward the goal of evaluating the effect of ephemeral gulley erosion control practices using RUSLE2, which is technology identified by NRCS as a critical need in conservation practice planning.
4. Improved Stream Bank Erosion Prediction for Meandering Streams. Computer models of river meandering currently use empirical equations that relate the migration of a river to local water velocity. These equations do not properly account for spatial variation in soil properties and riparian zone management practices, and therefore do not realistically estimate the complex planform patterns and meander bend shapes that develop over time and the resulting loadings of fine sediments produced from eroding stream banks. ARS scientists at Oxford, MS, in collaboration with scientists at the University of Illinois at Urbana-Champaign have integrated physically-based stream bank erosion components of the ARS channel evolution computer model CONCEPTS with multi-dimensional models flow and sediment transport in meandering streams. Application of the model to streams in California and Illinois have been performed to evaluate the impact of channel alignment on stream bank erosion. This provides an important tool for action agencies such as the USDA-Natural Resources Conservation Service, when designing new or restoring existing channels with a natural plan form.
5. Soil Erosion by Water Shown to Remain a Major Problem in Many Regions of the U.S. Estimates by the USDA suggest that ephemeral gully erosion ranges from 18 to 73% of the total erosion. Refilling or removing ephemeral gullies by tillage causes high erosion rates to continue damaging the soil next to the gully. The impact of filling ephemeral gullies on crop yield and the health or quality of the soil adjacent to gullies was measured on two gullies in Topashaw Canal Watershed, MS, two gullies in Cheney Lake watershed, KS, and two gullies in the Black Soils region of NE China. Soil physical, chemical, and biological indicators of soil quality were measured along transects perpendicular to the gullies and at background locations representing natural conditions. This dataset is being used to determine the best indicators of soil quality and the impact of ephemeral gully erosion on soil quality. This information is important when implementing conservation practices by action agencies when controlling ephemeral gully erosion.
6. Documented Sedimentation Rates in Mississippi Delta Lakes Impacted by Management Practices in Watersheds. Sediments stored in lakes represent a valuable archive that can be used to recover information on the erosion history on watersheds. Measuring the performance of costly watershed management practices is a problem with far-reaching impacts. The effects of civilization and intensive agriculture have caused erosion rates many times higher than that of undisturbed land; however, the return on the investment of funds used to control erosion problems is largely unknown since the affects of erosion control measures is not often measured. Sediments were studied from five natural oxbow cutoffs in the Mississippi River alluvial floodplain and were dated using 210Pb decay rates and bomb-pulse derived 137Cs to develop trends in sedimentation rates with management practices. Physical soil core data collection was completed, and 210Pb and 137Cs data collection have now been completed for all of the lakes in the study. Concentrations of 62 elements have also been collected for 4 of the lakes. As a result, the radioisotope dating methods were found to be best used in concert with known dates for implementation of management practices. Changes in sedimentation rate over time frames as short as 15 years have been detectable.
7. Developed Biologically Relevant Numerical Water-quality Targets for Sediment. “Aquatic life support” is the primary water-quality metric used by States, Territories and Tribes. Functional links between sediment-transport and biologic processes have now been identified by ARS researchers in cooperation with the University of Tennessee for an ecoregion in the upper Midwest using functional traits analysis. Biologically relevant numerical water-quality targets for sediment were developed based on sediment-transport magnitude, duration and frequency. This results in advancements from earlier, single numeric sediment-target values that were not linked to biologic processes or functional traits, providing meaningful sediment targets for total maximum daily load development.
8. The Growth, Development, and Evolution of Rill Networks were Quantified. Experiments were conducted using soil in a flume subjected to simulated rain and downstream flow controls to quantify the growth, development, and evolution of rill networks. Digital elevation models constructed using photogrammetric techniques greatly improved data acquisition and analysis. Results demonstrated that: (1) rill headcuts formed by lowering the downstream water level were the primary drivers of rill incision and network development, and the impact of this degradation wave occurred very quickly and effectively through the landscape; (2) rill networks extended upstream by headcut erosion, where channels split and were filled with sediment; (3) rill incision, channel development, and peaks in sediment transport occurred periodically, linked directly to the downstream water level adjustments; and (4) sediment discharge and rill drainage density approached nearly constant values with time following water level adjustments despite continuous application of rainfall. These findings have important implications for the prediction of soil loss, rill network development, and landscape evolution where headcut erosion can occur.
9. Sediment Transport in Shallow Flow. The most critical need in sediment transport is developing a better understanding of factors and processes affecting sediment transport capacity. In cooperation with the Department of Civil Engineering at the University of Mississippi, additional experimental studies with coarse and medium size sand were conducted to meet this need. In our basic studies, we have noted three modes of sediment movement: saltation (all sediment is in motion), sediment waves (sediment moves in waves), and meandering, each of which has different transport capacities. In previously reported works, a model was formulated that consisted of a two-phase continuum layer of water and sediment near the conveyance boundary in co-existence with a supernatent layer of water without sediment. The analysis, based on the continuity and momentum equation for the water sediment layer and the St. Venant equation for the supernatent, sediment free layer, complemented with expressions for the sediment boundary collisions and sediment particle fluid interactions, yielded a sediment velocity concentration relationship that is consistent with experimental observations. Also, an expression was found that allows the determination of the volume concentration at which the saltation mode changes into a transport mode by sediment waves. At this point, the sediment maximum transport capacity has been reached. This study also shows that sediment transport relationships as commonly used in erosion and runoff prediction models are simplistic and that sediment transport capacity evaluations on a far more complicated problem than so far has been assumed are needed. This conclusion suggests that more research efforts need to be made to arrive at more representative values of the sediment transport capacities for different flow regimes and material transport.
10. Non-contact Measurement of Soil Properties. Acoustic properties of soils have been shown to be indicative of soil physical properties. A rapid, multi-channel analysis of surface wave (MASW) method has been demonstrated using laser Doppler vibrometry as a non-contact sensor to obtain the sound speed profile in soil up to 3 meters below surface. The MASW method has been automated to provide mobility with increased scanning speed and spatial resolution. Using this method, the temporal variations of the soil profile due to changes in moisture content are being continuously monitored. From measurements of the soil profile, the surface layer of the soil (less than 5 cm below surface) has higher sound speeds than those of lower-layer soils, indicating sealing/crusting effects. Ongoing studies have been conducted to determine the thickness and structural nature of layered soils, including fragipans, hardpans and plowpans. Efforts have also been made to detect underground flows at different depths and diameters.
Wells, R.R., Bennet, S.J., Alonso, C.V. 2009. Effect of soil texture, tailwater height, and pore-water pressure on the morphodynamics of migrating headcuts in upland concentrated flows. Earth Surface Processes and Landforms. 34:1867-1877.