Location: Watershed Physical Processes Research2009 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 2009, research was performed by 8 scientists working in collaboration with many research institutions to assist agricultural landowners in the production of 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. The variation of the relative sources of sediment from land surface and channel banks has been explored on the Goodwin Creek Experimental Watershed. This is important information to ascertain the timing and placement of the dominant sediment producing regions in a watershed and the type of conservation practices that would be effective. Links between sediment and biologic processes were made from upper Midwest and Southeastern United States. This new analysis allowed the development of new sediment targets for the support of aquatic life. The effect of tillage on the processes of ephemeral gullies in agricultural watersheds have been incorporated into the USDA Annualized Agricultural Non-Point Source pollution model (AnnAGNPS). This will improve sediment yield calculations and improve the selection of effective conservation practices. Improvements have been made to RUSLE2 to more completely represent the processes of residue formation on hay and pasture lands. This will improve calculations of runoff and sediment from these types of lands. Wave height predictors for small reservoirs have been improved. This improved information will allow better estimates of where wave erosion will be a problem and offer means to control this erosion in small agricultural reservoirs. Subsurface flow processes have been investigated and found to be an important component in the formation of ephemeral gullies. This improved characterization of ephemeral gully formation will lead to improved methods of predicting their occurrence and conservation practices to prevent their formation. Improvements have been made to channel migration routines in the ARS channel evolution computer model CONservational Channel Evolution and Pollutant Transport System (CONCEPTS). This new approach will improve predictions of erosion and allow improved stability measures to be formulated for unstable channel systems in agricultural watersheds.
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 laboratory basic research studies. The influence of agricultural practices has been evaluated with the model 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, when evaluating the effect of agricultural conservation practices at field or watershed scales.
2. Title: RUSLE2-GRAZE. Calculating residue production only during periods of canopy decline or in response to operations underestimates residue additions, resulting in overestimates of soil erosion from pasture and hay lands. To solve this problem, new vegetation routines were developed and incorporated into RUSLE2 that better reflect the amount of residue added by perennial vegetation during its growth, and that make it is easier to model haying/grazing scenarios. The potential growth of vegetation is described in terms of: total annual production potential under good management, monthly production percentages, the average lifespan of vegetation, maximum canopy and biomass at peak live biomass, the cutting height that allows potential growth, and the tendency of the vegetation to thicken (form a sod) in response to repeated defoliations. The program then calculates the effect of alternative managements on the amount of forage harvested, above and below ground residues returned to the soil, and average annual soil erosion. USDA-NRCS grazing specialists are in the process of developing a grazing planner interface and regional databases of vegetation and management descriptions that will allow the new capabilities of RUSLE2 to be used in field offices nationwide. The new version of RUSLE2 will allow soil erosion to be a factor considered as part of grazing planning.
3. Simulating Channel Migration Using Physically-based Migration Rates. Current computer models of river meandering use empirical formulations that relate the migration of a river to local water velocity. These formulations are unable to properly account for spatial variation in soil properties and riparian vegetation, hence greatly reducing the accuracy of model predictions. The physically-based stream bank erosion component of the ARS channel evolution computer model CONCEPTS were combined with flow and sediment transport components of the RVR-MEANDER toolbox developed by the University of Illinois. The resulting model can 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.
4. Developed Biologically Relevant Numerical Water-quality Targets for Sediment. “Aquatic life support” is the primary water-quality metric used by States, Territories and Tribes. Unfortunately, functional links between sediment and biologic processes are not fully developed. Flow, suspended-sediment and fish data from ecoregions in the upper Midwest and the Southeastern United States were synthesized using functional traits analysis. Biologically relevant numerical water-quality targets for sediment were developed based on sediment-transport magnitude, duration and frequency. This marks advancement from earlier, single numeric sediment-target values that were not linked to biologic processes or functional traits, providing meaningful sediment targets and allows the formation of more realistic water quality targets.
5. Soil-pipe Flow Impacts on Ephemeral Gully Erosion. Ephemeral gully erosion accounts for around one third of the total soil erosion. Concentrated surface flow is generally the controlling process, however, subsurface flow can directly contribute to gully erosion by seepage exfiltration forces on the surface and by internal erosion as water flows rapidly through soil-pipes. Laboratory measurements demonstrated conditions under which preferential flow through soil pipes can serve to reestablish ephemeral gullies and initiate their development. This work has led to expanded opportunities for development of ephemeral gully erosion prediction models that include subsurface flow processes.
6. Improved Wave Height Prediction in Small Reservoirs. Predictions of wave height from wind speed in small reservoirs are inaccurate. Extensive data analyses led to an improved relationship that can be used to predict wave height from wind speed in small reservoirs. This piece of work was incorporated into the final report and is currently being reviewed for publication in a peer-reviewed journal. The work directly impacts farmers by providing a better method for predicting wave heights for the design of floating wave barriers, which are an economically viable method for protecting expensive earth levees used to store irrigation water.
Wilson, C.G., Kuhnle, R.A., Bosch, D.D., Steiner, J.L., Starks, P.J., Tomer, M.D., Wilson, G.V. 2008. Quantifying Relative Contributions from Sediment Sources in Conservation Effects Assessment Project Watersheds. Journal of Soil and Water Conservation. 63(6):523-532.