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The objective of this study was to establish the subwatershed size dependency of the Soil and Water Analysis Tool (SWAT) erosion model to adequately simulate annual runoff and fine sediment (< 0.063 mm) from the 21.3 km 2 Goodwin Creek Watershed (GCW). Results of the GCW application show that runoff volume is not appreciably affected by the number and size of subwatersheds. However, an upper limit to subwatershed size is required to adequately simulate fine sediment yield produced from upland sources. Decreasing the size of subwatersheds beyond this threshold does not substantially affect the computed fine sediment yield. The proper identification of this threshold size can optimize input data preparation requirements and computational resources needed for effective utilization of the SWAT model, and simplify the interpretation of results.

In recent years, there has been growing recognition of the importance of riparian buffers between agricultural fields and waterbodies. Riparian buffers play an important role in mitigating the impacts of land use activities on water quality and aquatic ecosystems. However, evaluating the effectiveness of riparian buffer systems on a watershed scale is complex, and watershed models have limited capabilities for simulating riparian buffer processes. Thus, the overall objective of this paper is to develop an understanding of riparian buffer processes towards water quality modelling/monitoring and nonpoint source pollution assessment. The paper provides a thorough review of relevant literature on the performance of vegetative buffers on sediment reduction. It was found that although sediment trapping capacities are site-and vegetation-specific, and many factors influence the sediment trapping efficiency, the width of a buffer is important in filtering agricultural runoff and wider buffers tended to trap more sediment. Sediment trapping efficiency is also affected by slope, but the overall relationship is not consistent among studies. Overall, sediment trapping efficiency did not vary by vegetation type and grass buffers and forest buffers have roughly the same sediment trapping efficiency. This analysis can be used as the basis for planning future studies on watershed scale simulation of riparian buffer systems, design of effective riparian buffers for nonpoint source pollution control or water quality restoration and design of riparian buffer monitoring programs in watersheds. Published in

The Goodwin Creek Research Watershed (21.3 km2) is located in the north central part of Mississippi in the bluff hills just east of the Mississippi River floodplain. Land use on the watershed has been surveyed annually and the percentage of cultivated land has decreased from 26% in 1982 to 12% in 1990. During this 9‐year period the concentration of fines (<0.062 mm) in Goodwin Creek have decreased by 62%, concentrations of sand (0.062–2.0 mm) have decreased by 66%, and concentrations of gravel (>2.0 mm) have decreased by 39%. The decrease in the percentage of cultivated land affects the sediment budget of the watershed in two ways. A source of readily eroded sediment is removed, and the energy of the flowing water available to erode and transport sediment is reduced. The reduced flow in the channels from the decrease in cultivated land in the watershed was probably the main cause for the lower transport rates of sand and gravel.

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Abstract:RUSLE2 (Revised Universal Soil Loss Equation) is the most recent in the family of Universal Soil Loss Equation (USLE)/RUSLE/RUSLE2 models proven to provide robust estimates of average annual sheet and rill erosion from a wide range of land use, soil, and climatic conditions. RUSLE2's capabilities have been expanded over earlier versions using methods of estimating time-varying runoff and process-based sediment transport routines so that it can estimate sediment transport/deposition/delivery on complex hillslopes. In this report we propose and evaluate a method of predicting a series of representative runoff events whose sizes, durations, and timings are estimated from information already in the RUSLE2 database. The methods were derived from analysis of 30-year simulations using a widely accepted climate generator and runoff model and were validated against additional independent simulations not used in developing the index events, as well as against long-term measured monthly rainfall/runoff sets. Comparison of measured and RUSLE2-predicted monthly runoff suggested that the procedures outlined may underestimate plot-scale runoff during periods of the year with greater than average rainfall intensity, and a modification to improve predictions was developed. In order to illustrate the potential of coupling RUSLE2 with a process-based channel erosion model, the resulting set of representative storms was used as an input to the channel routines used in Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS) to calculate ephemeral gully erosion. The method was applied to a hypothetical 5-ha field cropped to cotton in Marshall County, MS, bisected by a potential ephemeral gully having channel slopes ranging from 0Ð5 to 5% and with hillslopes on both sides of the channel with 5% steepness and 22Ð1 m length. Results showed the representative storm sequence produced reasonable results in CREAMS indicating that ephemeral gully erosion may be of the same order of magnitude as sheet and rill erosion.

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