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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 Soil Conservation Service (SCS) curve number (CN) estimates of direct runoff from rainfall for semiarid catchments can be inaccurate. Investigation of the Walnut Gulch Experimental Watershed (WGEW) (Southeastern Arizona) and its ten nested catchments determined that the inaccuracy is due to the original SCS ratio (λ) of 0.2 between initial abstraction and maximum potential retention. Sensitivity analyses indicate that runoff estimation can be very sensitive to the initial abstraction ratio, especially for relatively low rainfall amount and for watersheds covered by deep, coarse, and porous soil, conditions that dominate many semiarid watersheds worldwide. Changing the ratio of initial abstraction to the maximum potential retention to optimal values ranging from 0.01 to 0.53 for different Walnut Gulch catchments improved runoff estimates. The greater the channel area and the finer the soil, the smaller the initial abstraction ratio is. The variation of the initial abstraction ratio for the WGEW is due to the variation of maximum potential retention and initial abstraction, which are channel area and soil‐dependent parameters. The greater the channel area, the higher the maximum potential retention S is, and the coarser the soil, the larger the initial abstraction Ia is. In addition, the effect of initial abstraction ratio on runoff estimation increases with decreasing CN. Thus, impacts of initial abstraction ratio on runoff estimation should be considered, especially for semiarid watersheds where the CN is usually low. Copyright © 2012 John Wiley & Sons, Ltd.

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Both rural and urban development can lead to accelerated gully erosion. Quantifying gully erosion is challenging in environments where gullies are rapidly repaired, and in urban areas where microtopographic complexity complicates the delineation of contributing areas. This study used unmanned aerial vehicles (UAVs) and Structure-from-Motion (SfM) photogrammetric techniques to quantify gully erosion in the Los Laureles Canyon watershed, a rapidly urbanizing watershed in Tijuana, Mexico. Following a storm event, the gully network extent was mapped using an orthomosaic (0.038 m pixel size); the local slope and watershed area contributing to each gully head were mapped with a Digital Surface Model (0.3 m pixel size). Gullies formed almost exclusively on unpaved roads which had erodible soils and concentrated flow. Management practices (e.g. road maintenance that fill gullies after large storms) contributed to total sediment production at the watershed scale. Sediment production from gully erosion was higher and threshold values of slope and drainage area for gully incision were lower than ephemeral gullies reported for agricultural settings. This indicates high vulnerability of unpaved roads to gully erosion which is consistent with high soil erodibility and low critical shear stress measured in the laboratory with a mini jet-erosion-test device. Future studies that evaluate effects of different soil types on gully erosion rates for unpaved roads, as well as those that model effects of management practices such as road paving and their impact on runoff, soil erosion, and sediment loads are needed to advance sediment management and planning in urban watersheds.

Abstract:The Annualized Agricultural Non-Point Source (AnnAGNPS) model has been developed to quantify watershed response to agricultural management practices. The objective of this study was to identify critical areas where conservation practices could be implemented and to predict their impact on Beasley Lake water quality in the Mississippi Delta using AnnAGNPS. Model evaluation was first performed by comparing the observed runoff and sediment from a US Geological Survey gauging station draining 7 ha (17 ac) of Beasley Lake watershed with the AnnAGNPS simulated runoff and sediment. The model demonstrated satisfactory capability in simulating runoff and sediment at an event scale. Without calibration, the Nash-Sutcliffe coefficient of efficiency was 0.81 for runoff and 0.54 for sediment; the relative error was 0.1 for runoff and 0.18 for sediment, and the Willmott index of agreement was 0.94 for runoff and 0.80 for sediment. The quantity of water and sediment produced from each field within the Beasley Lake watershed, quantity of water and sediment reaching Beasley Lake, and the potential impact of various USDA Natural Resources Conservation Service conservation programs on water quality were then simulated. High sedimentproducing areas for nonpoint source pollution control were identified where sediment loads could be reduced by 15% to 77% using conservation practices. Simulations predicted that converting all cropland to no-till soybeans (Glycine max [L.] Merr.) would reduce sediment load by 77% whereas no-till cotton (Gossypium hirsutum L.) would reduce it 64%. The approach taken in this study could be used elsewhere in applying AnnAGNPS to ungauged watersheds or watersheds with limited field observations for conservation program planning or evaluation.

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