Sediment removal by prairie filter strips in row-cropped ephemeral watersheds

Twelve small watersheds in central Iowa were used to evaluate the effectiveness of prairie filter strips (PFS) in trapping sediment from agricultural runoff. Four treatments with PFS of different size and location (100% rowcrop, 10% PFS of total watershed area at footslope, 10% PFS at footslope and in contour strips, 20% PFS at footslope and in contour strips) arranged in a balanced incomplete block design were seeded in July 2007. All watersheds were in bromegrass ( L.) for at least 10 yr before treatment establishment. Cropped areas were managed under a no-till, 2-yr corn ( L.)-soybean [ (L.) Merr.] rotation beginning in 2007. About 38 to 85% of the total sediment export from cropland occurred during the early growth stage of rowcrop due to wet field conditions and poor ground cover. The greatest sediment load was observed in 2008 due to the initial soil disturbance and gradually decreased thereafter. The mean annual sediment yield through 2010 was 0.36 and 8.30 Mg ha for the watersheds with and without PFS, respectively, a 96% sediment trapping efficiency for the 4-yr study period. The amount and distribution of PFS had no significant impact on runoff and sediment yield, probably due to the relatively large width (37-78 m) of footslope PFS. The findings suggest that incorporation of PFS at the footslope position of annual rowcrop systems provides an effective approach to reducing sediment loss in runoff from agricultural watersheds under a no-till system

Modelagem da retenção de herbicidas em zonas ripárias Modeling of herbicide retention in riparian zones

O presente trabalho teve como objetivo investigar a retenção de atrazina e picloram, transportados via escoamento superficial, em zona ripária. Para isto, simulou-se um escoamento superficial, contendo uma mistura de caulinita, atrazina e picloram, dentro de zonas ripárias estabelecidas ao longo de plantações de pinheiros do Nordeste do Estado da Geórgia, EUA. Cinco parcelas foram instaladas dentro de zonas ripárias, apresentando declividades diferentes (2, 5, 10, 15 e 20%). Os efeitos da umidade do solo e da presença do horizonte O na retenção dos dois herbicidas foram avaliados. Um modelo exponencial, comumente utilizado na estimativa de redução da DBO e de nutrientes em tratamento por escoamento superficial, foi empregado na estimativa de redução de herbicidas e caulinita em zonas ripárias. O modelo possibilitou estimar com razoável precisão, a remoção de caulinita e atrazina da mistura em escoamento ao longo de zonas ripárias de 10 m de comprimento. Em geral, a declividade foi o parâmetro que apresentou melhor correlação com a retenção dos contaminantes presentes na mistura em escoamento na zona ripária. O horizonte O, mais espesso nas maiores declividades, favoreceu tanto a sedimentação da caulinita como a adsorção da atrazina.<br>This work aimed to investigate the retention of atrazine and picloram, carried by surface flow, in riparian zones. The surface flow, containing a mixture of both herbicides and kaolin, was then simulated within riparian zones established in pine plantations in north-eastern Georgia, USA. Five plots were established within riparian zones, each with a different slope (2, 5, 10, 15 and 20%). The influence of the initial moisture and of the O horizon condition in herbicide retention was analyzed. An exponential model, commonly used for the estimate of biochemical demand of oxygen (BOD) and nutrients attenuation in overland flow treatment, was used to estimate the attenuation of kaolin and herbicides in riparian zones. The model allowed an accurate estimate of the attenuation of kaolin and atrazine from the surface runoff mixture along 10 m of riparian zone. Generally, slope showed the best correlation with retention of the pollutants presented in the runoff mixture within the riparian zone. The O horizon present in the steeper slopes improved the sedimentation of kaolin and the atrazine adsorption

Evaluating Cover Crops (Sudex, Sunn Hemp, Oats) for Use as Vegetative Filters to Control Sediment and Nutrient Loading From Agricultural Runoff in a Hawaiian Watershed

A study was conducted to determine the effects of three land covers (sunn hemp -Crotalaria juncea, sudex, a sorghum-sudangrass hybrid -Sorghum bicolor x S. bicolor var. sudanese, and common oats -Avena sativa) planted as vegetative filter strips on the reduction of sediment and nutrient loading of surface runoff within the Kaika-Waialua watershed on the island of Oahu, Hawaii. Runoff samples were collected and analyzed for total suspended solids (TSS), total dissolved solids (TDS), phosphorous, and three forms of nitrogen (nitrate, ammonium, total nitrogen). Study results show that during seven out of 10 runoff events, the three cover crop treatments significantly reduced TSS as compared to the fallow treatment. Average removal efficiencies were 85, 77, and 73% for oats, sunn hemp, and sudex, respectively, as compared to the fallow treatment. Nutrient concentrations were low with phosphorous concentrations, lower than 1 (μg/ml) for all treatments, and total nitrogen (TN) concentrations below 7 (μg/ml) except in the sunn hemp treatment, where TN concentrations were less than 10 (μg/ml). Results of analysis of TDS showed that the cover crop treatments did not decrease dissolved solids concentrations in comparison with the fallow treatment. Analysis of nutrient concentrations in runoff samples did not detect any significant decreases in phosphorous, nitrogen, ammonium, or TN concentrations in comparison to the fallow treatment. However, a significant increase in TN concentrations in the sunn hemp treatment was detected and showed the nitrogen fixing capacity of sunn hemp. No treatment effects on runoff volume were detected, and runoff volumes were directly correlated with rainfall amounts showing no crops significantly impacted soil infiltration rates. These results were attributed to extremely low soil hydraulic conductivities (0.0001-7 cm/day at the soil surface, 15 and 30 cm below the soil surface). This study showed that cover crops planted as vegetative filters can effectively reduce sediment loads coming from idle and fallow fields on moderately steep volcanically derived highly weathered soils. © 2008 American Water Resources Association