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Assessment of erosion impact on soil carbon and nitrogen loss and redistribution within landscapes is needed to develop estimates of soil carbon sequestration potential, soil quality management plans, and to evaluate potential for transport of sediment bound agrochemicals. We used variable intensity rainfall simulations to quantify the effects of tillage, conventional and strip, and antecedent soil water content on sediment-bound carbon and nitrogen loss from a Tifton loamy sand located in the southeastern Atlantic Coastal Plain (USA). Carbon and nitrogen loss via erosion of silt + clay sized versus sand sized sediment particles were quantified. Antecedent water content had no effect on mean sediment loss within tillage treatments, but losses from conventional till treatments were significantly greater than from strip till. Sediment lost as silt + clay was from 58 to 78% of the total under conventional and from 30 to 39% under strip tillage. The fraction of sediment lost as silt + clay versus sand was greater under conventional tillage than under strip tillage. Within-event sediment carbon enrichment compared to the top 2 cm of soil was 0.9–7.2 for conventional and 0.6–3.7 for strip tillage. The strip till silt + clay fraction had significantly higher carbon content than the strip till sand fraction and the conventional till silt + clay fraction. Carbon loss from treatments was directly proportional to sediment loss. However, the conventional till treatments lost 4.6–6 times more carbon from the silt + clay sized fraction and 1.9–4.8 times more carbon from the sand sized fraction than strip till treatments. Results suggest that the higher proportion of silt + clay fraction sediment loss from conventional till may deplete nitrogen enriched organic matter while decreased erosion from strip till may serve to increase retention of organic nitrogen. Findings also indicate that an approach that adjusts loss estimates of organic carbon and nitrogen by using wet-sieved subsamples for analysis followed by standardization against total bulk sediment loss accounts for introduced errors from both sub-sampling efficiency and disturbance.

A rainfall simulator was used to apply 5 cm of rainfall in 2 hours to two replicate 624 m2 plots at six times during each of the growing seasons of 1992 and 1993. Because the simulator generated reproducible and time-invariant rainfall intensities, the resulting 24 hydrographs reproducibly reveal the effects of tractor wheel compaction, tillage, soil reconsolidation, surface sealing, and corn canopy development. A time series data set including weather, crop development, soils properties, evapotranspiration, and antecedent soil water is available. These data should provide hydrologie modelers, particularly those interested in modeling runoff with time resolutions of

Laboratory studies provide the best opportunity for studying water and pesticide partitioning for a range of soil conditions while maintaining reproducible conditions. We developed and evaluated a laboratory technique for determining pesticide partitioning among infiltration, runoff, and sediment. Simulated rainfall (44 mm h -1 ) was applied to a sandy clay loam 14 days after pesticide (atrazine, chlorpyrifos, and 2,4-D) application. The laboratory technique provided controlled test conditions and reproducible results on the partitioning of rainfall, runoff, and pesticides at the soil surface and on sediment and pesticide transport. Average atrazine and chlorpyrifos half-life (t 1/2 ) values determined from corresponding concentrations in the 0- to 1-cm soil layer were approximately 30 and 10 d. Atrazine, chlorpyrifos, and 2,4-D concentrations remaining in the 0- to 1-cm surface layer decreased logarithmically with rainfall duration. Atrazine and 2,4-D concentrations in the 0- to 1-cm layer were correlated positively with those associated with splash (S), runoff (R), and sediment yield (E) (R 2 = 0.69 to 0.99). Relationships for all pesticide concentrations in the 0- to 1-cm layer and infiltration (INF) were correlated negatively. Relatively poor correlations (R 2 = 0.10-0.63) were found between chlorpyrifos losses and corresponding concentrations in the surface layer even though positive correlations were found between R and E (R 2 = 0.91) and E and S, (R 2 = 0.77). Measured effective partitioning coefficients (K deff ) increased gradually for each event, and were at least 31 times greater than reported K d values for atrazine, chlorpyrifos, and 2,4-D. The methodologies and experimental design described provide a means of evaluating how changes in pesticide concentrations in the surface layer are influenced by soil, rainfall, and pesticide characteristics, which will enhance the development of process-based models to predict pesticide fate and transport for event-based storms.

Many pesticide fate and transport models, including the GLEAMS model, overestimate pesticide degradation during its later stages of residence in soil. Except for runoff events shortly after pesticide application, models using equilibrium sorption kinetics often underestimate sediment-transported pesticides. To address concerns about transport of low levels of pesticides by runoff to sensitive ecosystems over annual or longer time cycles, GLEAMS was modified using a two-compartment pesticide-state model and algorithms for adjustment of degradation rates for temperature and soil water contents. Two pesticide pools, labile and nonlabile, were linked using first-order kinetics, with the forward and reverse rate constants between these pools as the only two additional inputs required for the model, GLEAMS-TC. GLEAMS-TC was calibrated successfully using 3 years of field data. Long-term pesticide persistence in soil was represented. Comparing model predictions with observed data, GLEAMS-TC simulated observed pesticide sediment transport, whereas GLEAMS underestimated observed data by a factor of 4. Sensitivity to rates of pesticide exchange between the two pools and impact on runofflosses were demonstrated with 50-year simulations. GLEAMS-TC is a research tool for investigating changes in pesticide state in soils as a function of exposure time, environmental variables, and interrelationships between pesticide degradation, mobility, and fate and transport in the environment.

There is need to evaluate the effects of agricultural production activities on sedimentation, pesticide, and nutrient losses under controlled, simulated rainfall on plots large enough (generally exceeding 50 m2) to incorporate realistic slope lengths and dominant processes that control runoff and sediment yield from “field-size” areas. Therefore, a rainfall simulator system was developed for 600-m2 plots (mesoplots) to evaluate runoff (water and sediment) and agrichemical movement from fields for different tillage practices and chemical applications. The rainfall simulator applies water with irrigation sprinklers spaced 3 m apart on two irrigation laterals arranged 14.6 m apart along the plot length. Runoff and sediment were collected in a V-shaped trough and directed to a flume for measuring and sampling. Simulated rainfall of 25 mm/h with a median drop diameter of 1.52 mm had a coefficient of uniformity of 91 over the plot area. Water was applied and sediment and runoff collected from two mesoplots for six events over a corn-growing season.

Laboratory soil pans and rainfall simulators have been used to study fundamental erosion processes affecting interrill soil loss. Problems exist when laboratory results are extended to those determined under field conditions. Experimental methodology influences a soil's interrill erodibility and interrill erodibility ranking among soils. We compared interrill soil loss and erodibility data from two erosion pans with field data from identical soils. Three plot size-rainfall simulator methodologies were used: (1) 0.14-m 2 lab pan under a constant drop size (4.6 mm) rainfall simulator, (2)0.32-m 2 lab with soil border areas under an oscillating nozzle (2.3 mm median drop diameter) rainfall simulator, and (3) 1-m 2 field plots under the same rainfall simulator as used in Method 2. Soil loss was measured at 5-min intervals. Interrill erodibility (K i ) was calculated from two equations (E=K ii and E=K iq Iq) using measured soil loss (E), rainfall intensity (I), and flow discharge (q) values. The interrill earosion equation, methodology, time, and initial water content influenced calculated K i values. Soil loss and K i values from Method 1 did not correlate with and were greater tan corresponding values from Methods 2 and 3. Soil loss and K i values from Method 2 were correlated (r=0.56 to 0.79) with corresponding values from Method3. The K iq values decreased with time and were a function of soil properties related to soil detachment and sediment transport. The K ii values (i) increased with time, (ii) were primarily a function of soil properties related to soil detachment only, and (iii) did not account for infiltration and runoff differences among soils

Coastal Plain soils in Georgia are susceptible to runoff, sediment, and chemical losses from short duration-high intensity, runoff producing storms at critical times during the growing season. We quantified runoff and sediment losses from a Tifton loamy sand managed under conventional- (CT) and strip- (ST) tillage and planted to peanuts. Simulated rainfall was applied at planting, 30 days after planting, and after harvest during the peanut growing season with rainfall events comprised of variable intensity (Iv) patterns representative of each time or season (spring=IvSPR, summer=IvSUM, fall=IvFALL). Simulated rainfall was applied to 2x3m plots (n=3) for each treatment. Runoff and sediment were measured from each 6-m 2 plot. Runoff ranged from 9-22% of the rainfall applied for the three events. The most runoff occurred from CT-IvFALL plots; the least occurred from ST-IvSUM plots. Maximum runoff rates were 7-20% of the maximum intensity and occurred 3-8 min after maximum intensity peaks. Sediment yields ranged from 105-1420 kg ha -1 . The most sediment occurred from CT-IvSPR plots; the least occurred from ST-IvSUM plots. Runoff and sediment curves had similar shapes as their corresponding rainfall intensity pattern. As for tillage, CT plots had 38% more runoff and 2.7-fold more sediment than ST plots over the three events. The largest difference in runoff (2.4-fold) and sediment (3.8-fold) among CT and ST plots occurred in the fall (IvFALL). Results improve our understanding of when runoff, sediment, and chemical losses are highest at critical times during a peanut growing season, and show how ST is effective in limiting those losses.

Rainfall intensity (I), antecedent water content, and slope steepness affect soil loss from interrill areas. Objectives of this field study were to (i) test the validity of the assumption that interrill soil loss is a function of I 2 , (ii) determine effect of antecedent water content on interrill erodibilities (K i ) for two slope steepnesses, (iii) evaluate the slope factor used in the water erosion prediction project (WEPP), and (iv) evaluate the product of I, flow discharge, and slope steepness as a possible interrill erosion model. Four soils, ranging in texture from sandy loam to clay, were exposed to sequences of simulated rainfall at three intensities (50, 70, and 100 mm h −1 ). Soil loss and runoff were measured from airdried and prewetted flat plots and from air-dried and prewetted ridged plots [...]

Nutrients from agricultural fields can contaminate runoff and groundwater and enhance eutrophication. Runoff, soil loss, soluble P (SP), and bioavailable P (BP) associated with runoff and soil loss were measured from six 2 h simulated rainfall events (1 in./h) to determine amount of P transported during a corn (Zea mays L.) growing season and to develop relationships that predict P amounts and forms leaving different size field plots. Two 60-sq-ft microplots and two 6500-sq-ft mesoplots (3% slope) were located on a Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudult). Phosphorus was broadcast at 31 Ib P/acre, then incorporated to a depth of 6 in. Corn (conventional tillage) was planted after fertilizer application. Six simulated rainfall events occurred 5 d before and 1, 14, 29, 49, and 108 d after fertilizer application [...]

The relationship between rainsplash and three measures of soil strength was investigated for surface soil seals using a laboratory rainfall simulator and erosion pans. A significantly high correlation was found between pocket penetrometer and torvane strengths ( R 2 = 0.49) , and a lower correlation between both pocket penetrometer ( R 2 = 0.24) and torvane ( R 2 = 0.24) strengths and the fall-cone strength was found. Rainsplash was highly correlated ( R 2 = 0.49) with rainfall kinetic energy (KE) divided by the fall-cone shear strength (FC) but was not related to penetrometer and torvane strengths. Addition of a silt content term to the KE/FC linear equation slightly decreased R2. Improvements in the prediction of rainsplash can be achieved by including terms defining the macro- and microtopography, such as slope steepness, surface roughness, and depressional storage.

Potential pesticide loss in soil percolate is influenced by pesticide persistence and sorption by soil constituents (organic matter). Pesticide persistence, expressed as half-life (ti/2), changes with soil depth as microbial activity and soil properties change. Little is known, however, how these changes influence potential pesticide transport out of the root zone. Objectives of this study were to investigate relative differences in potential pesticide losses from the root zone by percolation due to 1) different soil surface and subsurface textures and pesticide ti/2» and 2) interactions between pesticide ti/2 and timing of rainfall after pesticide application. The GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model and a 50-year historical rainfall record at Tifton, Georgia, were used to simulate pesticide losses by percolation from three soils ranging in surface texture from sand to sandy clay loam. Hypothetical pesticides had surface ti/2 of 5, 15, 30, and 60 d and a range of subsurface ti/2 (2.5-360 d), and were applied to continuous com {Zea maize, L.) at 2 kg ha~ ^ as surface spray at planting each year on 1 April. Simulated pesticide losses by percolation increased with increased surface and subsuiface t|/2» and decreased with increased KQC (adsorption constant based on soil organic matter) values. Potential pesticide leaching was greatest for Lakeland sand and least for Greenville sandy clay loam. Rainfall timing affected simulated pesticide loss by percolation, especially for nonpersistent pesticides. For short pesticide ti/2 (0-5 d), excessive rainfall events within 1 ti/2 were largely responsible for simulated pesticide loss by percolation. Results indicate that changes in pesticide t|/2 in surface and subsurface horizons of different soils influence potential pesticide leaching from the root zone, and models (i.e., GLEAMS) can be used to provide comparative analysis of soil-pesticide-climate interactions. For example, depending on soil type and pesticide K^^ and surface Xyp^ values, potential leaching losses increased two to seven times as subsurface tiy2 increased six times.