Stephen H. Anderson

APEX Model Assessment of Variable Landscapes on Runoff and Dissolved Herbicides

Variability in soil landscapes and their associated properties can have significant effects on erosion and deposition processes that affect runoff and transport of pesticides and nutrients. Simulation models are one way in which the effects of landscapes on these processes can be assessed. This study evaluated the effects of variations in landscape position on runoff and dissolved atrazine using a calibrated APEX model. Fourteen agricultural plots (18 x 189 m2) in the Goodwater Creek watershed, a 7250 ha agricultural area in north-central Missouri, were simulated with the farm- and field-scale Agricultural Policy/Environmental eXtender (APEX) model. Plots were under three different tillage and herbicide management systems for three grain crop production systems. Each plot contained three landscape positions: summit, backslope, and footslope along with two transition zones. Runoff was measured and samples were collected from 1997 to 2002 during the corn year of the crop rotations. Runoff samples were analyzed for dissolved atrazine. The APEX model was calibrated and validated with event data from each plot during the corn growing years from 1997 to 1999 and 2000 to 2002, respectively. APEX reasonably simulated runoff and dissolved atrazine concentrations with annual coefficients of determination (r2) values ranging from 0.60 to 0.98 and 0.52 to 0.97, and Nash-Sutcliffe efficiency (NSE) values ranging from 0.46 to 0.94 and 0.45 to 0.86 for calibration and validation, respectively. The calibrated model was then used to simulate variable sequencing of landscape positions and associated soil properties as well as variable lengths of landscape positions. Simulated results indicate that as the length of the backslope increased while the steepness remained constant, so did the volume of runoff discharged and the atrazine concentrations at the plot outlet. In addition, the highest level of simulated runoff occurred when the backslope position was located adjacent to the outlet. Results from this study will be helpful to managers in placement of conservation practices on sensitive landscapes for improvement in water quality.

Comparison of concentrated-flow detachment equations for low shear stress

Abstract Several equations exist to describe the relationship between concentrated-flow detachment and shear stress ( τ ). However, the advantages and disadvantages of these equations for specific circumstances remain unclear. This study examines the performance of linear and power equations with and without a critical shear stress ( τ c ) term for concentrated-flow detachment at low shear stress. Equations were fit to data collected from experiments on five midwestern US soils using flume experimental data at low shear stress levels. Field experimental data were also available for these soils. The linear equation was simple to use and parameter values were easily estimated with linear regression. However, significant lack of fit was found when the linear equation was applied to data collected from low to medium shear stress levels. The value of soil erodibility ( K ) for a soil varied by a factor of 3 and critical shear stress ( τ c ) varied by a factor of 2.5. The linear equation prediction underestimated detachment ( D ) by 25% at high shear stress and overestimated detachment by 30% at low shear stress. In contrast, the power equations gave more stable erodibility parameters because these equations reduced the systematic nature of the observation residuals found with the linear equation. Correlation between rill detachment D and τ was generally lower with the linear compared to the power equations for conditions tested. For higher shear stresses and longer slopes, the linear equation may be acceptable where field experiments show a linear trend. It is suggested that τ c only be used when it has a value significantly different from zero.

Soil hydraulic properties influenced by agroforestry and grass buffers for grazed pasture systems

Agroforestry buffers have been introduced in temperate areas to improve water quality and diversify farm income. The objective of this study was to evaluate saturated hydraulic conductivity and water retention for soils managed under rotationally grazed (RG) pasture, continuously grazed (CG) pasture, grass buffers (GB), and agroforestry buffers (AgB). Pasture and GB areas included red clover (Trifolium pretense L.) and lespedeza (Kummerowia stipulacea Maxim.) planted into fescue (Festuca arundinacea Schreb.) while AgB included Eastern cottonwood trees (Populus deltoids Bortr. ex Marsh.) planted into fescue. Water retention data were measured at -0.4, -1.0, -2.5, -5.0, -10, -20, and -30 kPa (-0.058, -0.145, -0.363, -0.725, -1.45, -2.90, -4.35 pounds per square inch [psi]) soil water pressures using 76 mm (3.00 in) diameter by 76 mm long cores from the 0 to 10 cm, 10 to 20 cm, 20 to 30 cm, and 30 to 40 cm (0 to 3.94 in, 3.94 to 7.87 in, 7.87 to 11.81 in, and 11.81 to 15.75 in) depths. Soil bulk density was 12.6% higher for the RG and CG treatments (1.41 and 1.45 g cm-3 [88.02 and 90.52 lb ft-3]) than the GB and AgB treatments (1.25 and 1.29 g cm-3 [78.03 and 80.53 lb ft-3). Soil water content at high soil water potentials (0 and -0.4 kPa) was greater in the buffer treatments relative to the other treatments for the 0 to 10 cm soil depth. Soil macroporosity (>1,000 µm [>0.0394 in] diameter) was 5.7, 4.5, and 3.9 times higher, respectively, for the AgB, GB, and RG treatments compared to the CG treatment for the 0 to 10 cm soil depth. Buffer treatments had greater macroporosity (>1,000 µm diameter), coarse (60 to 1,000 µm [0.00236 to 0.0394 in] diameter) and fine mesoporosity (10 to 60 µm [0.000394 to 0.00236 in] diameter), but lower microporosity (<10 µm diameter) compared to RG and CG treatments. Saturated hydraulic conductivity values for GB and AgB treatments were 16.7 times higher (56.95 vs. 61.33 mm h-1 [2.24 vs. 2.41 in hr-1]) compared with RG and CG (3.98 vs. 3.11 mm h-1 [0.157 vs. 0.122 in hr-1]). This study illustrates that agroforestry and grass buffers maintained higher values for soil hydraulic properties compared to grazed pasture systems.

Influence of aggregate size on solute transport as measured using computed tomography

Abstract Application of computed tomography (CT) using selected tracers for in-situ monitoring of solute transport in soil cores is useful for identifying the influence of soil structure on solute movement. The objective of this study was to apply X-ray CT techniques for measurement of solute breakthrough curves for selected soil aggregates and to determine the two-dimensional velocity distributions within soil cores. Soil material from the A horizon of a Mexico silt loam (Udollic Ochraqualf) was brought to the laboratory, passed through selected sieves, and packed in soil cores (76 mm × 76 mm). Solute breakthrough experiments were conducted using a 1 % KI solution with the cores held in an apparatus used to control the bulk fluid velocity. Shapes of average breakthrough curves measured using CT generally compared well with curves measured from effluent samples. Aggregate sizes of 0.50–1.00 and 1.00–2.00 mm gave dispersion coefficients of 0.37 and 1.13 mm 2 /s, respectively, using a pore water velocity of 0.11 mm/s. Aggregate sizes of 0.25–0.50 and 1.00–2.00 mm gave dispersion coefficients of 0.042 and 0.123 mm 2 /s, respectively, using a pore water velocity of 0.05 mm/s. Techniques were developed for calculating the pore water velocity distribution as a function of location within the soil core. Velocities can be calculated at approximately 17,000 locations within a 76-mm diameter soil core, thus achieving a level of detail not previously obtained.

Overview of the Mark Twain Lake/ Salt River Basin Conservation Effects Assessment Project

The Mark Twain Lake/Salt River Basin was selected as one of the USDA Agricultural Research Service benchmark watersheds for the Conservation Effects Assessment Project because of documented soil and water quality problems and broad stakeholder interest. The basin is located in northeastern Missouri within the Central Claypan Region, and it is the source of water to Mark Twain Lake, the major public water supply in the region. At the outlet to Mark Twain Lake, the basin drains 6,417 km2 (2,478 mi2), including 10 major watersheds that range in area from 271 to 1,579 km2 (105 to 609 mi2). The basin is characterized by flat to gently rolling topography with a predominance of claypan soils that result in high runoff potential. The claypan soils are especially vulnerable to soil erosion, which has degraded soil and water quality throughout the basin, and to surface transport of herbicides. Results from cropping system best management practice studies showed that no-till cropping systems did not reduce surface runoff compared to tilled systems, and no-till led to increased transport of soil-applied herbicides. A major challenge is the need to develop cropping systems that incorporate herbicides yet maintain sufficient crop residue cover to control soil erosion. Results of the Soil and Water Assessment Tool model simulations showed that the model was capable of simulating observed long-term trends in atrazine concentrations and loads and the impact of grass waterways on atrazine concentrations. Current and future research efforts will continue to focus on best management practice studies, development of needed tools to improve watershed management, and refinements in the calibration and validation of the Soil and Water Assessment Tool model.

Estimation of porosity and hydraulic conductivity from X-ray CT-measured solute breakthrough

Abstract Heterogeneities are common in natural porous media and are present on different scales. Use of X-ray computed tomography (CT) may provide a tool for quantifying smallscale heterogeneities in porosity and hydraulic conductivity in porous media. Porosity and saturated hydraulic conductivity distributions were estimated using CT for a series of undisturbed soil core samples taken from a field site. CT measurements were collected during breakthrough experiments using an iodide tracer. Techniques were developed to estimate porosity and hydraulic conductivity from solute breakthrough data. Results were compared with bulk sample measurements. CT-measured porosity compared well with laboratory-measured porosity. Hydraulic conductivity estimated from CT methods slightly overestimated laboratory-measured values. These techniques provide a method to quantify the spatially variable porosity and hydraulic conductivity on a millimetre scale rather than on a core-averaged scale. Chemical transport through the soil was predicted using a finite element method for each core using the CT-measured soil properties. Comparisons between measured and predicted chemical transport suggest that small-scale heterogeneities cause departures between measured and simulated solute breakthrough curves, and that a smaller grid size may be needed to improve the simulation.

APEX model assessment of variable landscapes on runoff and dissolved herbicides.

Variability in soil landscapes and their associated properties can have significant effects on erosion and deposition processes that affect runoff and transport of pesticides. Simulation models are one way in which the effects of landscapes on these processes can be assessed. This simulation study evaluated the effects of variations in landscape position on runoff and dissolved atrazine utilizing a calibrated farm- and field-scale Agricultural Policy/Environmental eXtender (APEX) model. Twelve agricultural plots (18 m × 189 m) in the Goodwater Creek watershed, a 7250 ha agricultural area in north-central Missouri, were simulated. Plots were treated with three tillage and herbicide management systems for two grain crop rotations. Each plot contained three landscape positions (summit, backslope, and footslope) along with two transition zones. Runoff was measured and samples were collected from 1997 to 2002 during the corn year of the crop rotations. Runoff samples were analyzed for dissolved atrazine. The model was calibrated and validated for each plot with event data from 1997 to 1999 and from 2000 to 2002, respectively. APEX reasonably simulated runoff and dissolved atrazine concentrations, with coefficients of determination (r2) values ranging from 0.52 to 0.98 and from 0.52 to 0.97, and Nash-Sutcliffe efficiency (NSE) values ranging from 0.46 to 0.94 and from 0.45 to 0.86 for calibration and validation, respectively. The calibrated model was then used to simulate variable sequencing of landscape positions and associated soil properties as well as variable lengths of landscape positions. Simulated results indicated that the runoff and the atrazine load at the plot outlet increased when the backslope length increased while keeping the steepness constant. The maximum simulated runoff among different sequences of landscape positions occurred when the backslope position was located adjacent to the outlet. Results from this study will be helpful to managers in placement of conservation practices on sensitive landscapes for improvement in water quality.

Effects of long-term soil and crop management on soil hydraulic properties for claypan soils

Various land management decisions are based on local soil properties. These soil properties include average values from soil characterization for each soil series. In reality, these properties might be variable due to substantially different management, even for similar soil series. This study was conducted to test the hypothesis that for claypan soils, hydraulic properties can be significantly affected by long-term soil and crop management. Sampling was conducted during the summer of 2008 from two fields with Mexico silt loam (Vertic Epiaqualfs). One field has been under continuous row crop cultivation for over 100 years (Field), while the other field is a native prairie that has never been tilled (Tucker Prairie). Soil cores (76 × 76 mm [3.0 × 3.0 in]) from six replicate locations from each field were sampled to a 60 cm (24 in) depth at 10 cm (3.9 in) intervals. Samples were analyzed for bulk density, saturated hydraulic conductivity (Ksat), soil water retention, and pore-size distributions. Values of coarse (60 to 1,000 μm [0.0024 to 0.039 in] effective diameter) and fine mesoporosity (10 to 60 μm [0.00039 to 0.0024 in] effective diameter) for the Field site (0.044 and 0.053 m3 m−3 [0.044 and 0.053 in3 in−3]) were almost half those values from the Tucker Prairie site (0.081 and 0.086 m3 m−3 [0.081 and 0.086 in3 in−3]). The geometric mean value of Ksat was 57 times higher in the native prairie site (316 mm h−1 [12.4 in hr−1]) than in the cropped field (5.55 mm h−1 [0.219 in hr−1]) for the first 10 cm (3.9 in) interval. Differences in Ksat values were partly explained by the significant differences in pore-size distributions. The bulk density of the surface layer at the Tucker Prairie site (0.81 g cm−3 [50.6 lb ft−3]) was two-thirds of the value at the Field site (1.44 g cm−3 [89.9 lb ft−3]), and was significantly different throughout the soil profile, except for the 20 to 30 cm (7.9 to 12 in) depth. These results show that row crop management and its effect on soil loss have significantly altered the hydraulic properties for this soil. Results from this study increase our understanding of the effects of long-term soil management on soil hydraulic properties.

Polyacrylamide and gypsum amendments for erosion and runoff control on two soil series

Application of polyacrylamide (PAM), gypsum, or their combination generally decreases erosion and runoff. However, their benefits are uncertain for soils with varying properties. The objectives of this study were to evaluate the effects of 5 Mg ha−1 (2 tn ac−1) gypsum (5G), 20 kg ha−1 (18 lb ac−1) PAM (20P), 40 kg ha−1 (36 lb ac−1) PAM (40P), and 20 kg ha−1 (18 lb ac−1) PAM with 5 Mg ha−1 gypsum (20P+5G) for increasing time to initial runoff (TRO), decreasing cumulative runoff (RO), and decreasing cumulative sediment loss (SL) after 1 hour of a simulated rainfall on soil samples from two soil series (Hoberg and Brussels). Soils were packed to a bulk density of 1.3 Mg m−3 (81 lb ft−3) in test beds 0.3 × 0.3 × 0.15 m (12 × 12 × 6 in) set to a slope of 20% and subjected to a 61 mm h−1 (2.4 in hr−1) simulated rainfall with a kinetic energy of 1.5 kJ m−2 h−1 (103 ft lb ft−2 hr−1). Differences in TRO, RO, and SL for four soil samples (two sample depths per soil series) and four amendments plus an unamended check were all significantly different (p < 0.01). Amendments of 20P, 40P, and 20P+5G increased TRO for soil samples with ≤0.5% organic matter (OM). The 5G amendment increased TRO for an acid soil sample (pH 4.1) with low OM (0.2%). The 20P+5G amendment produced an average 25% reduction in RO. Other amendments reduced RO by an average of 9% to 10%. The 40P amendment did not reduce RO, except for a Brussels silt loam surface soil that showed a 41% decrease. The 20P+5G amendment reduced SL by an average of 47% for soil samples, except for a high OM (3.7%) soil sample where OM likely interfered with soil-PAM bonding. The order of the amendment effectiveness for increasing TRO, and reducing RO and SL was 20P+5G > 40P > 20P > 5G. Generally, the 20P+5G amendment was the best, irrespective of soil calcium (Ca++) content. On average, this amendment increased TRO by 69%, decreased RO by 25%, and decreased SL by 36%. When this amendment was used on an acid soil sample with low OM (0.2%) and low cation exchange capacity (9.2 cmolc kg−1), it increased TRO by 71% and reduced RO and SL by 45% and 74%. The amendment effectiveness was influenced by soil properties, including texture, clay mineralogy, cation exchange capacity, and OM.

Atrazine and Alachlor Transport in Claypan Soils as Influenced by Differential Antecedent Soil Water Content

Increased attention to ground water contamination has encouraged an interest in mechanisms of solute transport through soils. Few studies have investigated the effect of the initial soil water content on the transport and degradation of herbicides for claypan soils. We investigated the effect of claypan soils at initial field capacity vs. permanent wilting level on atrazine and alachlor transport. The soil studied was Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualf) with a subsoil clay content, primarily montmorillonite, of >40%. Strontium bromide, atrazine, and alachlor were applied to plots; half were at field capacity (Wet treatment), and half were near the permanent wilting point (Dry treatment). Soil cores were removed at selected depths and times, and cores were analyzed for bromide and herbicide concentrations. Bromide, atrazine, and alachlor were detected at the 0.90-m depth in dry plots within 15 d after experiment initiation. Bromide was detected 0.15 m deeper (P < 0.05) in the Dry compared with the Wet treatment at 1, 7, and 60 d after application and >0.30 m deeper (P < 0.01) in the Dry treatment at 15 and 30 d after application; similar treatment results were found for atrazine and alachlor, although on fewer dates with significant differences. The mobility order of the applied chemicals was bromide > atrazine > alachlor. The atrazine apparent half-life was significantly longer in the Dry plots compared with the Wet plots. The retardation factor determined from the relative velocity of each herbicide to that of bromide was higher for alachlor than for atrazine. This study identifies the impact that shrinkage cracks have for different moisture conditions on preferential transport of herbicides in claypan soils.