Allen L. Thompson

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.

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.

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.

PREDICTING RUNOFF AND SEDIMENT YIELD FROM A STIFF-STEMMED GRASS HEDGE SYSTEM FOR A SMALL WATERSHED

Grass hedges planted at regular intervals on the landscape offer many opportunities to reduce runoff and sediment from leaving fields. Objectives of this study were (1) to evaluate the ability of the WEPP watershed model to simulate grass hedge system effects of sediment trapping (TE), bench terracing (BT), and variable effective soil hydraulic conductivity (HC) on simulated hillslope runoff and sediment yield, and (2) to model the effects of measured effective hydraulic conductivity (Keff) values from a grass hedge management system by comparing predicted runoff and sediment yield values to those measured in a small watershed over an 11-year period. The study was conducted on a 6.6 ha watershed located in the deep loess hills region of southwestern Iowa. Narrow grass hedges of predominantly switchgrass (Panicum virgatum) were planted at 15.4 m intervals in 1991. The WEPP model simulated greater reductions in runoff (9%) and sediment yield (58%) from BT compared to TE and HC effects. Combination of all three effects gave the highest reductions in runoff (22%) and sediment yield (79%) compared to individual effects or any combination of two effects. The watershed model did not adequately simulate slope length reduction effects from the grass hedges. Runoff (r 2 = 0.78) and sediment yield (r 2 = 0.75) were comparable to observed data when measured Keff values for grass hedge, row crop, and channel areas were used as input data. Measured Keff data from grass hedge, row crop, and channel areas should be used for improved runoff and sediment yield predictions.

Polyacrylamide efficacy for reducing soil erosion and runoff as influenced by slope

Anionic polyacrylamide (PAM) can reduce soil erosion. Slope is an important factor determining erosion rate; however, PAM guidelines have not been well developed for different slopes. The objective of this study was to evaluate the extent which 20 kg ha−1 (18 lb ac−1) PAM (20P) and 40 kg ha−1 (36 lb ac−1) PAM (40P) increase the time to initial runoff (TRO), decrease cumulative runoff (RO), and decrease cumulative sediment loss (SL) on a Mexico silt loam soil adjusted to slopes of 10%, 20%, and 40%. 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) and were 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) for 1 h. Differences in TRO and SL for all slopes and PAM rates were all highly significant, as were all two-way interactions (p < 0.01). Applications of 20P and 40P significantly increased TRO compared to the untreated control (0P) at slopes 20% and greater but not at a 10% slope. No difference in TRO was found for soils at slopes up to 20% between 20P and 40P. The TRO was significantly greater at 40% slope for the 40P compared to the 20P application. Values of TRO with 20P and 40P were not increased at a 10% slope compared to 0P. Slope was not a factor in reducing RO. Applications of 40P for all slopes had significantly greater RO than either 0P or 20P. Polyacrylamide reduced SL for all slopes. A higher rate of PAM (40P) had less SL than a lower rate of PAM (20P) at slopes of 20% and 40%. With 40P, the reduction in SL was 72% greater than 0P at 40% slope. Therefore, slope is a critical factor in determining a PAM rate for reducing soil erosion and should be considered for proper application.

Phosphate Treatment of Lead-Contaminated Soil: Effects on Water Quality, Plant Uptake, and Lead Speciation.

Water quality threats associated with using phosphate-based amendments to remediate Pb-contaminated soils are a concern, particularly in riparian areas. This study investigated the effects of P application rates to a Pb-contaminated alluvial soil on Pb and P loss via surface water runoff, Pb accumulation in tall fescue ( Schreb; Kentucky 31), and Pb speciation. An alluvial soil was treated with triple superphosphate at P to Pb molar ratios of 0:1 (control), 4:1, 8:1, and 16:1. After a 6-mo reaction period, rainfall simulation (RFS) studies were conducted, followed by tall fescue establishment and a second set of RFS studies (1 yr after treatment). Results from the first RFS study (unvegetated) demonstrated that the total Pb and P concentrations in the effluents of 8:1 and 16:1 (P:Pb molar ratio) treatment levels were significantly greater ( < 0.05) than the control. One year after P treatment and 6 mo after vegetation establishment, total P and Pb concentrations of the effluents from a second RFS decreased by one to three orders of magnitude. Total and dissolved P concentration in runoff from the 16:1 P:Pb treatment remained significantly greater than all other treatments. However, total Pb concentration in the runoff was comparable among the treatments. Phosphorus treatment also reduced Pb uptake into tall fescue by >55%. X-ray absorption near-edge structure spectroscopy data showed that pyromorphite [Pb(PO)OH,Cl,F] abundance ranged from 0% (control) to 32% (16:1 P:Pb; 1 yr after treatment) of the total soil Pb. Although P treatment stimulated pyromorphite formation, pyromorphite abundance was comparable between the P-treated soils. These findings suggest that a 4:1 (P:Pb molar ratio) P treatment may be a sufficient means of reducing Pb bioavailability while minimizing concerns related to P loss in an alluvial setting.

Irrigation Scheduling Based on Crop Canopy Temperature for Humid Environments

The use of infrared thermometers (IR) to measure canopy temperatures for irrigation scheduling has been successfully applied in arid environments. Functionality of this technique in humid areas has been limited due to the presence of low vapor pressure deficits (VPD) and intermittent cloud cover. This study evaluated an alternate scheduling method for humid environments based on comparing measured canopy temperature with calculated canopy temperature of a well-watered crop. Irrigation was applied when the measured canopy temperature was greater than the predicted canopy temperature for more than three consecutive hours on two consecutive days. This method was evaluated against well-watered, semi-stressed, and dryland treatments of corn, soybean, and cotton on the basis of yield, irrigation amount, and irrigation water use efficiency (IWUE). Canopy temperature was underpredicted when the VPD was greater than 2 kPa. Limiting data to conditions when the solar radiation was greater than 200 W m-2 and the Richardson number was less than 0.2 resulted in very good prediction of canopy temperatures for cotton and soybean, particularly in the later growing period, but corn temperatures were consistently underpredicted. Although soybean and cotton yields were not significantly different across treatments, IWUE was improved for corn and cotton by use of this technique. Corn yield was greater for the well-watered crop, but the IR method resulted in 85% of the maximum yield while requiring less than 50% of the irrigation water. Results from this study suggest that the threshold temperature may be up to 1°C greater for corn and soybean and up to 0.5°C greater for cotton for humid compared to arid environments. This method shows potential as a tool for irrigation scheduling in humid environments. Further work is suggested to determine if conditions of excessive cloud cover and high VPD can be better accommodated, and to refine the threshold temperatures for corn, soybean, and cotton for humid environments.

Evaluating BMPs in a Claypan Watershed

The United States Department of Agriculture Conservation Effects Assessment Project (CEAP) was initiated to quantify the benefits of conservation practices on a national scale. This paper will address water quality benefits from conservation practices implemented at the field level using measured water quality data from the Goodwater Creek watershed in the claypan area of north central Missouri. Eleven years of hydrologic and climatic data from 1993-2003 were analyzed to identify trends and possible effects of best management practices (BMPs), including grassed waterways and terraces, on atrazine stream loadings and concentrations. During this period, area implemented with BMPs increased by 12%. Trends were identified using regression models. Over the time period, atrazine levels have been affected by drier, warmer springs, and increased no-till practices causing earlier application of atrazine, increased levels of atrazine in April, and decreased levels as the season progressed. Atrazine loading was reduced significantly over the entire eleven year time period (P<0.10). Over the months of April, May, and June significant decreases were detected for atrazine concentrations (P<0.05). Data at this time does not attribute the reduction of atrazine to BMPs. SWAT modeling is planned to determine if the SWAT model is sensitive enough to produce data that follow the statistical trends found in this study and to determine what level of BMP implementation would be required to see greater decreases in atrazine.

Polyacrylamide Influence on Runoff and Sediment Detachment from a Silt Loam Soil

Alternative erosion control practices using polyacrylamide (PAM) have received increased attention. This study examines the effect of PAM applied to soil when rainfall intensity, rainfall kinetic energy (KE), and PAM applied all interact, influencing time to runoff (ROt), cumulative runoff (ROc), and cumulative soil detachment (Dc). This study evaluated the response of a silt loam soil to aqueous PAM rates of 0 (untreated control), 20, and 40 kg ha-1 when applied to an initially air-dried soil. A total of 0.5 L of reverse osmosis water was sprayed on all soil beds. Simulated rainfall using a gravity-fed rainfall simulator was applied from 13.8 and 0.8 m fall heights at application rates of 64, 96, and 128 mm h-1. Results show significant benefit, as evidenced by increased ROt and reduced ROc and Dc for 20 and 40 kg ha-1 PAM application levels. This was true for all rain rates and durations up to 75 min whether rainfall was at or less than terminal velocity. Polyacrylamide was less effective at increasing ROt as rain rate increased. There was no difference in ROt with 40 kg ha-1 PAM compared to 20 kg ha-1 PAM; thus, the greater PAM application rate would not be recommended for this purpose. The benefits of reduced ROc and Dc with PAM were always greater for lower KE-rain (29% lower KE) than for higher KE-rain at a given rainfall intensity. Although both PAM application levels reduced ROc, for high KE-rain, only application of 40 kg ha-1 was significantly less than the untreated soil. For low KE-rain, ROc was significantly less with either PAM application level. Results show that PAM alone has a limited effect for maintaining infiltration rate; therefore, it is likely that regions with fewer intense convective storms would receive greater benefit from an increased PAM application rate. The greatest benefit of PAM was in reduced Dc where justification can be made for using increased PAM rates for high KE-rain, as documented by significantly less Dc for 40 kg ha-1 PAM compared to 20 kg ha-1 PAM at rain rates of 96 and 128 mm h-1. Under these conditions, both PAM rates significantly reduced Dc compared to untreated soil. With low KE-rain, Dc was significantly less for 20 kg ha-1 PAM compared to the untreated control, but 40 kg ha-1 PAM did not significantly reduce Dc since soil loss was so small.

Irrigation Scheduling Using Infrared Thermometers to Measure Canopy Temperature on Cotton and Soybean

The use of infrared thermometers to measure canopy temperatures for irrigation scheduling purposes has been successfully applied in arid environments, but has had complications in humid areas where the vapor pressure deficit is low and intermittent cloud cover occurs. In this study, four methods of irrigation scheduling were examined. These included: 1) well-watered; 2) dryland; 3) 50% of the amount applied to the well-watered treatment; and 4) irrigation based on cropcanopy temperatures. This last method compared the canopy temperature with a predicted temperature calculated from weather data collected at the site. Irrigation was triggered when the canopy temperature was above the predicted temperature for more than three consecutive hours for two consecutive days. The model under-predicted canopy temperatures in some conditions, but overall results indicate the potential for equal or higher yields with less water using this irrigation scheduling method.