E.J. Sadler

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.

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.

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.

Herbicide transport to surface runoff from a claypan soil: Scaling from plots to fields

Streams and drinking water reservoirs throughout the claypan soil region of Missouri and Illinois are particularly vulnerable to herbicide contamination from surface runoff during spring. This study follows a plot-scale study conducted on claypan soils to quantify and compare edge-of-field herbicide losses from a corn–soybean rotation under mulch tillage and no-tillage systems. The objectives of the present study were to confirm at field scale (34.4 ha [85 ac] and 7.8 ha [19.3 ac]) the plot-scale findings (0.37 ha [0.92 ac]) on the effects of tillage and herbicide incorporation on herbicide transport and to evaluate the applicability of plot-scale exponential models in calculating atrazine and metolachlor concentrations as a function of application rate, runoff volume, and days after application at the field scale. Herbicide transport to surface runoff was studied (1997 to 2001) from two fields with cropping systems similar to those on the plots. Field 1 (F1) was a mulch tillage corn–soybean rotation system with surface-applied herbicides, which are then incorporated. Field 2 (F2) was a no-tillage corn–soybean rotation system with surface-applied herbicides that were not incorporated. During each event, runoff volumes were measured, and water samples were collected and analyzed for atrazine and metolachlor concentrations. The percentages of applied atrazine and metolachlor transported to surface runoff from no-tillage (F2) were 3.2 and 2.0 times those from mulch tillage (F1), respectively. Throughout the study period, 1.0% and 3.2% of total atrazine and 1.0% and 2.0% of total metolachlor applied to F1 and F2 were lost to surface runoff, respectively. Similar to the results from the plot study, the model performed well in calculating field atrazine concentrations from both mulch and no-tillage systems with coefficient of determination ≥ 0.70 and Nash and Sutcliffe efficiency ≥ 0.64. However, model performance in calculating metolachlor concentrations was poor for both tillage systems (Nash and Sutcliffe efficiency < 0.35). When the model was modified to include cumulative temperature instead of days after application, performance in calculating atrazine and metolachlor concentrations was improved, particularly metolachlor concentrations at the field scale. The coefficient of determination and Nash and Sutcliffe efficiency values for metolachlor relative to cumulative temperature and days after application were 0.62 and 0.61 versus 0.41 and −0.13 for F1, and 0.73 and 0.55 versus 0.53 and 0.34 for F2, respectively. Overall, the study confirmed plot-scale results that atrazine concentrations and losses were greater for a no-tillage system than for a mulch-tillage system, in which the herbicide was incorporated. The study also showed that the model developed using plot-scale data was applicable in calculating concentrations at the field scale, particularly for atrazine.

Conservation effects on soil quality indicators in the Missouri Salt River Basin

The Conservation Effects Assessment Project (CEAP) was initiated in 2002 to quantify the potential benefits of conservation management practices throughout the nation. Within the Central Claypan Region of Missouri, the Salt River Basin was selected as a benchmark watershed for soil and water quality assessments. This study focuses on two objectives: (1) assessing soil quality for 15 different annual cropping and perennial vegetation systems typically employed in this region, and (2) evaluating relationships among multiple measured soil quality indicators (SQIs). Management practices included annual versus perennial vegetation, and varying grass species composition (cool-season versus warm-season), tillage intensity (no-till versus mulch-till), biomass removal, rotation phase, crop rotation (corn [Zea mays L.]–soybean [Glycine max L. Merr] versus corn–soybean–wheat [Triticum aestivum L.]) and incorporation of cover crops into the rotation. Soil samples were obtained in 2008 from 0 to 5 cm (0 to 2 in) and 5 to 15 cm (2 to 6 in) depth layers. Ten biological, physical, chemical, and nutrient SQIs were measured and scored using the Soil Management Assessment Framework (SMAF). Across SQIs, biological and physical indicators were the most sensitive to management effects, reflecting significant differences in organic carbon (C), mineralizable nitrogen (N), β-glucosidase, and bulk density. In the 0 to 5 cm layer, perennial systems demonstrated the greatest SMAF scores, ranging from 93% to 97% of the soils inherent potential. Scores for annual cropping systems ranged from 78% to 92%: diversified no-till, corn–soybean–wheat rotation with cover crops (92%) > no-till, corn–soybean rotation without cover crops (88%) > mulch-till corn–soybean rotation without cover crops (84%). Conversely, in the 5 to 15 cm layer, no-till cropping systems scored lower for overall soil function (58% to 61%) than mulch-till systems (65% to 66%). In the 0 to 5 cm layer, biological soil quality under the diversified no-till system with cover crops was 11% greater than under no-till without cover crops, and 20% greater than under mulch-till without cover crops. The effect of rotation phase was primarily reflected in 64% lower mineralizable N following corn relative to soybean. Additionally, soil nutrient function was significantly affected by biomass removal. The results of this study demonstrate that the benefits of conservation management practices extend beyond soil erosion reduction and improved water quality by highlighting the potential for enhanced soil quality, especially biological soil function. In particular, implementing conservation management practices on marginal and degraded soils in the claypan region can enhance long-term sustainability in annual cropping systems and working grasslands through improved soil quality.

WATERSHED-SCALE CROP TYPE CLASSIFICATION USING SEASONAL TRENDS IN REMOTE SENSING-DERIVED VEGETATION INDICES

Analysis and simulation of watershed-scale processes requires spatial characterization of land use, including differentiation among crop types. If this crop type information could be obtained accurately from remote sensing data, the effort required would be significantly reduced, especially for large watersheds. The objective of this study was to compare two methods using multiple satellite remote sensing datasets to differentiate land cover, including crop type, for the Salt River/Mark Twain Lake basin in northeast Missouri. Method 1 involved unsupervised classification of Landsat visible and near-infrared satellite images obtained at multiple dates in the growing season, followed by traditional, manual class identification. Method 2, developed in this research, employed the same unsupervised classification but also used normalized difference vegetation index (NDVI) maps obtained on a 16-day cycle from MODIS satellite images as ancillary data to derive seasonal NDVI trends for each class in the classification map. Tree analysis was applied to the NDVI trend data to group similar classes into clusters, and crop type for each cluster was determined from ground-truth data. Additional ground-truth data were used to assess the accuracy of the procedure, and crop acreage estimates were compared to county-level statistics. The overall classification accuracy of Method 2 was 3% higher than that of Method 1. Method 2 was also more efficient in terms of analyst time and ground-truth data requirements. Therefore, this method, employing variations in seasonal NDVI trends, is suggested for differentiation of crop type. The 30-m resolution crop type maps developed using this process will be useful as input data to environmental analysis models.

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.