Field Trial Assessment of Biological,Chemical, and Physical Responses of Soil to Tillage Intensity,Fertilization, and Grazing

Soil microbial populations can fluctuate in response to environmental changes and, therefore, are often used as biological indicators of soil quality. Soil chemical and physical parameters can also be used as indicators because they can vary in response to different management strategies. A long-term field trial was conducted to study the effects of different tillage systems (NT: no tillage, DH: disc harrow, and MP: moldboard plough), P fertilization (diammonium phosphate), and cattle grazing (in terms of crop residue consumption) in maize (Zea mays L.), sunflower (Heliantus annuus L.), and soybean (Glycine max L.) on soil biological, chemical, and physical parameters. The field trial was conducted for four crop years (2000/2001, 2001/2002, 2002/2003, and 2003/2004). Soil populations of Actinomycetes, Trichoderma spp., and Gliocladium spp. were 49% higher under conservation tillage systems, in soil amended with diammonium phosphate (DAP) and not previously grazed. Management practices also influenced soil chemical parameters, especially organic matter content and total N, which were 10% and 55% higher under NT than under MP. Aggregate stability was 61% higher in NT than in MP, 15% higher in P-fertilized soil, and also 9% higher in not grazed strips, bulk density being 12% lower in NT systems compared with MP. DAP application and the absence of grazing also reduced bulk density (3%). Using conservation tillage systems, fertilizing crops with DAP, and avoiding grazing contribute to soil health preservation and enhanced crop production.     

Phosphorus runoff: effect of tillage and soil phosphorus levels

Continued inputs of fertilizer and manure in excess of crop requirements have led to a build-up of soil phosphorus (P) levels and increased P runoff from agricultural soils. The objectives of this study were to determine the effects of two tillage practices (no-till and chisel plow) and a range of soil P levels on the concentration and loads of dissolved reactive phosphorus (DRP), algal-available phosphorus (AAP), and total phosphorus (TP) losses in runoff, and to evaluate the P loss immediately following tillage in the fall, and after six months, in the spring. Rain simulations were conducted on a Typic Argiudoll under a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Elapsed time after tillage (fall vs. spring) was not related to any form of P in runoff. No-till runoff averaged 0.40 mg L(-1) and 0.05 kg ha(-1) DRP and chisel-plow plots averaged 0.24 mg L(-1) and 0.02 kg ha(-1) DRP concentration and loads, respectively. The relationship between DRP and Bray P1 extraction values was approximated by a logistic function (S-shaped curve) for no-till plots and by a linear function for tilled plots. No significant differences were observed between tillage systems for TP and AAP in runoff. Bray P1 soil extraction values and sediment concentration in runoff were significantly related to the concentrations and amounts of AAP and TP in runoff. These results suggest that soil Bray P1 extraction values and runoff sediment concentration are two easily measured variables for adequate prediction of P runoff from agricultural fields.     

Climatic and agricultural factors in nutrient exports from two watersheds in Ohio

Export of agricultural nutrients and sediment to lakes and oceans is of great environmental concern in many agricultural watersheds. Recent years have seen efforts to reduce loads through agricultural practices such as conservation tillage, efficient fertilization, and reservation of erodible areas. Monitoring the efficacy of such efforts is complicated by the fact they take place against a varying climatic and hydrologic background. In this study, statistical analysis was used to identify those climatic, hydrologic, and agricultural variables that best explained variations in nitrate, phosphorus, and total suspended solids over the period 1976-1995 in two large agricultural watersheds that feed Lake Erie, those of the Maumee and Sandusky Rivers. The dominant variable was stream discharge; after curvefits to remove its influence, the residual loads were tested via stepwise linear regression to reveal the most significant explanatory variables. Loads of nitrate, total suspended solids, and total phosphorus tended to decrease when previous months were wet, except in the summer, and to decrease when snow cover was extensive. It is speculated that stores of nitrate in the soil were lost during wet periods through increased crop uptake and/or leaching. Nitrogen fertilizer application in the Maumee watershed decreased following dry periods, but not enough to decrease stream loads. Soluble reactive phosphorus loads were negatively correlated to conservation tillage and reserves, and positively correlated to fertilizer and manure sources. Results for total phosphorus were similar to those for total suspended solids, on which most transported phosphorus is adsorbed.     

MODELING SEDIMENT AND PHOSPHORUS LOSSES IN AN AGRICULTURAL WATERSHED TO MEET TMDLs1

ABSTRACT: This paper studies the effectiveness of alternative farm management strategies at improving water quality to meet Total Maximum Daily Loads (TMDLs) in agricultural watersheds. A spatial process model was calibrated using monthly flow, sediment, and phosphorus (P) losses (1994 to 1996) from Sand Creek watershed in south‐central Minnesota. Statistical evaluation of predicted and observed data gave r² coefficients of 0.75, 0.69, and 0.49 for flow (average 4.1 m³/s), sediment load (average 0.44 ton/ha), and phosphorus load (average 0.97 kg/ha), respectively. The calibrated model was used to evaluate the effects of conservation tillage, conversion of crop land to pasture, and changes in phosphorus fertilizer application rate on pollutant loads. TMDLs were developed for sediment and P losses based on existing water quality standards and guidelines. Observed annual sediment and P losses exceeded these TMDLs by 59 percent and 83 percent, respectively. A combination of increased conservation tillage, reduced application rates of phosphorus fertilizer, and conversion of crop land to pasture could reduce sediment and phosphorus loads by 23 percent and 20 percent of existing loads, respectively. These reductions are much less than needed to meet TMDLs, suggesting that control of sediment using buffer strips and control of point sources of phosphorus are needed for the remaining reductions.     

Evaluating Conservation Program Success with Landsat and SWAT

In the United States, many state and federally funded conservation programs are required to quantify the water quality benefits resulting from their efforts. The objective of this research was to evaluate the impact of conservation practices subsidized by the Oklahoma Conservation Commission on phosphorus and sediment loads to Lake Wister. Conservation practices designed to increase vegetative cover in grazed pastures were evaluated using Landsat imagery and the Soil and Water Assessment Tool (SWAT). Several vegetative indices were derived from Landsat imagery captured before and after the implementation of conservation practices. Collectively, these indicators provided an estimate of the change in vegetative soil cover attributable to conservation practices in treated fields. Field characteristics, management, and changes in vegetative cover were used in the SWAT model to simulate sediment and phosphorus losses before and after practice implementation. Overall, these conservation practices yielded a 1.9% improvement in vegetative cover and a predicted sediment load reduction of 3.5%. Changes in phosphorus load ranged from a 1.0% improvement to a 3.5% increase, depending upon initial vegetative conditions. The use of fertilizers containing phosphorus as a conservation practice in low-productivity pastures was predicted by SWAT to increase net phosphorus losses despite any improvement in vegetative cover. This combination of vegetative cover analysis and hydrologic simulation was a useful tool for evaluating the effects of conservation practices at the basin scale and may provide guidance for the selection of conservation measures subsidized in future conservation programs.     

Spatial Optimization of Six Conservation Practices Using Swat in Tile‐Drained Agricultural Watersheds

Targeting of agricultural conservation practices to the most effective locations in a watershed can promote wise use of conservation funds to protect surface waters from agricultural nonpoint source pollution. A spatial optimization procedure using the Soil and Water Assessment Tool was used to target six widely used conservation practices, namely no‐tillage, cereal rye cover crops (CC), filter strips (FS), grassed waterways (GW), created wetlands, and restored prairie habitats, in two west‐central Indiana watersheds. These watersheds were small, fairly flat, extensively agricultural, and heavily subsurface tile‐drained. The targeting approach was also used to evaluate the model's representation of conservation practices in cost and water quality improvement, defined as export of total nitrogen, total phosphorus, and sediment from cropped fields. FS, GW, and habitats were the most effective at improving water quality, while CC and wetlands made the greatest water quality improvement in lands with multiple existing conservation practices. Spatial optimization resulted in similar cost‐environmental benefit tradeoff curves for each watershed, with the greatest possible water quality improvement being a reduction in total pollutant loads by approximately 60%, with nitrogen reduced by 20‐30%, phosphorus by 70%, and sediment by 80‐90%.     

OPTIMAL PROFITS UNDER ENVIRONMENTAL CONSTRAINTS: IMPLICATIONS OF NUTRIENT AND SEDIMENT STANDARDS1

To reduce nonpoint source pollution from nutrient, chemical, and sediment runoff, a number of environmental policy standards have been proposed. Such standards could be used to reduce nonpoint source pollution from nutrient, chemical, and sediment runoff to impaired water bodies. State governments can use voluntary approaches to meet nonpoint source pollution reduction goals. However, the practices that lower net returns will not be voluntarily adopted by farmers. Crop rotations and tillage practices may help producers to comply with the environmental standards while minimizing losses in farm profits. This study compares runoff from crop rotation practices and conventional continuous row cropping systems in Mississippi. The results are compared for different tillage systems in order to examine robustness of results. Nutrient runoff and sediment runoff are simulated using the Erosion Productivity Impact Calculator (EPIC). Sensitivity analysis of the sediment and nitrate reductions at 15 percent, 25 percent, and 35 percent are conducted. Under these scenarios, net returns are optimized under environmental constraints, and the marginal cost of sediment reduction ranges from US$1.61 to US$9.63 per ton depending on soil conditions, while the corresponding nitrate and phosphorus reductions costs range from US$1.21 to US$7.08 per kg and from US$0.09 to US$31.91, respectively. The empirical results from this study indicate that a nitrate reduction policy is relatively less costly than a sediment reduction policy. The results also demonstrate the importance of geophysical conditions and policy costs, which vary across regions.     

Effect of Nutrient Management Planning on Crop Yield, Nitrate Leaching and Sediment Loading in Thomas Brook Watershed

Government priorities on provincial Nutrient Management Planning (NMP) programs include improving the program effectiveness for environmental quality protection, and promoting more widespread adoption. Understanding the effect of NMP on both crop yield and key water-quality parameters in agricultural watersheds requires a comprehensive evaluation that takes into consideration important NMP attributes and location-specific farming conditions. This study applied the Soil and Water Assessment Tool (SWAT) to investigate the effects of crop and rotation sequence, tillage type, and nutrient N application rate on crop yield and the associated groundwater ${\text{NO}_{3}}^{ - } {\text{-N }}$ leaching and sediment loss. The SWAT model was applied to the Thomas Brook Watershed, located in the most intensively managed agricultural region of Nova Scotia, Canada. Cropping systems evaluated included seven fertilizer application rates and two tillage systems (i.e., conventional tillage and no-till). The analysis reflected cropping systems commonly managed by farmers in the Annapolis Valley region, including grain corn-based and potato-based cropping systems, and a vegetable-horticulture system. ANOVA models were developed and used to assess the effects of crop management choices on crop yield and two water-quality parameters (i.e., ${\text{NO}_{3}}^{ - } {\text{-N }}$ leaching and sediment loading). Results suggest that existing recommended N-fertilizer rate can be reduced by 10–25 %, for grain crop production, to significantly lower ${\text{NO}_{3}}^{ - } {\text{-N }}$ leaching (P > 0.05) while optimizing the crop yield. The analysis identified the nutrient N rates in combination with specific crops and rotation systems that can be used to manage ${\text{NO}_{3}}^{ - } {\text{-N }}$ leaching while balancing impacts on crop yields within the watershed.     

Nutrient and sediment losses under simulated rainfall following manure incorporation by different methods

Incorporation of manure into cultivated soils is generally recommended to minimize nutrient losses. A 3-yr study was conducted to evaluate sediment and nutrient losses with different tillage methods (moldboard plow, heavy-duty cultivator, double disk, and no-incorporation) for incorporation of beef cattle manure in a silage barley (Hordeum vulgare L.) cropping system. Runoff depths, sediment losses, and surface and subsurface nutrient transfers were determined from manured and unmanured field plots at Lethbridge, Alberta, Canada. A Guelph rainfall simulator was used to generate 30 min of runoff. Sediment losses among our tillage treatments (137.4-203.6 kg ha(-1)) were not significantly different due to compensating differences in runoff depths. Mass losses of total phosphorus (TP) and total nitrogen (TN) in surface runoff were greatest from the no-incorporation (NI) treatments, with reductions in TP loads of 14% for double disk (DD), 43% for cultivator (CU), and 79% for moldboard plow (MP) treatments. Total N load reductions in 2002 were 26% for DD, 70% for CU, and 95% for MP treatments compared to the NI treatments. Nutrient losses following incorporation of manure with the DD or CU methods were not significantly different from the NI treatments. Manure treatments generally had lower runoff depths and sediment losses, and higher phosphorus and nitrogen losses than the control treatments. Subsurface concentrations of NH4-N, NO3-N, and TN were greatest from the MP treatments, whereas subsurface phosphorus concentrations were not affected by tillage method. Tillage with a cultivator or double disk minimized combined surface and subsurface nutrient losses immediately after annual manure applications.     

Effects of conservation tillage on the performance of Lake Erie basin farms

This paper summarizes research that investigates the effects of alternative farming practices on the performance of Lake Erie basin farms. First, data from a representative panel of about 100 farmers is analyzed to determine how conservation tillage, rotations, and other factors affected farms' economic returns during 1987-1992. Statistical analysis of these data is unable to demonstrate that there is any significant relationship between farming system (i.e., tillage and rotation) variables and farm profitability. Next, a farm-level bioeconomic simulation model is used to analyze the effects of conservation tillage adoption on farm profitability, farm size, and pollutant emissions. Findings are that tillage system, farm size, and crop selection are determined jointly and may substantially improve economic performance of farms. Conservation tillage enables farms to be larger and more specialized, and as a result, farm profitability improves. Statistical analysis of farm panel data is unable to show the effect of tillage on profitability because it neglects to account for endogeneity of variables (or joint effects of tillage, size, crop selection, and performance) in production decisions.     

Using the soil and water assessment tool to estimate achievable water quality targets through implementation of beneficial management practices in an agricultural watershed

Runoff from crop production in agricultural watersheds can cause widespread soil loss and degradation of surface water quality. Beneficial management practices (BMPs) for soil conservation are often implemented as remedial measures because BMPs can reduce soil erosion and improve water quality. However, the efficacy of BMPs may be unknown because it can be affected by many factors, such as farming practices, land-use, soil type, topography, and climatic conditions. As such, it is difficult to estimate the impacts of BMPs on water quality through field experiments alone. In this research, the Soil and Water Assessment Tool was used to estimate achievable performance targets of water quality indicators (sediment and soluble P loadings) after implementation of combinations of selected BMPs in the Black Brook Watershed in northwestern New Brunswick, Canada. Four commonly used BMPs (flow diversion terraces [FDTs], fertilizer reductions, tillage methods, and crop rotations), were considered individually and in different combinations. At the watershed level, the best achievable sediment loading was 1.9 t ha(-1) yr(-1) (89% reduction compared with default scenario), with a BMP combination of crop rotation, FDT, and no-till. The best achievable soluble P loading was 0.5 kg ha(-1) yr(-1) (62% reduction), with a BMP combination of crop rotation and FDT and fertilizer reduction. Targets estimated through nonpoint source water quality modeling can be used to evaluate BMP implementation initiatives and provide milestones for the rehabilitation of streams and rivers in agricultural regions.     

Carbon sequestration in dryland soils and plant residue as influenced by tillage and crop rotation

Long-term use of conventional tillage and wheat (Triticum aestivum L.)-fallow systems in the northern Great Plains have resulted in low soil organic carbon (SOC) levels. We examined the effects of two tillage practices [conventional till (CT) and no-till (NT)], five crop rotations [continuous spring wheat (CW), spring wheat-fallow (W-F), spring wheat-lentil (Lens culinaris Medic.) (W-L), spring wheat-spring wheat-fallow (W-W-F), and spring wheat-pea (Pisum sativum L.)-fallow (W-P-F)], and Conservation Reserve Program (CRP) planting on plant C input, SOC, and particulate organic carbon (POC). A field experiment was conducted in a mixture of Scobey clay loam (fine-loamy, mixed, Aridic Argiborolls) and Kevin clay loam (fine, montmorillonitic, Aridic Argiborolls) from 1998 to 2003 in Havre, MT. Total plant biomass returned to the soil from 1998 to 2003 was greater in CW (15.5 Mg ha(-1)) than in other rotations. Residue cover, amount, and C content in 2004 were 33 to 86% greater in NT than in CT and greater in CRP than in crop rotations. Residue amount (2.47 Mg ha(-1)) and C content (0.96 Mg ha(-1)) were greater in NT with CW than in other treatments, except in CT with CRP and W-F and in NT with CRP and W-W-F. The SOC at the 0- to 5-cm depth was 23% greater in NT (6.4 Mg ha(-1)) than in CT. The POC was not influenced by tillage and crop rotation, but POC to SOC ratio at the 0- to 20-cm depth was greater in NT with W-L (369 g kg(-1) SOC) than in CT with CW, W-F, and W-L. From 1998 to 2003, SOC at the 0- to 20-cm depth decreased by 4% in CT but increased by 3% in NT. Carbon can be sequestered in dryland soils and plant residue in areas previously under CRP using reduced tillage and increased cropping intensity, such as NT with CW, compared with traditional practice, such as CT with W-F system, and the content can be similar to that in CRP planting.     

Net global warming potential and greenhouse gas intensity in irrigated cropping systems in northeastern Colorado

The impact of management on global warming potential (GWP), crop production, and greenhouse gas intensity (GHGI) in irrigated agriculture is not well documented. A no-till (NT) cropping systems study initiated in 1999 to evaluate soil organic carbon (SOC) sequestration potential in irrigated agriculture was used in this study to make trace gas flux measurements for 3 yr to facilitate a complete greenhouse gas accounting of GWP and GHGI. Fluxes of CO2, CH4, and N2O were measured using static, vented chambers, one to three times per week, year round, from April 2002 through October 2004 within conventional-till continuous corn (CT-CC) and NT continuous corn (NT-CC) plots and in NT corn-soybean rotation (NT-CB) plots. Nitrogen fertilizer rates ranged from 0 to 224 kg N ha(-1). Methane fluxes were small and did not differ between tillage systems. Nitrous oxide fluxes increased linearly with increasing N fertilizer rate each year, but emission rates varied with years. Carbon dioxide efflux was higher in CT compared to NT in 2002 but was not different by tillage in 2003 or 2004. Based on soil respiration and residue C inputs, NT soils were net sinks of GWP when adequate fertilizer was added to maintain crop production. The CT soils were smaller net sinks for GWP than NT soils. The determinant for the net GWP relationship was a balance between soil respiration and N2O emissions. Based on soil C sequestration, only NT soils were net sinks for GWP. Both estimates of GWP and GHGI indicate that when appropriate crop production levels are achieved, net CO2 emissions are reduced. The results suggest that economic viability and environmental conservation can be achieved by minimizing tillage and utilizing appropriate levels of fertilizer.     

Addressing data challenges in riverine nutrient load modeling of an intensively managed agro-industrial watershed

Data limitations often challenge the reliability of water quality models, especially in intensively managed watersheds. While numerous studies report successful hydrological model setup and calibration, few have addressed in detail the data challenges for multisite and multivariable model calibration to an intensively managed watershed. In this study, we address some of these challenges based on our reflective experience calibrating the Soil and Water Assessment Tool (SWAT) to the Upper Sangamon River Watershed in central Illinois based on daily flow, annual crop yield, and monthly sediment, nitrate, and total phosphorus loads. We highlight some challenges in SWAT calibration processes due to data errors and inconsistencies, and insufficient precipitation and water quality observations. Following, we demonstrate the merits of additional weather and water quality observations that could help reduce input uncertainties, and we provide suggestions for selecting appropriate observations for the model calibration. After dealing with the data issues, we show that the SWAT model could be calibrated with acceptable results for the case study watershed.     

Manure history and long-term tillage effects on soil properties and phosphorus losses in runoff

Manure additions to cropland can reduce total P losses in runoff on well-drained soils due to increased infiltration and reduced soil erosion. Surface residue management in subsequent years may influence the long-term risk of P losses as the manure-supplied organic matter decomposes. The effects of manure history and long-term (8-yr) tillage [chisel plow (CP) and no-till (NT)] on P levels in runoff in continuous corn (Zea mays L.) were investigated on well-drained silt loam soils of southern and southwestern Wisconsin. Soil P levels (0-15 cm) increased with the frequency of manure applications and P stratification was greater near the surface (0-5 cm) in NT than CP. In CP, soil test P level was linearly related to dissolved P (24-105 g ha(-1)) and bioavailable P (64-272 g ha(-1)) loads in runoff, but not total P (653-1893 g ha(-1)). In NT, P loads were reduced by an average of 57% for dissolved P, 70% for bioavailable P, and 91% for total P compared with CP. This reduction was due to lower sediment concentrations and/or lower runoff volumes in NT. There was no relationship between soil test P levels and runoff P concentrations or loads in NT. Long-term manure P applications in excess of P removal by corn in CP systems ultimately increased the potential for greater dissolved and bioavailable P losses in runoff by increasing soil P levels. Maintaining high surface residue cover such as those found in long-term NT corn production systems can mitigate this risk in addition to reducing sediment and particulate P losses.     

SURFACE WATER QUALITY IMPACTS OF TILLAGE PRACTICES UNDER LIQUID SWINE MANIJRE APPLICATION1

ABSTRACT: Field studies were conducted to investigate the effects of tillage practices on the saturated hydraulic conductivity, and quantity and quality of surface runoff water resulting from the application of the liquid swine manure as a fertilizer. As part of the study, infiltration experiments were conducted on silt-loam soil with no-tillage (NT) and disk tillage (DT) practices. Liquid swine manure was applied on test plots, and the rainfall was applied by the portable rainfall simulator. The infiltration data was analyzed for the saturated hydraulic conductivity (K8) and runoff volume determinations. The surface runoff water was analyzed for total N, total P, ammonia, and nitrate concentration determinations. The study indicated that the tillage had significant effects on K_s, and quantity and quality of runoff water. The K_s values of the NT plots were found to vary from 0.693 to 1.734 mm/min, with a mean of 1.494 mm/min, while they varied from 1.056 to 2.543 mm/min, with a mean of 2231 mm/mm in the DT plots. The total N, total P, ammonium nitrogen and nitrate nitrogen concentrations were lower in runoff generated from DT plots, compared to that from the NT plots. The chemical concentration levels were significantly different in runoff waters collected one-day after manure application than in those collected 40-days after the manure application. Study suggested that the DT practice must be preferred over the NT practice if liquid swine manure is used as the fertilizer. The study is further continued to assess the long-term impacts of swine manure application and tillage on the quantity and quality of surface runoff water.     

WATER QUALITY MODELING OF ALTERNATIVE AGRICULTURAL SCENARIOS IN THE U.S. CORN BELT1

ABSTRACT: Simulated water quality resulting from three alternative future land‐use scenarios for two agricultural watersheds in central Iowa was compared to water quality under current and historic land use/land cover to explore both the potential water quality impact of perpetuating current trends and potential benefits of major changes in agricultural practices in the U.S. Corn Belt. The Soil Water Assessment Tool (SWAT) was applied to evaluate the effect of management practices on surface water discharge and annual loads of sediment and nitrate in these watersheds. The agricultural practices comprising Scenario 1, which assumes perpetuation of current trends (conversion to conservation tillage, increase in farm size and land in production, use of currently‐employed Best Management Practices (BMPs)) result in simulated increased export of nitrate and decreased export of sediment relative to the present. However, simulations indicate that the substantial changes in agricultural practices envisioned in Scenarios 2 and 3 (conversion to conservation tillage, strip intercropping, rotational grazing, conservation set‐asides and greatly extended use of best management practices (BMPs) such as riparian buffers, engineered wetlands, grassed waterways, filter strips and field borders) could potentially reduce current loadings of sediment by 37 to 67 percent and nutrients by 54 to 75 percent. Results from the study indicate that major improvements in water quality in these agricultural watersheds could be achieved if such environmentally‐targeted agricultural practices were employed. Traditional approaches to water quality improvement through application of traditional BMPs will result in little or no change in nutrient export and minor decreases in sediment export from Corn Belt watersheds.     

SWAT‐LUT: A Desktop Graphical User Interface for Updating Land Use in SWAT

Long‐term simulations of agricultural watersheds have often been done assuming constant land use over time, but this is not a realistic assumption for many agricultural regions. This paper presents the soil and water assessment tool (SWAT)‐Landuse Update Tool (LUT), a standalone, user‐friendly desktop‐based tool for updating land use in the SWAT model that allows users to process multi‐year land use data. SWAT‐LUT is compatible with several SWAT model interfaces, provides users with several options to easily prepare and incorporate land use changes (LUCs) over a simulation period, and allows users to incorporate past or emerging land use categories. Incorporation of LUCs is expected to provide realistic model parameterization and scenario simulations. SWAT‐LUT is a public domain interface written in Python programming language. Two applications at the Fort Cobb Reservoir Experimental Watershed located in Oklahoma and pertinent results are provided to demonstrate its use. Incorporating LUCs related to implementation of recommended conservation practices over the years reduced discharge, evapotranspiration, sediment, total nitrogen, and total phosphorus loads by 59%, 9%, 68%, 53%, and 88%, respectively. The user’s manual is included in this article as Supporting Information. The SWAT‐LUT executable file and an example SWAT project with three land use rasters and the user’s manual are available at the United States Department of Agriculture‐Agricultural Research Service Grazinglands Research Laboratory website under Software. Editor’s note : This paper is part of the featured series on Optimizing Ogallala Aquifer Water Use to Sustain Food Systems. See the February 2019 issue for the introduction and background to the series.     

Climatic and agricultural contributions to changing loads in two watersheds in Ohio

Trends in climatic variables, streamflow, agricultural practices, and loads of nutrients and suspended solids were estimated for 1976-1995 in the Maumee and Sandusky watersheds, two large agricultural basins draining to Lake Erie. To understand the contributions that various factors may have made to the trends in loads, earlier results of models linking loads to explanatory variables were combined with estimated trends in those variables. The study period was characterized by increases in temperature, wintertime precipitation and streamflow, conservation farming, and loads of nitrate and total suspended solids; decreases in snowfall and snow cover, fertilizer, manure from livestock, and loads of soluble reactive phosphorus; and relatively steady exports of total phosphorus. After removing the effects of trends in streamflow, nitrate loads increased much less while total suspended solids and total phosphorus loads declined. The analysis suggests that the nitrate increases were due largely to climatic factors, particularly increases in winter streamflow, decreases in snowfall and snow cover, and declining annual precipitation. Decreases in soluble reactive phosphorus were associated with changes in agricultural practices, particularly declines in fertilizer deliveries and head of livestock.     

Assessing Sediment Yield for Selected Watersheds in the Laurentian Great Lakes Basin Under Future Agricultural Scenarios

In the Laurentian Great Lakes Basin (GLB), corn acreage has been expanding since 2005 in response to high demand for corn as an ethanol feedstock. This study integrated remote sensing-derived products and the Soil and Water Assessment Tool (SWAT) within a geographic information system (GIS) modeling environment to assess the impacts of cropland change on sediment yield within four selected watersheds in the GLB. The SWAT models were calibrated during a 6 year period (2000–2005), and predicted stream flows were validated. The R ² values were 0.76, 0.80, 0.72, and 0.81 for the St. Joseph River, the St. Mary River, the Peshtigo River, and the Cattaraugus Creek watersheds, respectively. The corresponding E (Nash and Sutcliffe model efficiency coefficient) values ranged from 0.24 to 0.79. The average annual sediment yields (tons/ha/year) ranged from 0.12 to 4.44 for the baseline (2000 to 2008) condition. Sediment yields were predicted to increase for possible future cropland change scenarios. The first scenario was to convert all “other” agricultural row crop types (i.e., sorghum) to corn fields and switch the current/baseline crop rotation into continuous corn. The average annual sediment yields increased 7–42 % for different watersheds. The second scenario was to further expand the corn planting to hay/pasture fields. The average annual sediment yields increased 33–127 % compared with baseline conditions.