George J. Sabbagh

Economic and Environmental Impacts of Limiting Nitrogen Use to Protect Water Quality: A Stochastic Regional Analysis

Potential economic and environmental effects of broad versus targeted nitrogen use policies are evaluated in five Central High Plains subregions. Results indicate that per-acre restrictions are more effective than total nitrogen restrictions in reducing expected nitrogen losses in runoff and percolation, and reducing percolation losses at all probability levels. Because of the distribution of soils within subregions, targeting nitrogen reductions to more permeable soils may not produce the anticipated reductions in percolation. It may be more effective to target nitrogen restrictions on production systems than on soil types. Reductions in producer income are less for targeted than for broad policies.

Parameter Importance and Uncertainty in Predicting Runoff Pesticide Reduction with Filter Strips

Vegetative filter strips (VFS) are an environmental management tool used to reduce sediment and pesticide transport from surface runoff. Numerical models of VFS such as the Vegetative Filter Strip Modeling System (VFSMOD-W) are capable of predicting runoff, sediment, and pesticide reduction and can be useful tools to understand the effectiveness of VFS and environmental conditions under which they may be ineffective. However, as part of the modeling process, it is critical to identify input factor importance and quantify uncertainty in predicted runoff, sediment, and pesticide reductions. This research used state-of-the-art global sensitivity and uncertainty analysis tools, a screening method (Morris) and a variance-based method (extended Fourier Analysis Sensitivity Test), to evaluate VFSMOD-W under a range of field scenarios. The three VFS studies analyzed were conducted on silty clay loam and silt loam soils under uniform, sheet flow conditions and included atrazine, chlorpyrifos, cyanazine, metolachlor, pendimethalin, and terbuthylazine data. Saturated hydraulic conductivity was the most important input factor for predicting infiltration and runoff, explaining >75% of the total output variance for studies with smaller hydraulic loading rates ( approximately 100-150 mm equivalent depths) and approximately 50% for the higher loading rate ( approximately 280-mm equivalent depth). Important input factors for predicting sedimentation included hydraulic conductivity, average particle size, and the filters Mannings roughness coefficient. Input factor importance for pesticide trapping was controlled by infiltration and, therefore, hydraulic conductivity. Global uncertainty analyses suggested a wide range of reductions for runoff (95% confidence intervals of 7-93%), sediment (84-100%), and pesticide (43-100%) . Pesticide trapping probability distributions fell between runoff and sediment reduction distributions as a function of the pesticides sorption. Seemingly equivalent VFS exhibited unique and complex trapping responses dependent on the hydraulic and sediment loading rates, and therefore, process-based modeling of VFS is required.

Economic and environmental impacts of water quality protection policies; 2. Application to the central high plains

A three-stage modeling framework is applied to evaluate the potential economic and environmental impacts of agricultural groundwater protection policies in the Central High Plains Region. Three alternative policies (limitations on total nitrogen applications, limitations on unit-area nitrogen applications, and restrictions on the use of selected herbicides) are compared to a baseline scenario that reflects the absence of any form of groundwater quality protection measures. In general, nitrogen restrictions are more effective in reducing nitrate loadings in percolation water if implemented on a unit-area basis rather than as a total (farm level) restriction. In contrast, the total restriction is more effective in controlling runoff losses of nitrogen. Both nitrogen restrictions have significant impacts on crop production levels and regional agricultural income, while the economic consequences of the pesticide restriction are much less pronounced. The proposed regional modeling framework provides critical information necessary to assess the economic and environmental tradeoffs of policy alternatives aimed at controlling agricultural nonpoint source pollution.

Economic and environmental impacts of water quality protection policies: 1. Framework for regional analysis

Agricultural production systems provide some unique challenges for assessing the regional impacts of water quality protection policies. A modeling framework is proposed for assessing the environmental and economic consequences of groundwater quality protection policies at the regional level. The model consists of three components: (1) a crop simulation/chemical transport model, (2) a regional economic optimization model, and (3) an aquifer groundwater flow model. The three submodels are linked and run recursively to simulate producer response to alternative water quality policies over a multiple-year time horizon. Model solutions provide projections of production practices employed on various resource situations across the region. Economic evaluation of alternative policies may be based upon regional agricultural income, crop production levels, input use, and changes in aquifer water levels over time. Measures of agricultural nonpoint source pollution provided by the model include nitrate, phosphorus and pesticide loadings in deep percolation and runoff water, as well as sediment losses.

Revised framework for pesticide aquatic environmental exposure assessment that accounts for vegetative filter strips.

For pesticides that do not pass higher-level environmental exposure assessments, vegetated filter strips (VFS) are often mandated for use of the compound. However, VFS physiographic characteristics (i.e., width) are not currently specified based on predictive modeling of VFS performance. This has been due to the lack of predictive tools that can explain the wide range of field-reported efficacies. This research hypothesizes that mechanistic modeling of VFS runoff and sediment trapping, integrated with an empirical, regression-based pesticide trapping equation and the U.S. Environmental Protection Agencys (EPA) exposure framework, is able to effectively derive these VFS characteristics. To test this hypothesis, a well-tested process-based model for VFS (VFSMOD) was coupled with the pesticide trapping equation and integrated with EPAs PRZM/EXAMS exposure package. The revised framework was applied to a prescribed U.S. EPA assessment scenario for four hypothetical pesticides: more mobile (i.e., organic carbon (OC) sorption coefficients, K(oc), of 100 L/kg OC) and less mobile (2000 L/kg OC) pesticides that are fast degrading or stable (i.e., 10 or 10,000 d aquatic dissipation half-lives). A nonlinear and complex relationship was observed between pesticide reduction, VFS length, and rainfall plus runon event size. The impact of VFS on environmental exposure concentrations (EECs) was found to be dependent on the pesticide sorption and dissipation half-life and whether calculating an acute or chronic EEC. While acute and chronic EECs were equivalent for stable pesticides, for fast degrading pesticides the acute EEC depended on specific loading events. Therefore, while VFS may reduce the cumulative pesticide loading, a corresponding reduction in the acute EEC may not always be observed. Such results emphasize the need to incorporate physically based modeling of VFS reductions for pesticides that do not pass the current U.S. EPA exposure assessment framework.

Distinct influence of filter strips on acute and chronic pesticide aquatic environmental exposure assessments across U.S. EPA scenarios

Vegetative filter strips (VFS) are proposed for protection of receiving water bodies and aquatic organisms from pesticides in runoff, but there is debate regarding the efficiency and filter size requirements. This debate is largely due to the belief that no quantitative methodology exists for predicting runoff buffer efficiency when conducting acute and/or chronic environmental exposure assessments. Previous research has proposed a modeling approach that links the U.S. Environmental Protection Agencys (EPAs) PRZM/EXAMS with a well-tested process-based model for VFS (VFSMOD). In this research, we apply the modeling framework to determine (1) the most important input factors for quantifying mass reductions of pesticides by VFS in aquatic exposure assessments relative to three distinct U.S. EPA scenarios encompassing a wide range of conditions; (2) the expected range in percent reductions in acute and chronic estimated environmental concentrations (EECs); and (3) the differential influence of VFS when conducting acute versus chronic exposure assessments. This research utilized three, 30-yr U.S. EPA scenarios: Illinois corn, California tomato, and Oregon wheat. A global sensitivity analysis (GSA) method identified the most important input factors based on discrete uniform probability distributions for five input factors: VFS length (VL), organic-carbon sorption coefficient (K(oc)), half-lives in both water and soil phases, and application timing. For percent reductions in acute and chronic EECs, VL and application timing were consistently the most important input factors independent of EPA scenario. The potential ranges in acute and chronic EECs varied as a function of EPA scenario and application timing. Reductions in acute EECs were typically less than percent reductions in chronic EECs because acute exposure was driven primarily by large individual rainfall and runon events. Importantly, generic specification of VFS design characteristics equal across scenarios should be avoided. The revised pesticide assessment modeling framework offers the ability to elucidate the complex and non-linear relationships that can inform targeted VFS design specifications.

Estimating nitrogen percolation relationships: An application of tobit analysis

Abstract Regression analysis is used to complement a simulation model by synthesizing a large amount of data into concise results. The crop growth/chemical fate simulation model EPIC-PST is used to estimate annual nitrogen percolation loadings under irrigated wheat and corn production for various soil types, irrigation systems, and management practices. Tobit analysis synthesized these results and provided information about the effect of selected variables (e.g. irrigation level and nitrogen applied) on the expected value and the probability of nitrogen percolation events. The tobit results can be used to make field recommendations and policy prescriptions based on EPIC-PST output.

An Improved Express Fraction for Modeling Macropore/Subsurface Drain Interconnectivity

The rapid transport of contaminants through macropores and into subsurface drains is a concern. Recent research has proposed methods for incorporating this direct connectivity into contaminant transport models. For example, the one-dimensional pesticide fate and transport model, Root Zone Water Quality Model (RZWQM), was modified to include an express fraction parameter based on the percentage of macropores in direct hydraulic connection to subsurface drains. When macropore flow first reached the top of the water table (point midway between the drains), a macropore express fraction of water and chemical was routed directly into the subsurface drain, which improved predictions of concentration peaks. The remaining water and chemical was allowed to fill and mix with the water table, resulting in a concentration bulge at the water table. This research proposes an updated express fraction for RZWQM, which distributes water across all saturated layers between the drain and water table. Implicitly assumed is a uniform spatial distribution of macropores. This updated express fraction is evaluated using data from two isoxaflutole/metabolite field experiments in Allen County and Owen County IN (2000), where concentrations of parent and metabolite were measured in the drain flow. The results showed a slight improvement in the prediction of chemical concentrations on the recession limbs of drainage hydrographs.