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.Agricultural chemicals and fertilizers have been cited as significant groundwater contaminants (Office of Technology Assessment). EPA reported that over seventy pesticides were detected in the groundwater of thirty-eight states, and forty-six pesticides found in groundwater in twenty-six states were from agricultural use (Office of Technology Assessment). Nitrates are the most commonly detected agricultural chemicals in groundwater. EPA estimated that more than half of the nation's wells contain nitrates, with about 1.2% of the community wells and 2.4% of the rural wells having concentrations above

Targeting certain soils and cropping systems may be necessary in consideration of regional water quality protection policies. However, little information is available relating soils and cropping practices to regional water quality problems. This study evaluates crop yield and NO3‐N movement to surface and groundwater on four soils and nine principal cropping systems in the High Plains region of Oklahoma. The cropping systems involve wheat (Triticum aestivum L.), grain sorghum [Sorghum bicolor (L.) Moench], and corn (Zea mays L.), and are part of a regional data base also containing soils and chemical management information. For each combination of crop, soil, cropping system, and chemical alternative, a 20‐yr simulation was made. The simulation was based on a modeling system that includes EPIC‐PST (crop growth/chemical movement model) interfaced with a Geographic Information System (GIS), Earthone. Results of each simulation included crop yield and NO3‐N movement in runoff and percolation. Results show wide variations in NO3‐N losses for different soils, irrigation systems, and cropping systems. When compared with continuous irrigated wheat and grain sorghum cropping systems, double‐cropped wheat‐grain sorghum resulted in greater NO3‐N loss in percolation. Compared with sprinkler and LEPA (low energy precision application) irrigation systems, furrow irrigation resulted in high NO3‐N loss on both fine‐textured and coarse‐textured soils, with significantly greater loss on the coarser‐textured soils. The modeling framework can be used to compare alternative water quality policies. Broad policies such as a restriction on the amount of N that can be applied per hectare can be compared with targeted policies, such as limiting N applications or irrigation water use on coarser soils or under furrow irrigation.

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.

No abstract

No abstract