The difference between the 1992 and 1996 needs estimates can be attributed partially to the increase in total area of timberland harvested in the United States, but mostly to the increase in average BMP implementation cost per acre of timberland harvested in each State. The number of States that have a formal regulatory structure for forestry operations increased from 1992 to 1996, thereby increasing BMP implementation costs. The methodologies used to develop the modeled estimates are presented in the paragraphs that follow. These methodologies are similar to those used in the 1992 CWNS, with some refinements, and use of the most recent data. Runoff from crop production and grazing land carries primarily sediments, salts, nutrients, and pesticides to the downstream receiving waters.
Sediments generally result from erosion of cropland and grazing land.
Excessive application of chemical fertilizer or animal manure on land frequently results in high concentrations of nitrogen and phosphorus in runoff or in leaching of nitrogen to ground water. Pesticide applications on cropland and pastures can introduce toxic pollutants into both surface water and ground water. The cost estimating methodology addressed the control of erosion and pollutant export from cropland and grazing land. Water quality management was incorporated into the model by applying additional control measures, such as nutrient management, pesticide management, and irrigation water management. The model estimated costs by determining an appropriate set of BMPs and estimating the implementation costs.