The Progress Scenarios provide an estimate of the projected nutrient and sediment reductions towards the cap load allocation in any given year based on the reported cumulative implementation of control measures for that year. In the Progress Scenarios, the estimated loads are based on the hydrology for the ten-year period of 1985 to 1994. The model simulates nutrient and sediment loads for each day of the ten-year period and reports the results as an average annual load. Using a ten-year average allows for a simulation representative of the loads that would occur during an average hydrology year.
Average hydrology provides a consistent hydrology condition against which reductions in nutrients and sediment resulting from management actions are measured. Actual hydrology changes year to year. In low flow drought years, such as in 2002, nutrient and sediment loads are low. In high rainfall years, such as the in 2003, the flows and loads are high. For this reason, the average nutrient and sediment loads of the Progress Scenarios should not be compared to actual field measurements of hydrology and loads of a given single year. Further, lag times in groundwater nitrogen and in sediment transport are poorly simulated in the Watershed Model, so the Progress Scenario estimates are best interpreted as a total load, assuming average conditions, that will occur sometime in the future. Estimated land use change, animal populations, BMP implementation, and point source reductions for a given year are included in the Progress Scenarios.