Most models and approaches toward cost accounting of Stormwater Control Measures (SCMs) use standardized engineering economics. These models use the first cost and annualized O&M costs, brought forward to present value using assumed interest rates, etc. Models that are more sophisticated will break the first costs down to show engineering design, project administration, land, and construction costs. O&M costs can be segmented for minor and major operations. In some models, costs are estimated from a statistical database based on costs from other projects. For example, the cost of pond maintenance is estimated on data from ten case history’s which are normalized to the area of the pond. Others are based on unit costs of labor, equipment, disposal costs, administration, etc. Each method has its advantages and drawbacks. However, my guess is that both methods lack the precision that people are looking for, and is some cases, are underestimating or overestimating costs.