Is modeling needed to scope or quantify a problem?

When a water quality issue is first identified, the level of understanding of the severity and sources of the problem is often limited. Modeling is frequently used to help build understanding of a water quality problem. Typically, simple "scoping" models are useful to help you quickly estimate the extent and severity of a problem.

For example, consider a lake which is believed to receive excess loads of nutrients. Does this loading result in a degree of eutrophication which impairs uses?

Both monitoring and modeling can be used to help answer this question. Monitoring actual lake responses would be more reliable, but there is not always the luxury of time and funding necessary to collect the data. Further, a few scattered measurements may not be very informative. Lakes often exhibit a high degree of variability in algal response both within a season, and year to year, so a few chlorophyll a measurements of algal concentrations may not provide a clear answer.

In some cases, a simple scoping model can quickly be applied to give a general answer or estimate of the risk posed to water quality. For example, you could use an empirical model, which predicts trophic state based on a statistical relationship between phosphorus load and lake retention time, and algal concentration.

Simple models can also help to estimate runoff flows or contaminant loads for the purpose of assessing relative magnitudes, and thus targeting areas of greatest risk. This is another area where modeling can be more cost-effective than monitoring if a high degree of accuracy is not required for initial estimates. For example, simple models, such as loading coefficients, can aid in identifying areas where runoff is greatest, and areas which are likely to generate the largest loads of a given pollutant. Such modeling is particularly useful for obtaining initial estimates of nonpoint loads; It is difficult to gather monitoring data on nonpoint runoff flow and pollutant loads. This makes modeling of runoff-generated loading an attractive option.

For further reading: Mills, W.B. et al. 1985. Water Quality Assessment: A Screening Procedure for Toxic and Conventional Pollutants in Surface and Ground Water. EPA/600/6-85/002.