How is surface water runoff determined?
There are a number of ways to determine the amount, or quantity, of water that runs off of a surface. In addition to collections that are done in the field, we can also use computer models and simulations to estimate runoff. One of the methods, the Soil Conservation Service Runoff Curve Number (CN) method, is currently the most appropriate and authentic of the numerical models in use by soil scientists. The underlying mathematics of the SCS CN method are described in the runoff algorithm page. but some of the key concepts of the model are described here. Over the years, data collected in the field has been analyzed. This data has been produced in a number of forms. One is graphically as an “X-Y” graph, shown below. On the “x” axis is the independent variable, in this case the amount of rainfall in inches. On the “y” axis is the dependent variable, the amount of direct runoff of water in inches. The graph shows generally that as the rainfall increases, the amount of water that runs off
