All the source variables were strongly significant as indicated by probability levels of less than or equal to 0.01 in all cases (table 1). Urban area was the weakest of the five source variables, but was statistically significant. Because sources are already in units of load, parameter estimates for sources should approximate the value 1, if all other total nitrogen sources are described, and if losses are accounted for through land-to-water delivery factors. For example, point sources discharge directly to streams and because point-source loads are in the same units as stream loads, the point-source parameter should be close to the value 1. In contrast, urban area is in units of area because the loading rate from urban area is not known. Thus, the urban-area parameter estimate must compensate for the differences in units, and is not expected to approximate the value 1.