There can be subwatersheds within watersheds. A lake is a reflection of its watershed. See also the section on conductivity . Lakes having a defined surface inlet and outlet. Figure 8 Landuse has an important impact on the quality and quantity of water entering a lake.
Retention time is important in determining the impact of nutrient inputs.
Long retention times result in recycling and greater nutrient retention in most lakes. Thus, water quantity affects water quality. Additionally, as water flows over roads, parking lots and rooftops, it accumulates nutrients and contaminants in both dissolved and particulate form. Not included here, but also important, is the influence of soil type and slope. One can see that, all other things being equal, converting a forest into a city can increase the phosphorus export to a lake more than ten times.