Watershed models simulate the hydrologic processes by which precipitation is converted to streamflow. The watershed is the system being modeled; precipitation is provided as input; and the runoff characteristics are computed. Water quality is changed during these hydrologic processes. Some models consider only water quantities, whereas others simulate both water quality and quantity. Simulation results essentially always include streamflow hydrographs and sometimes include the associated pollutographs as well. The watersheds being modeled include streams, reservoirs, drainage improvements, and stormwater management facilities as well as the land and land cover on which the precipitation falls.