Precipitation provides nearly all of the influx of water to a watershed. It is obvious to engineers and hydrologists that precipitation must be accurately estimated before any estimates of channel flow can be made. It is much less common for equal consideration to be given to ways water leaves a watershed other than as outflow. In many areas of the United States, more than 50% of precipitation returns to the atmosphere through evaporation and transpiration. Transpiration makes up most of the return. These two processes are key to accurate long-term hydrologic simulations.