Permission to use illustration from Physics 161 Online granted by Greg Bothun, University of Oregon Figure 1. The hydrologic cycle consists of a system of water-storage compartments, and solid, liquid, or gaseous flows of water within and between the storage points. The water budget is a concept in which components of the hydrologic cycle are categorized into input, output, and storage components. It is essentially an accounting procedure that quantifies and balances these hydrologic components for a watershed. Coupled with energy, precipitation is the primary input to a watershed system.
Infiltration is the process of water entering the soil surface.
Evapotranspiration , which represents the sum of all of the water evaporated and transpired from a watershed, is the most difficult of all of the components of a water budget to quantify. However, the evapotranspiration component and its linkage to soil water storage and the movement of water off of a watershed is one of the hydrologic processes most affected by vegetative manipulations. Relationships of precipitation, infiltration, and soil water storage affect volumes and rates of water movement downstream. That part of the precipitation input that runs off a land surface and the part that drains from the soil and, as a consequence, is not consumed through evapotranspiration is the water-flow component of the hydrologic cycle. The most direct pathway from precipitation to streamflow is that part of the precipitation that falls into stream channels, called channel interception .