Input consists of files, information from databases and information from a GIS interface . More specific information can be entered singly, for each area or for the watershed as a whole. The SWAT hydrology model is based on the water balance equation. A soil database is used to obtain information on soil type, texture, depth, and hydrologic classification. In SWAT, soil profiles can be divided into ten layers.
Infiltration is defined in SWAT as precipitation minus runoff.
Infiltration moves into the soil profile where it is routed through the soil layers. A storage routing flow coefficient is used to predict flow through each soil layer, with flow occurring when a layer exceeds field capacity. The irrigation algorithm developed for SWAT allows irrigation water to be transferred from any reach or reservoir to any other in the watershed. Based on surface runoff calculated using the SCS runoff equation, excess surface runoff not lost to other functions makes its way to the channels where it is routed downstream. For sediment routing in SWAT, deposition calculation is based on fall velocities of various sediment sizes.