Quoted from: https://pubs.usgs.gov/tm/tm6-a31/tm6a31.pdf 

      The SWB code uses a modified Thornthwaite-Mathersoil-water accounting method (Thornthwaite and Mather,1957) to calculate recharge. Recharge is calculated separately for each grid cell in the model domain. Sources and sinks of water within each grid cell are determined on the basis of input climate data and landscape characteristics; recharge is calculated as the difference between the change in soil moisture and these sources and sinks (eq. 1):

      Each of the water-budget components given in equation1 is handled by one or more modules within the SWB model. Specific water-balance components are discussed briefly below.

      precip—Precipitation data are input as daily values either as a time series at a single gage or as a series of daily ArcASCII or Surfer grid files created by the user. Precipitation-gage records from an unlimited number of sites may be used if the user supplies precipitation as a series of grid files.

      snowmelt—Snow is allowed to accumulate and/or melton on a daily basis. The daily mean, maximum, and minimum air temperatures are used to determine whether precipitation takes the form of rain or snow. Precipitation that falls on a day when the mean temperature minus one-third the difference between the daily high and low temperatures is less than or equal to the freezing point of water is considered to fall as snow (Drippsand Bradbury, 2005).

      Snowmelt is based on a temperature-index method. In the SWB code, it is assumed that 1.5 mm (0.059 in.) of snow melts(expressed as snow water equivalent) per day per average degree Celsius that the daily maximum temperature is above the freezing point (Dripps and Bradbury, 2005).

      inflow——Inflow is calculated by use of a flow-direction grid derived from a digital elevation model to route outflow(surface runoff) to adjacent downslope grid cells. Inflow is considered to be zero if flow routing is turned off.

      interception—-Interception is treated simply by means of a "bucket" model approach——a user-specified amount of rainfall is assumed to be trapped and used by vegetation and evaporated or transpired from plant surfaces. Daily precipitation values must exceed the specified interception amount before any water is assumed to reach the soil surface. Interception values may be specified for each land-use type and season(growing and dormant).

      outflow—Outflow (or surface runoff) from a cell is calculated by use of the U.S. Department of Agriculture, natural resources Conservation Service (NRCS) curve number rain-fall-runoff relation (Cronshey and others,1986). This rainfall-runoff relation is based on four basin properties: soil type, land use, surface condition, and antecedent runoff condition.

      The curve number method defines runoff in relation to the difference between precipitation and an "initial abstraction"'term.Conceptually, this initial abstraction term represents the summation of all processes that might act to reduce runoff, including an interception by plants and fallen leaves, depression storage, and infiltration (Woodward and others, 2003).Equation 2 is used to calculate runoff volumes (Woodward and others, 2002):