A soil moisture budget tracks the inputs and outputs of water in soil. At it's simplest form it tracks actual evapotranspiration, potential evapotranspiration and precipitation across a year for a given location. The model assumes that the first 100 mm of precipitation after P>Ep recharges the soil, the remainder of precipitationin this category is then surplus. The first 100 mm of precipitation when P<Ep is counted as use when the system is drawing on water stored within the soil. the remainder in this category becomes deficit.