Isotopic variations of soil water and rainfall are studied through rainfall infiltration experiments under isotopically depleted rainfall inputs condition. The isotopic compositions of rainfall and soil water mix in the process of rainfall infiltration

and the isotopic compositions of outflow decrease with time and then keep constant. The slope of hydrogen and oxygen isotopes of outflow is determined by that of mixing line connecting the isotopic compositions of rainfall and the initial soil water. Using curve fitting method to fit isotopic variations of outflow

it is found that the transfer function distribution of isotopic composition of outflow is exponentialpiston flow model distribution

and the transfer function of hydrogen isotope is similar to that of oxygen isotope.