A procedure to estimate the mass balance in the superficial soil layer, using soil water content readings, is presented. The volumetric water content has been determined by the Time Domain Reflectometry technique. Under suitable assumptions the proposed procedure computes the loss or gain of water in a soil column of cross section equal to one. This allows estimating evaporation during drying periods and infiltration after precipitation events. The method was applied to soil moisture data collected during several field experiments realized both in a physical model at the Lido beach of Venice and in natural sites in mountain and suburban environment.