In places where water resource happens to be scarce, direct sowing in a mulch of residues appears to be a promising technique, that can improve significantly soil water management. This work aims at quantifying in detail and modeling the effects of direct sowing with a mulch of residues on soil water management. It was carried out at la Tinaja (Mexico) on corn-cropped plots, under a semi-arid tropical climate. To start with, three specific effects of direct sowing with mulch were studied in detail: i) reduction of evaporation, ii) increase in infiltration rate, and iii) reduction of runoff. Theoretical tools, which are not presented in this paper, were used to address each effect (Findeling, 2001). Then the latter were taken into account in the frame of a global functioning model, which simulates simultaneously all the effects of direct sowing with mulch, and provides a detailed water balance of the soil - mulch - plant continuum. Results showed that the effects on infiltration and consequently on runoff were decisive, whereas those on evaporation were slighter.