To try to explain the spectacular results obtained by conservation tillage (CT) in the western semi-arid zone of Mexico, using even a small amount of residue as a mulch (2 t/ha), the mechanisms involved in water dynamics in the soil-mulch-plant system were analysed carefully. Runoff is the principal mechanism involved, as CT decreases runoff losses by 30 to 65% in a stable way, throughout the crop cycle. In the short run (cycle scale), small residue heaps act as a succession of barriers giving the water more time to infiltrate. In the long run (~5 years), conservation tillage, by impeding crust formation, leads to infiltration rates up to 10 times higher on average than under traditional tillage. In total, available water during the crop cycle can be increased by 30% under conservation tillage, ensuring a big advantage for maize production. Studies have shown the potential merits of this technique even for Mexican dry zones.