Cow-calf production is the main practice within marginal agricultural lands of the world, like the Flooding Pampa of Argentina, where it promotes the transformation of native tall-tussock grasslands Paspalum quadrifarium into native short-grass grasslands or sown pastures. The effect of these land use changes on water dynamics are not well under-stood, especially in regions subjected to marked interannual drought and flooding cycles. Here we measured soil prop-erties (infiltration rate, bulk density and soil organic matter), rainfall interception by the canopy, and soil moisture during two years with different annual rainfall. Then, we parameterized a hydrological model (HYDRUS) for inferring consequences of soil water fluxes on water regulation. Infiltration rate was significantly higher in native tall-tussock grasslands than native short-grass grasslands and sown pastures, bulk density was significantly lower in native tall-tussock grasslands than native short-grass grasslands and sown pastures, and soil organic matter was significantly higher in native tall-tussock grasslands than sown pastures. Simulated water dynamics during years of low annual pre-cipitation (summer rainfall deficit), show that transpiration and evaporation from native short-grass grasslands repre-sented 59 % and 23 % of total water balance, whereas transpiration and evaporation from native tall-tussock grasslands represented 70 % and 12 %, respectively. This result reflects the high productive capacity of native tall- tussock grasslands under dry conditions. In contrast, under high annual precipitation (excess during fall and winter), transpiration and evaporation from native short-grass grasslands represented 48 % and 26 % of total water balance, whereas in native tall-tussock grasslands represented 35 % and 9 %, respectively. These results suggest a low capacity of native tall-tussock grasslands to evacuate water excess, especially during fall and winter. The observed differences in water fluxes between native tall-tussock grasslands and native short-grass grasslands are important to understand water dynamics under different climatic conditions and could be useful for adaptation to climate change through ecosystem-based management