In this article the relative roles of precipitation and soil moisture in in?uencing runoff variability in the Mekong River basin are addressed. The factors controlling runoff generation are analysed in a calibrated macro-scale hydrologic model, and it is demonstrated that, in addition to rainfall, simulated soil moisture plays a decisive role in establishing the timing and amount of generated runoff. Soil moisture is a variable with a long memory for antecedent hydrologic ?uxes that is in?uenced by soil hydrologic parameters, topography, and land cover type. The in?uence of land cover on soil moisture implies signi?cant hydrologic consequences for large-scale deforestation and expansion of agricultural land. Copyright ? 2007 John Wiley & Sons, Ltd.