In the first of this two-paper series, the main mechanisms for generation of runoff from rainfall in large basins with small to negligible slopes are analyzed using available data from the Río Negro basin in Uruguay. Topography and soils were examined in order to identify physical features that may influence the flow patterns. Soil moisture storage in space and soil moisture variability in time were also evaluated to relate rainfall and runoff generation. The study revealed the existence of strongly developed horizontal layers. Soil moisture depends essentially on vertical water transport processes due to the low morphological energy of the terrain. Surface and subsurface flow occurs during the season of low evapotranspiration where soils become progressively wet. Extreme storms in terms of accumulated rainfall are required to produce surface and subsurface flow during the season of high evapotranspiration. In the following paper, these observations and hypotheses are used to model a large basin with small to negligible slopes.

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