Instability driven flow and runoff formation in a small catchment
Keywords:Hydrology, Rainfall, Runoff, Soil, Catchment, Preferential flow.
AbstractTwo anomalous phenomena were observed in a small catchment: 1) In some situations, the water supplied by rain caused a pronounced decrease in the soil water content. 2) In these periods, the soil water movement could be explained only by assuming an irregularly oscillating outflow of soil water into lower horizons. In these situations a large volume of water flows through the soil; therefore on the hydrological scale, this phenomenon forms a great part of the outflow from a watershed. These phenomena are described in the frame of the instability driven flow theory and explained as consequences of the porous soil body's capacity to become conductive as a result of a very little change of its moisture. Therefore the soil profile can attenuate or amplify the rainfall pulses during their transformation to the outflow below the soil profile. If the soil water content is lower than the threshold value, the rainfall pulses can be suppressed down to zero. If the soil profile contains more water, the soil does not attenuate the rainfall pulses, it can even amplify them by adding the released soil water. This is the mechanism of rapid growth of rising hydrograph limb during a storm event. The rapid transport of the soil water can occur in any part of the porous soil body regardless of the pore size and can be caused by any rainfall event with any intensity, duration or total volume.
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