BYPASS WATER-FLOW THROUGH UNSATURATED MICROAGGREGATED TROPICAL SOILS

Recent evidence suggests that bypass flow occurs in many soils even under unsaturated conditions, but experimental confirmation is lacking. We measured bypass flow in two microaggregated Inceptisols from the humid Atlantic region of Costa Rica under water-application rates below those needed to produce ponding. Water was applied with a constant-head rainfall simulator to undisturbed soil cores taken when soils were near field capacity. Mass of soil core and of water exiting the core (outflow) and time to first appearance of water at the bottom of the core (breakthrough) were recorded. For each application rate, steady-state outflow was reached quickly at a core mass well below that corresponding to soil saturation. (In contrast, classical theory for sandy soils without macropores, based on the Richards' equation, predicts that breakthrough should take more than twice the time actually observed.) Successive step increases in application rate produced successively smaller increases in core mass and decreases in the lag time before outflow increased. Our results suggest that bypass flow will occur in the noncapillary interpedal pore space whenever the application rate exceeds the infiltration rate of individual microaggregates. Because values for matrix conductivity are extremely low (much less than typical rainfall rates), we suggest that bypass flow may be the rule rather than the exception in microaggregated soils with extensive interpedal noncapillary pore space.