Estimates by the USDA for 17 States suggest that ephemeral gully erosion ranges from 18 to 73% of the total erosion with a median of 35%. Concentrated flow is generally considered the controlling process and subsurface flow is often overlooked. Pipe-erosion may occur with no visible evidence until pipe collapse results in a fully developed ephemeral gully. Tillage operations fill-in the ephemeral gully thereby leaving the soil-pipe, which was previously at the gully head, buried and discontinuous. Quantification of the conditions underwhich discontinuous soil-pipes reestablish ephemeral gullies and continuous soil-pipes initiate ephemeral gullies is seriously lacking. The objective of this study was to quantify these hydrologic conditions. Experiments were conducted on a discontinuous soil-pipe (2 cm i.d.) that extended 50 cm into the soil bed with 30 cm topsoil depth and a 5% slope. Experiments were also conducted on a continuous soil-pipe (1 cm i.d.) that extended the entire 150 cm length of the soil bed with 10 cm topsoil and 15% slope. Preliminary findings did not exhibit sudden development of mature ephemeral gullies by tunnel collapse but did exhibit sudden re-establishment of filled in gullies. When pipe flow occurs with rainfall a synergistic effect is produced that results in cataclysmic pop-out failures which may be up to 20 times higher than sheet erosion.