DISCUSSION ON PREFERENTIAL FLOW AND PISTON FLOW IN THICK LOESS VADOSE ZONE

The mode of water migration in thick loess vadose zone has always been argued on preferential flow and piston flow. In this paper,the authors conducted in-situ observations and two model tests to demonstrate that piston flow is the unique way of water migration in vadose zone in thick loess. Based on flow behavior of unsaturated soil,it is also verified that preferential flow can only exist in saturated soils. Loess in natural condition mainly exists as unsaturated state,so piston flow is the dominant flow mode in loess. Preferential flow just occurs in shallow saturated zone locally and in the raining periods temporally. In addition,preferential paths in loess,such as vertical joints,load release cracks,animal caves and root pores are the channels for surface water discharge rather than those for groundwater recharge.