Preference flow at rock-soil interface and its influence on soil water dynamics in the karst troughs

Karst grike and troughs are widely developed in epi-karst zone, in which the soil water and fissure water are important sources for vegetation. The preferential flow through rock-soil(R-S) interface and plant roots are important sources of groundwater recharge. In this paper, two typical fresh limestone karst trough profiles (A and B) in the Puding karst experiment area were selected, where the filled soil were yellow soil clay and yellow calcareous soil, respectively. Especially, the upper and middle parts of profile B contain more plant roots. The field infiltration experiments on the two typical karst profiles were done (profile A:3 times; profile B:1 time), and the soil moisture content at the R-S interface and intermediate soil (SZ) were measured with TDR. Meanwhile, combined with the infrared imaging and dyeing tracer of the infiltration front of the profile, the influence of the dominant rock-soil interface flow, the initial moisture state (degree of wetness and non-uniform distribution) and the vegetation root system on the movement of the soil moisture front in the trough with different initial moisture content was analyzed. The results show that the rapid dominant flow at R-S interface significantly affects the infiltration and water movement in the soil of the karst troughs, and the degree of its influence is related to the size and distribution of the initial soil moisture content. The increase of soil moisture content in the vicinity of R-S interface where the infiltration front reaches during infiltration is 1.1 to 14.5 times of that in the middle of karst trough; but the lateral water potential gradient formed by the preferential flow in the R-S interface can reduce the downward velocity of the dominant flow front. Whereas, in the middle part of the soil far away from the R-S interface, the wetting front moving down speed in infiltration mainly depends on the degree of soil moisture at the initial stage. The lower the soil moisture content, the greater the wetting front matrix potential gradient and the faster the wetting front moving down. For another, the macro-pore dominant flow of vegetation root system mainly acted on the surface soil moist front movement in the initial infiltration and the preferential flow of the R-S interface is not significant. Then, with the increase of infiltration depth, the lateral dispersion of preferential flow in the R-S interface has a stronger influence on wetting front migration and soil moisture content.