The Kaixian study area is located in Kaixian County, in the Chongqing Municipality. It has an area of about 250 km2 and elevations from 145 to 1070 m above sea level, with an average of 390 m (Fig. 6). The area has a high local relief, with an average elevation of about 300 m and a maximum about 700 m. Most of the slopes in this area are very steep with an average gradient of about 20°. The lithology over the area is of three major types: the lower to middle Jurassic system (including J2s, J2xs, J2x, J1–2z, and J1z), which is mainly made of sandstone, siltstone, mudstone, and shale; the upper Jurassic system (including J3s and J3p), which primarily consists of sandstone and siltstone; and the Quaternary system (Q) which is mainly composed of relatively recent deposits along the river valleys (Fig. 7).