Soil loss through fissures and its responses to rainfall based on drip water monitoring in karst caves

•137Cs technique, high frequency online monitoring and UAV survey were used.•Underground soil loss through fissures was little monitored in cave drip water.•Underground soil loss was controlled by surface land coverage and rainfall process. Owing to the binary geologic structure, soil loss in the karst region is characterized as a combination of surface erosion and underground soil loss. However, we know little about the process and amount of underground soil loss by now due to the difficulty in observation and identification. To predict soil loss accurately and control rocky desertification effectively, it is urgent to collect data of underground soil loss directly by new ways. In this study, 137Cs tracer technique, high frequency online monitoring and UAV survey were used to investigate soil loss through fissures. The soil samples were collected in five karst caves of Guizhou Province for determining 137Cs activity. In two of these caves, the improved monitoring devices were adopted to monitor drip water online, and the sediment content in drip water was measured once a month. Furthermore, depending on 5 exposed karst sections, the distribution density of fissures was surveyed with UAV to calculate the average underground soil loss modulus in the watershed. The results showed that (1) the soil materials deposited in caves were mainly transported by runoff from the cave entrance, rather than through fissures above the caves. (2) The initial response time and duration of drip water in caves to rainfall were controlled by surface land coverage and the process of precipitation. And the variation of sediment content in drip water kept almost constant with rainfall except at the beginning of the rainy season. (3) The directly measured underground soil loss rate monitored through the fissure above a cave with high bedrock exposure rate and a cave with high vegetation coverage was 2.42 g·yr−1 and 0.36 g·yr−1, respectively. (4) The average underground soil loss modulus in the area with high bedrock exposure rate and with high vegetation coverage was 0.35 t·km−2·yr−1 and 0.05 t·km−2·yr−1, respectively. This study is helpful for understanding the process and intensity of underground soil loss.