Analysis of soil water movement inside a footslope and a depression in a karst catchment, Southwest China

Soil water movement is difficult to explain with event-scale approaches, especially in karst regions. This paper focuses on investigating seasonal recharge and mean residence time (MRT) of soil water based on temporal variation of stable isotopes (delta D and delta O-18) and a dispersion model (DM), and discussing their differences along a footslope and a depression in a small karst catchment of southwest China. Temporal variations of the stable isotopes in precipitation and soil water within 0-100 cm profiles were monitored weekly for approximately 43 and 99 weeks. Results show that the seasonal recharge of soil water inside the footslope and the depression were similar, but the vertical flow velocity was higher implying a faster hydrological process in the footslope. The MRT of soil water (2-64 weeks) increased roughly, suggesting decreasing velocity of water displacement with increasing depth. However, the MRT at 60-100 cm depths in the depression (47-64 weeks) was obviously longer than at other sites, revealing more intensive water mixing. Furthermore, a shallower isotopic damping depth was found in the depression, indicating stronger delay and attenuation effects on base flow recharge. These results provide new insights into research on hydrological processes in karst areas.