Spatiotemporal variations of soil water stable isotopes in a small karst sinkhole basin

Soil water plays an important role in the ecological restoration of karst areas impacted by rocky desertification (KRD) caused by deforestation. A better understanding of the spatiotemporal dynamics and modes of water flow in KRD areas is essential for satisfactory soil water management. This study examines the soil water content (SWC) and stable isotope (delta O-18 and delta D) patterns in seven sampling sites along an E-W transect in a typical small karst basin in southwest China over the hydrologic year, February, 2018-January, 2019. The results of this study show that the spatial differences of SWC and soil water delta O-18 (delta D) values were smaller but in with clear seasonal variations. Seasonal vertical profiles of soil water delta O-18 (delta D) showed a decreasing trend from the surface to a depth of 100 cm in February, April, and June, which reversed August and October. Shallow soil water delta O-18 (delta D) and line-conditioned excess (lc-excess) (<20 cm depth) showed significantly stronger seasonality than the deeper soil water (>20 cm depth). Enriched soil water delta O-18 (delta D) values in the upper 20 cm depth was closely related to the potential evaporation over the 7 days prior to each sampling (PET7). Piston flow was the dominant mode of flow in the sampling sites, inferred from the vertical profile characteristics of the soil water delta O-18 (delta D). The influence of evaporative processes was limited to the upper 20 cm depth of the soil. The fraction of evaporation losses from soil water storage at the 20 cm depth varied from - 15.8% to 12.3%, based on delta D estimates, and from - 13.3% to 14.0% based on delta O-18. These results are important for effective soil water management in KRD areas that experience seasonal drought.

Automated summary

Soil water plays a crucial role in the ecological restoration of karst areas affected by rocky desertification caused by deforestation. This study investigated the spatiotemporal dynamics and modes of water flow, as well as the seasonal variations in soil water content and stable isotopes in a small karst basin in southwest China. The results showed clear seasonal variations in soil water content and stable isotopes, with shallow soil water exhibiting stronger seasonality than deeper soil water. This information is important for effective soil water management in areas experiencing seasonal drought.