Characterizing three-dimensional soil hydraulic properties and water flow in the thick unsaturated zone of Yungang Grottoes

Context Yungang Grottoes, a UNESCO World Heritage Site in Datong, Shanxi Province, China, are a series of Buddhist cave temples carved into a sandstone cliff during the Northern Wei dynasty (386–534 CE). Unfortunately, the grottoes have suffered from water damage over the years. Understanding the water cycle in the mountain (sandstone and the overlying loess) is essential for identifying the sources of this damage and implementing effective conservation measures. However, due to the complex structure of the terrain and the lack of observational data, there are few quantitative simulation studies on water transport. Aims This study focuses on Grottoes 5–20, analyzing the water dynamics of the mountain mass over these grottoes and thus further identifying whether precipitation infiltration is a source of water damage. Methods We characterized the three-dimensional (3D) spatial variability of subsurface hydraulic properties by first estimating vertical parameters using PEST coupled with HYDRUS-1D on three soil profiles. Then, we scaled these parameters horizontally using scaling theory and extensive soil sample data. This enabled the development of a 3D water migration model of the thick unsaturated zone using HYDRUS-3D. Key results Model effectiveness was validated by comparing simulated and measured water content during May–October 2021, confirming the rationality of our approach to integrating multiple data sources to build the model in data-scarce regions. We then used the model to analyze the 3D moisture migration within the hill under two scenarios: short-term, intense rainfall and low, long-term infiltration. Conclusions The results indicated that, in both cases, the surface soil layer plays a ‘protective role’ by preventing water damage to the grottoes. Implications Our study provides a quantitative method for simulating and predicting the soil water dynamics within the mountain mass where the grottoes are situated.