Increase of moisture content in Mogao Grottoes from artificial sources based on numerical simulations

Abstract Moisture is a key factor that can lead to deterioration of cultural heritages in arid regions. The Mogao Grottoes at the center of the ancient Silk Road suffer from moisture problem due to a multiple natural water sources, including the nearby river and flood, underground water migration, and rainfall. However, contributions by artificial sources remain unclear, which range from irrigation to the sand control vegetation belts on the sand dunes, plus the green field in the tourist and administrative area. Here, we use numerical simulations to examine if and how much these artificial sources increase the moisture in and around the grottoes. A wide range of factors are considered including both liquid and vapor phases of water, both steady and transient scenarios, local climate, and inputs by different water/vapor values and wind speeds. The results show that the drip irrigation on the sand dunes has little influence, because of its far distance to the grottoes, limited irrigation water supply, and strong evaporation on the desert surface; however, a threating potential exists if the water amount becomes too large. The green field close to the east of the grottoes with flood irrigation can increase the soil water content of the grottoes rock by about 1.5%, and the water vapor concentration by about 45% under a gentle easterly wind. Rainfall also increases the water content, with fluctuation, on the cliff surface where the grottoes are located. To prevent the manmade moisture-related damages to the grottoes, it is necessary to control the size of the green field, to convert flood irrigation to drip irrigation, and to change the trees to desert shrubs. A shield system can be applied to the cliff surface to reduce the damage from rainfall.