Research on Causes of Water System Changes Inside and Outside the Mountain Resort

Objective As the largest royal garden in the world today, the Mountain Resort is renowned for its classic natural landscape construction. From the initial construction of the Mountain Resort in 1703 to the collapse of Qing Dynasty in 1911, the water systems inside and outside the Mountain Resort underwent significant changes. However, the historical process of water system changes inside and outside the Mountain Resort hasn't been clearly sorted out, and the causes thereof haven't been researched in depth. These deficiencies have brought obstacles to the comprehensive understanding of the Mountain Resort, and also brought challenges to the recognition of multiple values of landscape heritage. Methods This research reviews water system changes inside and outside the Mountain Resort over the past 200 years since its initial construction. The central research topic is developed by using the method of historical data analysis and relevant calculation results with a focus on the specific process of water system construction inside and outside the Mountain Resort in history. Through the interpretation of ancient drawings and documents and the implementation of field investigation, the boundary of the palace wall of the Mountain Resort before the lake expansion project is roughly drawn. Based on the interpretation of the archival records and the principle of river dynamics, the influence of erosion and sediment deposition in the channel confluence on the water diversion area of the Mountain Resort is expounded. According to the changes of the flood section area of the Wulie River before and after the lake expansion project of the Mountain Resort, it can be seen that the flood discharge capacity of the Wulie River has decreased significantly. This research sorts out the river management work during the rapid change of water system inside and outside the Mountain Resort, including the excavation of internal rivers and lakes, the annual maintenance of dry rivers, and the maintenance of external Wulie River channels, and summarizes the general process of the establishment, modification and abandonment of relevant river governance systems, In addition, the restrictive effects of the diversion project, the lake extension project and the river engineering policy on waterscape change are summarized. Based on historical documents and maps, remote sensing data of land use/land cover monitoring in the early 1980s, and the soil loss data in Chengde during the same period, this research discusses the source of river sediment, which is a more important factor causing water system change, from the perspective of Wulie River basin. Results Although the water system construction of the Mountain Resort has created a classic waterscape, it has also promoted the water system regression of the Mountain Resort. Specifically, the main water diversion project is located near the confluence of the Lion Ditch and the Wulie River, where the fluvial facies changes are far more frequent than other river sections and river sediment is more likely to accumulate river near the diversion point, leading to the significant accretion of river channel while bringing the risk of diversion channel cut-off. During the reign of Emperor Kangxi in Qing Dynasty, the lake expansion project was implemented to excavate the Mirror Lake and the Silver Lake within the palace wall of the Mountain Resort, which reduced the flood section of the Wulie River on the east side of the Mountain Resort by more than 50%, forming a bottle-neck reach for river discharge, causing backwater and sedimentation in the upper reaches of the Mountain Resort, and raising the river bed year by year. The establishment, improvement and abandonment of such systems as "annual repair of dry river", "river and lake dredging" and "annual repair of major river" with respect to the Mountain Resort, coupled with the rapid water system change inside and outside the Mountain Resort, played a non-negligible role in the decline of local waterscape. Based on historical archives and photographic images, it can be found that since the middle and late Qing Dynasty, the problem of land overdevelopment in the Wulie River basin has become more and more serious. According to the annual soil loss scale of cultivated land at different slopes in Chengde compiled by Chengde Institute of Soil and Water Conservation in the 1980s, and relevant combined with DEM data and the earliest remote sensing data of land use/land cover in China in the same period, it is calculated that the total amount of annual soil loss of cultivated land in the Wulie River basin is about 1,025,422.30 t, roughly equivalent to the annual total mean sediment runoff of the Wulie River basin collected by Chengde Hydrographic Station in the 1970s, which is about 1,070,000.00 t. Conclusion Despite its creation of a classic waterscape, the water-related construction of the Mountain Resort also plays a significant role in the disappearance of water systems inside and outside the Mountain Resort. However, since the late Qing Dynasty, the serious soil and water loss arising from the overcultivation in the Wulie River basin is the most critical factor causing the water system changes inside and outside the Mountain Resort, which serves as a precious sample of the temporal and spatial changes of landscape. An in-depth research on the causes of such changes is the prerequisite for the recognition of multiple values of landscape heritage, and also an important way to excavate and cultivate the ecological wisdom.