Comprehensive study on characteristics of leakage in the source water area and landscape conservation by artificial leakage reduction of Huanglong Wucai pool

The study area is Huanglong Wucai pool in China. In recent years, due to the influence of Salix cupularis thicket in Zhuanhua pond of the upstream source water area, there has been strong surface water leakage, resulting in a decrease of water conservation and shrinkage of water landscape, which in turn has caused the degradation of pools such as blackening and podzolization. Through analyzing the characteristics of water conservation in Wucai pool and their influence on the evolution of travertine landscape, this study discusses the effect on landscape restoration by artificial intervention in the reduction of surface water leakage, so as to provide systematic results and technical reference for subsequent landscape conservation and serve the protection of the core value of Huanglong World Natural Heritage Site.

By means of field investigation, flow monitoring, water quality monitoring, particle analysis, SEM characterization, and experiment on water quantity regulation and conservation, the characteristics and causes of water runoff leakage in the source water area of Wucai pool were identified, and the effects of artificial leakage reduction on the evolution of travertine landscape in the study area were analyzed. Five monitoring stations were set up to monitor the total flow in Zhuanhua pond, the water flow entering Wucai pool and the corresponding changes in hydrochemical characteristics. Meanwhile, pH, temperature and conductivity were measured by a handheld WTW 3310 pH meter, and Ca²⁺ and HCO$_3^{−}$ were measured by titration on-site. The monitoring period was from April to November, 2019. Collected from loose travertine at the forest area of Zhuanhua pond and from the newly formed loose travertine at the bottom of landscape pool, the particle analysis samples and SEM samples were used to analyze the transformation and destruction of travertine body by the root system of Salix cupularis thicket. Thin-walled rectangular weir and triangular weir were used for flow measurement. After water flow regulation of Zhuanhua pond, isolation of channel with strong leakage, and conservation of Wucai pool, the impact of the increased water flow on the improvement of local degradation of Wucai pool was assessed on June 5, 2020.

The research findings show, (1) The quality of spring water from Zhuanhua pond in the source water area is stable, with high partial pressure of carbon dioxide and high calcium. Degassing reaction occurs along the river, and calcite is oversaturated at a low level to the southern edge of Wucai pool. (2) Due to activities of thicket root and influences of freeze-thawing, strong runoff leakage occurs in the source water area, and only 49% of the total water flow can enter Wucai pool to form a landscape conservation water source. The main site of strong leakage is located at the west branch channel composed of loose calcareous gravel and sand in Zhuanhua pond. (3) The leakage in the source water area caused a relative decline in the water conservation capacity of Wucai pool. The northern edge of Wucai pool became blackened and water in the east part was displaced by surface water of Huanglonghou Ditch to form podzolization. The scale of the landscape pools shrank to about half of its early size. The maximum natural landscape water conservation of Wucai pool, with the capacity of 5,115 m³·d⁻¹, generally occurs in September. (4) Through artificial conservation, the impact of surface water leakage in the source water area of Wucai pool landscape is reduced. During the main conservation period (July–October), the proportion of the water conservation into Wucai pool increased to 83%, up from 49% before artificial conservation, and the average monthly water flow increased from 4,892 m³·d⁻¹ to 8,674 m³·d⁻¹, with an increase of water conservation capacity of 77%. Before and after the conservation of Wucai pool, the concentration of calcium and bicarbonate ions in water changed little; SIc changed from oversaturation to equilibrium; carbon dioxide degassing in the middle and lower reaches increased; the deposition capacity of travertine generally increased. (5) During the main conservation period, the average monthly water flow after conservation increased by 3,559m³·d⁻¹. 2,544 m³·d⁻¹ of water was consumed to restore the blackened pool in the north and alleviate podzolization in the east, accounting for 29% of the total capacity of water conservation in Wucai pool. The landscape water area of Wucai pool increased by 16.3% and remained at the same level after the four-month landscape restoration and conservation.

The degradation such as blackening and podzolization of Wucai pool caused by increasing surface water leakage in the source water area by activities of root system of Salix cupularis contributed to the decrease of water conservation capacity in the landscape area. However, through the artificial leakage reduction and conservation experiment, the water conservation capacity increased, and thus the degradation was contained and the travertine landscape was restored to a certain extent in the study area, which indicates that appropriate artificial intervention is beneficial to landscape protection.