Response of hydrological systems to the intensity of ecological engineering

The rapid global construction of ecological engineering has had an important impact on hydrological processes, especially in China. However, previous studies have largely regarded the hydrological system as a "black box" and have formed conclusions by comparing changes before and after ecological engineering; additionally, managers have assumed that the intensity of ecological engineering (IEE) is proportional to the hydrological system effect. The influence processes and mechanisms of the IEE on hydrological systems are unclear. Here, the source region of the Yangtze River (SRYR) in China was adopted as the empirical research area. Based on various data, such as DEM, soil, land use and climate data from 1980 to 1987 and 2004-2015, a Soil and Water Assessment Tool (SWAT) model was constructed, and the response of the hydrological system to the IEE was quantitatively explored. The results showed that the IEE generally displayed an increasing trend, increasing from 10.209 in 2005 to 10.649 in 2015. There was no linear relationship between the IEE and hydrological effects, and the hydrological effects did not increase with increasing IEE but did exhibit obvious stages. Specifically, the four phases - the lag phase, transition phase, sensitive phase, and adaptive phase - had different hydrological response characteristics, sensitivities, structures, and functions. Based on the feedback of the hydrological system, ecological engineering managers should abandon the incorrect assumption of a linear relationship between the two and realize that there is usually a lag phase in the early implementation of ecological engineering that requires a continuous increase in the IEE. Additionally, managers should choose the appropriate IEE, evaluation nodes and periods and prioritize the most sensitive hydrological variables in different stages based on the evaluation purpose and hydrological response, thereby improving the efficiency of ecological engineering and realizing the original goal of ecological protection and restoration.