www.nature.com/scientificreports OPEN Determination of controlled limit value of groundwater level depth and management practice in Xi’an, China En‑lin Mu1,2, Long Yan3*, Ai‑zhong Ding1,4*, Wei Deng3 & Yong Wang3 Based on the diferent types of geomorphic units in Xi’an, China, and the groundwater recharge methods of the diferent geomorphic units, the goal is to ensure the sustainable development and utilization of groundwater, to ensure the natural attributes and to prevent salinization. According to diferent rainfall conditions, the upper and lower limits of the controlled limit value of groundwater level in diferent regions are calculated to defne the control targets of the diferent geomorphic units. Based on the calculated controlled limit value of groundwater level and the administrative divisions of Xi’an City, the red lines of groundwater control in each county and city are developed. Four management lines are delineated from the surface to the bottom (from top to bottom, the upper limit of groundwater depth, the upper limit of infltration of groundwater depth, the lower limit of groundwater depth and the risk line in extremely dry years), and fve management areas are delineated (from top to bottom, the prevent soli salinization area, the normal extraction area, the careful extraction area, the permit only in extreme dry years area and the prohibited extraction area) to provide technical support for groundwater management in Xi’an. Groundwater resources are not only irreplaceable strategic resources for maintaining socioeconomic develop- ment but also important factors for stabilizing the ecological environmental system 1. Due to the unreason- able exploitation of groundwater and the increasingly serious environmental geological problems caused by it, groundwater overexploitation is widespread in the United States, Spain, India, North Africa and the Middle East and other countries and regions. In order to efectively manage the groundwater resources, strengthen the management of groundwater in the overexploited area, and prevent the overexploitation of groundwater in the area without overexploitation, some countries or regions have designated the area according to the local hydro- geological conditions, the current situation of groundwater development and utilization and overexploitation groundwater management zones, such as Arizona groundwater management zone, Kansas key groundwater exploitation control zone, etc. Te Central Shaanxi Plain, where Xi’an is located, is an important grain-producing area in China. Long-term overexploitation and irrational use have led to the serious overexploitation of groundwater, the continuous expansion of the groundwater depression cone, and a series of problems, such as ecological degradation, land subsidence, ground fssures and groundwater pollution, which have become important bottlenecks restricting the sustainable development of the economy and society in the area. Te groundwater level and its fuctuation are not only important indexes that refect the exploitation and replenishment balance of groundwater but also the basis for measuring the rationality of groundwater resource development2.To meet the water demands for socioeconomic development and ecological environmental protection, China has been studying the critical groundwater level (buried depth) since the 1960s and using it as a control index to guide, supervise and manage the development and utilization of groundwater resources. Te goal of groundwater level management has gradu- ally shifed from the balance of the exploitation and replenishment of the groundwater system to the coordinated and sustainable development of social, economic, ecological and environmental multiuser and multisystem 1College of Water Sciences, Beijing Normal University, Beijing, China. 2Water Resources Management Center of Ministry of Water Resources, Beijing, China. 3China Institute of Water Resources and Hydropower Research, Beijing, China. 4Engineering Research Center of Groundwater Pollution Control and Remediation Ministry of Education, Beijing, China. *email: [email protected]; [email protected] SCIENTIFIC REPORTS | (2020) 10:15505 | https://doi.org/10.1038/s41598-020-72523-4 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1. Study area Te map were created using ArcGIS 10.2 (https ://www.esri.com/sofwa re/arcgi s/arcgi s-for- deskt op). management3–13. To protect groundwater resources, Shaanxi Province formulated and implemented the Imple- mentation Plan for Assessment of controlled limit value of groundwater level in Key Areas of Shaanxi Province in 2014. Under the qualifcation condition that the average decline in a single-well water level is no more than 0.5 m, the analysis and evaluation, assessment of achievement, evaluation of assessment results and formulation of opinions for groundwater monitoring data of various cities (districts) including Xi’an City were performed. However, the evaluation index confrmation lacks a scientifc basis, and the groundwater characteristics are not well considered; therefore, it is difcult to interpret the geomorphic type, the relationship between groundwater replenishment and discharge, and the impact of precipitation and human activities on groundwater. Moreover, the current assessment method focuses on the assessment of regional groundwater levels where groundwater levels drop and groundwater overexploitation may occur, but the problems of soil salinization caused by shal- low groundwater depth have not yet been considered. Tese two problems coexist and have seriously afected regional production and development. Tere is mounting concern about how to support decision makers in driving sustainable groundwater resource management; science needs to support the decision-making process to promote evidence-based decisions 14. Terefore, the scientifc and reasonable determination of groundwater level control indexes is of great signifcance to improve the level of groundwater management and promote the sustainable use of groundwater in Xi’an. Materials and methods Study areas. Xi’an is located in the Central Shaanxi Basin in the middle of the Weihe River Basin between 107.40°–109.49° east longitude and 33.42°–34.45° north latitude, bordering the Weihe River and Loess Plateau in the north and the Qinling Mountains in the south. Xi’an is approximately 204 km long from east to west and 116 km wide from south to north, covering an area of 9,983 km2. According to the Plan for the Delimitation and Protection of Groundwater Over-exploitation Areas in Shaanxi Province, there are 5 overexploitation areas in Xi’an, with an overexploitation area of 601.3 km2. Based on the data of the existing monitoring wells, 68 moni- toring wells with continuous data are selected as investigation wells for analysis and calculation, including 50 investigation wells in the river terrace, 14 investigation wells in the alluvial fan and 4 investigation wells in the loess tableland (Fig. 1). Te aquifer of terrace, which is mainly distributed on both sides of Weihe River and its tributaries, is com- posed of sand and gravel rock with a thickness of 10–70 m and a groundwater depth of 3–70 m. Te aquifer of diluvial fan, which is mainly distributed in the diluvial sector in the north of Qinling mountains, is composed SCIENTIFIC REPORTS | (2020) 10:15505 | https://doi.org/10.1038/s41598-020-72523-4 2 Vol:.(1234567890) www.nature.com/scientificreports/ of sand and sand gravel with a thickness of 10–50 m and a groundwater depth of 5–25 m. Te loess tableland is mainly distributed in the Loess Plateau and river valley Alluvium on the northern edge of the Weihe Fault Basin, belonging to the pore and fssure water of the Loess Plateau. Te lithology of the groundwater aquifer is mainly loess like sub clay, with a thickness of 50–100 m, and the depth of groundwater level is generally 20–60 m. Methods. Based on the diferent types of geomorphic units in Xi’an and the groundwater recharge methods of the diferent geomorphic units, the goal is to ensure the sustainable development and utilization of ground- water, to ensure the natural attributes and to prevent salinization. In the alluvial fan and loess tableland area, the continuous fow of rivers from mountainous areas will be ensured, the minimum ecological fow of each tributary will be determined, the relationship between the ecological base fow and groundwater level will be analyzed, and the lower limit value of groundwater pipe control for the alluvial fan and loess tableland area will be determined in combination with the principle of the river channel and groundwater recharge and discharge. For the terrace, the study mainly considers that rainfall can continuously recharge groundwater without cutting of the connection between the rainfall and groundwater. Tis study also uses the method of soil stratifcation to calculate the depth when the rate of rainfall infltration into the wetting front is zero to determine the lower limit value of the terrace underground water pipe control. In addition, the theoretical calculation formula of the maximum height of capillary water rise is used to determine the control index of groundwater depth that causes soil salinization. According to diferent rainfall conditions, the upper and lower limits of the groundwater level control indexes in diferent regions are calculated to defne the control targets of diferent geomorphic units. Finally, reasonable groundwater level control indexes for diferent regions in Xi’an are delimited by using the geomorphic units