## 1 Introduction  
A tailings pond refers to the places where dams are built to intercept valleys or enclosures to stack the tailings or other industrial waste residues from metal or non-metal mines after ore separation. A tailings pond is one of the three basic projects of mining facilities and is also a major hazard source of mining projects. Tailings pond accidents occur from time to time all over the world, causing significant damage to the economy and, downstream, people's lives and properties [1, 2, 3, 4, 5]. The tailings dam of the Corrego do Feijao iron mine in Brazil broke on 25 January 2019, with a dam breaking volume of 12 million m\({}^{3}\), causing more than 230 deaths and serious environmental pollution [6]. Compared with a traditional hydraulic-filled tailings pond, a dry-stack tailings pond has become a tailings storage form strongly advocated by the Chinese government [7] due to its obvious advantages of safety and environmental protection [8, 9]. In the design of a tailings pond, seepage field analysis is an important basis for the design and stability analysis of tailings ponds [10, 11, 12]. China's current Code for Design of Tailings Facility (GB50863-2013) does not specify the calculation method of seepage field analysis of dry-stack tailings ponds [13], which brings uncertainty to the design of dry-stacktailings ponds. As there is no standard method for calculating the seepage field, discharge pipes are usually set in the dam body of dry-stack tailings ponds to control unpredictable phreatic lines. When the phreatic line is very low, it may be a waste. Therefore, accurate determination of the seepage field in a dry-stack tailings pond can optimize its design and save costs. The saturation line in the standard seepage field is the lifeline of a tailings pond [13], and accurate determination of the seepage field is crucial to the design of the slope of the dry-stack tailings pond. In brief, the study on the seepage field of dry-stack tailings ponds under rainfall is very important to guide the design of dry-stack tailings ponds. For a dry-stack tailings pond, there is no water stored. Hence, the seepage field is mainly affected by rainfall infiltration [14; 15]. In the analysis of seepage field of dams or slopes under rainfall, differential equations can be established based on seepage theory [16], and the boundary conditions and initial conditions can be determined according to the actual situation. The seepage field can be solved by theoretical derivation [16; 17] or numerical simulation [18; 19]. The distribution characteristics and evolution law of the seepage field can be reflected by indicators such as pore water pressure [20] and the change in groundwater level (phreatic line) [20; 21]. The above research methods have been widely used in the stability analysis of slope seepage under rainfall [17; 18]. [PERSON] [17] used the theoretical analysis method to determine the saturation line equation of the slope, but the saturation line equation is applicable to specific boundary conditions. [PERSON] [18] used the finite element method to analyze the distribution law of seepage field of slopes under different rainfall modes. In general, the theoretical method to determine the seepage field is suitable for simpler boundary conditions [16]. Correspondingly, the numerical simulation method can be applied to complex boundary conditions and initial conditions, which is why it is more widely used [18; 19]. At present, there are many achievements in slope seepage and stability analysis under rainfall. Relevant literature shows that different rainfall patterns [18; 22], amounts of rainfall [23], rainfall periods [20], prophase rainfall [24], soil materials [25; 26], and different slopes [27] have a greater impact on the slope seepage field under rainfall. Compared with general slopes, there are only a few studies focused on the seepage field of dry-stack tailings ponds under rainfall.