Study on non-linear rainfall infiltration recharge of numerical karst water model based on MODFLOW-CFP：A case study of Xianghualing area, Hunan Province

Carbonate rocks are widely distributed in southwest China, and these strata constitute sophisticated karst aquifers, which are local important water and tourism resources. However, groundwater system has strong heterogeneity there, which makes it difficult to characterize subterranean runoff. At the same time, the characteristic that the karst aquifer can be recharged by rainfall infiltration through different ways has not been considered in most of the numerical simulation work. The study of karst groundwater and solute transport plays a key role in the development and utilization of karst groundwater and in the prevention and control of pollution. The purpose of this study is to discuss the relationship between rainfall intensity and rainfall recharge infiltration coefficient in the numerical model, and to use practical method to deal with the variation of coefficient to make the simulation more accurate.The Xianghualing area is located in Chenzhou City of Hunan Province, with low latitude. The aquifer in the study area is composed of Carboniferous limestone. It is a relatively independent karst basin surrounded by mountains with an area of about 14km2, and the surface is covered by luxuriant vegetation. The average annual temperature is 17.9 °C. The annual rainfall ranges from 1，000 mm to 2，400 mm, but most of the rainfall events usually occurs in summer, and there are often rainstorms between July and October. The groundwater level fluctuates with the change of rainfall conditions, and the groundwater table is too low in most areas that the evaporation barely exists. The main artificial extraction of groundwater are mine drainage, the drainage tunnel intersects with the underground river in the discharge area.According to the binary theory of karst system, the karst aquifer can be generalized into two subsystems. The fissure can be generalized into the equivalent porous media system recharged by diffuse infiltration. It is assumed that the groundwater moves in the form of seepage. Underground rivers and karst large-size passages are generalized as conduit system recharged by point infiltration, where is the main discharge pathway of karst groundwater system. MODFLOW-CFPM1 was used to develop the equivalent pore-conduit coupling model in Xianghualing, and its CRCH module was used to simulate the centralized point infiltration. Then the nonlinear infiltration process was represented by rainfall coefficient method and its applicability in the model was well discussed. Depend on data size, six rainfall coefficients of different rainfall intensity intervals were set up and obtained by artificial trial-and-error inversion parameters. After the nonlinear infiltration treatment, the Nash coefficient of the simulation results of underground river flow is increased to 0.91. The results turn out that the rainfall coefficient method can make the model better simulate the change of underground river discharge, and the method is also suitable for the numerical simulation of some karst small watershed.