Abstract: In the groundwater flow-land subsidence model based on MODFLOW-SUB, the vertical hydraulic conductivity of clay interbed is set as a constant. However, during the compression process of clay interbed, its vertical hydraulic conductivity will change accordingly, so the prediction of long-term land subsidence by using this model may be not realistically suitable. In this study, a variable hydraulic conductivity model was established through improving the source code of SUB combined with the change law of vertical hydraulic conductivity and the head of clay interbed during subsidence. Taking the typical structure of Antelope Valley in California established by the USGS （U.S. Geological Survey）as an example, the constant hydraulic conductivity model and the variable hydraulic conductivity model were used to simulate the process respectively. The results show that the two models have good consistency in the first 20 years of mining. With the continuous development of mining, the accumulated subsidence calculated by the variable system model is gradually smaller than that calculated by the constant model, and the accumulated subsidence in 80 years decreases by 15.6%. With the increase of mining amount, the time at which the two models differed is advanced and the final subsidence difference value is increased. With the increase of the interbed thickness, the time the two models differed gradually delays but the final subsidence difference value increases. The results indicate that both the constant model and the variable model have good applicability in the early stage of mining with desirable accuracy, but with the extension of mining time, the variable model can better reflect the actual compressive subsidence process.

Key words: land subsidence model, vertical hydraulic conductivity, variable element, source code improvement