Temporal and spatial evolution of layered subsidence in Hengshui City and its response mechanism to environmental factors

[Objective]Affected by climate change and human activities, the groundwater resources in Hengshui area are in an over-exploited state all year round, which directly leads to severe ground subsidence, which has become one of the main geological disaster problems in this region. [Methods]Based on the accumulated settlement data of the main urban area of Hengshui City from January 2009 to December 2022 obtained from layered monitoring, this paper analyzes and studies the ground subsidence mechanism of three compression layers (F1, F2, F3 layers from shallow to deep). Firstly, the Gompertz model is used to fit the cumulative settlement of each compression layer. The first-order derivative of the fitting results is obtained to obtain the settlement rate of each layer. With 0.5mm as the threshold, the settlement growth, rapid settlement rate growth, slow settlement rate decline and stable settlement period of each layer are obtained. Then, the zero growth (ZG) model is used to divide the settlement of each layer into irreversible settlement period (GRC) sequence and reversible rebound period (SWD) sequence, and analyze the fluctuation characteristics of settlement and rebound periods. Finally, the linear mixed model (LMM) is used to analyze the contribution of environmental factors such as precipitation, evapotranspiration, shallow groundwater level and deep groundwater level to the compression settlement (GRC_rate) during each layer's settlement period. [Result]The results show that: (1) The cumulative settlement fitting curves of each layer show an "S" shape, and the settlement rate curve shows a single peak pattern, with a "slow-fast-slow" trend. The main settlement period start time, end time, cumulative settlement and settlement rate of each layer are different; (2) In terms of cumulative settlement and monthly settlement increment amplitude, F3 layer＞F2 layer＞F1 layer. In terms of rebound amplitude, F1 layer＞F2, F3 layer; (3) Evapotranspiration and shallow groundwater level contribute most to the compression settlement of F1 and F2 layers, while shallow groundwater level and deep groundwater level contribute most to the compression settlement of F3 layer.