A physical model test has been carried out to study the groundwater dynamics, water balance, and deformation characteristics of the aquitard in response to an abrupt hydraulic head decline in the adjacent confined aquifer. The hydraulic head in the clay layer is initially constant. The hydraulic head in the lower sand layer declines suddenly by φ  at the beginning and then remains unchanged during testing. We measured the drainage flux at the bottom of the aquifer and the subsidence of the clay layer.This supporting information document provides: (1)Supplementary Table 1 (Table S1), which presents time series data of the deformation of the clay layer (mm); (2) Supplementary Table 2 (Table S2), which gives time series data of the fluxes at the bottom of the clay layer (cm³/s).

The hydraulic head in the clay layer is initially constant. The hydraulic head in the lower sand layer declines suddenly by φ  at the beginning and then remains unchanged during testing. We measured the drainage flux at the bottom of the aquifer and the subsidence of the clay layer. The testing steps are given as follows:

(1) Initial inspection of the testing device: The consolidation container must be ensured to be sealed, and the inlet and outlet pipes must not be obstructed. The testing instrument must be ensured to operate normally.

(2) Soil sample preparation: Measure the weight of sand and clay with an electronic balance scale. The inner wall of the consolidation container is cleaned and smeared with a layer of grease or Vaseline; water is injected into the consolidation container through the overflow tank and the water table is raised slowly; no bubble is allowed in the filter layer during the water injection process; the consolidation container is successively filled with sand layer, clay layer, and sand layer, and the device to measure the settlement is placed at the top of and below the clay layer after each soil layer is consolidated under its self-weight for a duration of 24 hours. Turn off all the valves except the exhaust valve, and pump out the air in the container with a vacuum pump; increase the negative pressure within the consolidation container gradually at a given pressure sequence (e.g., − 0.02, − 0.04, − 0.06, − 0.08, and −0.1 MPa); the time interval for maintaining each pressure level is 12 hours.

3) After the soil layers are saturated, water is added slowly to reach the target elevation; open the valve of the inlet pipe, and supply water into the device; mount the water supply tank at the designed height, and fill in it with water; discharge the excess water from the overflow nozzle; adjust the position of dial gauge, fasten it, and record the initial reading; observe the readings of the dial indicator until they do not vary. This process generally takes a few days.

(4) Record the readings of the dial indicator during testing. Start the test by opening the valve of outlet pipe; decrease the hydraulic head below the confined soil layer by φ= 1 m at the beginning of test, and then maintain it to be constant; at the same time, record the readings of the dial indicator to calculate the deformation (S) of clay layer (Table S1) and measure the flux from the outlet of overflow tank using an electronic balance (the water discharge is held with a measuring cup on an electronic balance) to calculate the flux (Q_b) at the bottom of the clay layer (Table S2). Observe the readings continuously until they do not vary, and the test is then terminated; the observation duration is 480 minutes; the flux becomes stable at the end of observation.