In the middle-to-late Holocene, Earth’s monsoonal regions experienced catastrophic precipitation decreases that produced green to desert state shifts. Resulting hydrologic regime change negatively impacted water availability and Neolithic cultures. Whereas mid-Holocene drying is commonly attributed to slow insolation reduction and subsequent nonlinear vegetation–atmosphere feedbacks that produce threshold conditions, evidence of trigger events initiating state switching has remained elusive. Here we document a threshold event ca. 4,200 years ago in the Hunshandake Sandy Lands of Inner Mongolia, northern China, associated with groundwater capture by the Xilamulun River. This process initiated a sudden and irreversible region-wide hydrologic event that exacerbated the desertification of the Hunshandake, resulting in post-Humid Period mass migration of northern China’s Neolithic cultures. The Hunshandake remains arid and is unlikely, even with massive rehabilitation efforts, to revert back to green conditions.

Electromagnetic tomography and electrical resistance tomography (EMT/ERT) dual-modality sensors can combine the advantages of single-modality sensors and detect the distribution of a three-phase medium in a gas-liquid-solid three-phase fluidized bed containing magnetic catalyst. However, the detailed design and optimization of the structure of the EMT/ERT dual-modality sensor arranged in the same cross-section are lacking. In this paper, this problem is solved from the perspective of mutual interference with both EMT and ERT. In numerical simulations, the boundary measurement data and the quality of reconstructed images are compared between single-modality and dual-modality. It is concluded that the EMT has no interference with the ERT, whereas the EMT is interfered by the ERT with the increase of duty ratio. In addition, it is determined that the position of the center of the coil facing the center of the electrodes and the electrode duty ratio of 0.5 are the optimal dual-modality structure. A dual-modality sensor experiment is carried out, and the optimal structure of dual-modality sensor is further validated.

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