It gives us great pleasure to introduce this Geophysics special section on hydrogeophysics, which is jointly sponsored by the Society of Exploration Geophysicists (SEG) and the American Geophysical Union (AGU). Significant advances have been made in the field of hydrogeophysics since the last special section on this topic in 2010, in particular with regard to data acquisition, analysis, and applications. In addition, there has been a realization within the geophysical community of the value of hydrogeophysics, as evidenced by the collaboration between SEG and AGU that made this special section possible. Hydrogeophysics is the integration of hydro(geo)logic and geophysical data sets collected at multiple scales in time and space to improve the understanding of water in the subsurface. Ground-based, airborne, and spaceborne geophysical data sets can provide important information for advancing our fundamental understanding of hydrogeologic systems at multiple scales in time and space. These data enable improvements to hydrologic conceptual model development and groundwater management decision making. Three papers published in the special section illustrate the value of electrical resistivity tomography (ERT) data for studying hydrobiogeochemical processes over time. Rickel et al. explore the effects of ferricrete on seasonal groundwater-surface