The International Union of Geodesy and Geophysics (IUGG) was established on 28 July 1919 in Brussels, Belgium at the Inaugural General Assembly of the International Research Council (now the International Science Council) to promote activities of already-existing international scientific societies dealing with geodesy, terrestrial magnetism and electricity, meteorology, physical oceanography, seismology, and volcanology.

To celebrate the centenary of IUGG a special issue was compiled in the open-access journal History of Geo- and Space Sciences.
The International Union of Geodesy and Geophysics: from different spheres to a common globe (https://www.hist-geo-space-sci.net/special_issue996.html)
Editor(s): A. Ismail-Zadeh and J. A. Joselyn

The special issue (SI) is comprised of 11 articles describing the history of the International Union of Geodesy and Geophysics (IUGG) and its eight International Associations. The SI is dedicated to the centennial anniversary of the Union, which has been coordinating and promoting international efforts in Earth and space sciences since 1919.
The purpose of this SI is not to relay a definitive history of the development of international cooperation in geosciences, but to summarize IUGG's remarkable role during the 100-year span of its existence. The first three articles present the history of IUGG from its founding through the years of extraordinary development of geophysical sciences and the concurrent evolution of the Union's structure and programs extending up to its advances at the beginning of the 21st century. Outstanding histories of each of the IUGG Associations are then presented.

Two articles are of special relevance to IAHS:

IAHS: a brief history of hydrology
Dan Rosbjerg and John Rodda
Hist. Geo Space. Sci., 10, 109-118, https://doi.org/10.5194/hgss-10-109-2019, 2019

Short summary
After describing the hydrological cycle and defining hydrology in the introduction, the early historical development of hydrology is briefly presented. Then the incorporation of hydrology within the IUGG and the subsequent development of the association are described chronologically. Finally, in the conclusions, the present state of the association is discussed together with an outlook for the future.

And

IACS: past, present, and future of the International Association of Cryospheric Sciences
Ian Allison, Charles Fierz, Regine Hock, Andrew Mackintosh, Georg Kaser, and Samuel U. Nussbaumer
Hist. Geo Space. Sci., 10, 97-107, https://doi.org/10.5194/hgss-10-97-2019, 2019

Short summary
The International Association of Cryospheric Sciences (IACS) became the eighth and most recent association of IUGG in July 2007. IACS was launched in recognition of the importance of the cryosphere, particularly at a time of significant global change. The forbears of IACS, however, start with the 1894 Commission Internationale des Glaciers (CIG). This paper traces the transition from CIG to IACS; scientific objectives that drove activities and changes, and key events and individuals involved.

More information on the history of IAHS can be found at https://iahs.info/About-IAHS/History-of-IAHS.do