August Colding contributed the first law of thermodynamics, evaporation from water and grass, steady free surfaces in conduits, the cross-sectional velocity distribution in conduits, a complete theory for the Gulf Stream, air speed in cyclones, the piezometric surface in confined aquifers, the unconfined elliptic water table in soil between drain pipes, and the wind-induced set-up in the sea during storms.

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.

In northern China, rivers run dry and groundwater tables drop, causing economic losses for all water use sectors. We present a groundwater-surface water allocation decision support tool for cost-effective long-term recovery of an overpumped aquifer. The tool is demonstrated for a part of the North China Plain and can support the implementation of the recent China No. 1 Document in a rational and economically efficient way.