NSF  Hydrologic Sciences  
Directorate for Geosciences   Division of Earth Sciences
Updated to  NSF 22-540    https://www.nsf.gov/pubs/2022/nsf22540/nsf22540.htm
NSF 20-560  https://www.nsf.gov/pubs/2020/nsf20560/nsf20560.htm

Accepted anytime.

The Hydrologic Sciences Program supports basic research on the fluxes of water in the terrestrial environment that constitute the water cycle as well as the mass and energy transport function of the water cycle. The Program supports the study of processes including (but not limited to): rainfall, runoff, infiltration and streamflow; evaporation and transpiration; the flow of water in soils and aquifers; and the transport of suspended, dissolved, and colloidal components. The Program is interested in how water interacts with the landscape and the ecosystem as well as how the water cycle and its coupled processes are altered by land use and climate. Studies may address physical, chemical, and/or biological processes that are coupled directly to water transport. Observational, experimental, theoretical, modeling, synthesis and field approaches are supported. Projects submitted to Hydrologic Sciences commonly involve expertise from physical and ecosystem sciences, engineering and/or mathematics; and proposals may require joint review with related programs.