system
Given the following post, respond as if you're commenting on Reddit:
human
How would the water cycle be affected if we were to switch to hydrogen as a fuel for the majority of cars?

Would there be a net change in the amount of water on the planet? What would be the ramifications long term (100 years, or more into the future)?
gpt
It depends on the source of the hydrogen. If hydrogen is obtained through electrolysis of water then the net change would be zero. However electrolysis of water is a relatively expensive way of obtaining hydrogen. 

Currently the most common way of obtaining hydrogen is through refining natural gas (natural gas reforming) or biomass gasification. Both process requires water in their chemical process However neither is carbon neutral and the refining process produces CO and CO2 so neither process really abates the accumulation of greenhouse gases. 

The hope is that renewable energy can be made cheap enough that electrolysis of water can be done economically to displace fossil or biomass sources of hydrogen. There are possibilities of genetic engineering bacteria’s that produces hydrogen as a byproduct through some kind of biological process which would be carbon neutral but this is still an area of active research.

All of that is the preface and context needed to answer your question. The amount of changes to the water cycle depends on how the hydrogen is primarily going to be sourced in the future. Both natural gas reforming and biomass gasification are water intensive chemical processes and may have impact on the local water source near where the refineries are located. On a planetary scale the consumption of natural gas derived hydrogen means an effective mass transfer of hydrogen in the form of hydrocarbons to the form of water. The amount of hydrocarbons on the earth is pretty insignificant compared to the amount of water on the planet so the net change in water through these process would be negligible (my assumption as I’m an engineer and not a geologist; please correct me if I’m wrong here).

If hydrogen is sourced from electrolysis using renewable energy then the local impact on the water cycle will again be dependent on where the water sources are drawn from. On a planetary scale there will be no net change in the amount of water on the planet. The water to hydrogen+water back to water cycle is essentially a means to store and transport the energy used for electrolysis.