In my area, I think drought monitoring would be a very  severe problem in the future and more attention should be paid.

To me, human impacts. With changing time, climate variability is playing a dominant role in changing the hydrological systems, but I think human impacts are far more important than those of climate. We analyse historical hydrological data  without really realizing how much change is due to climate and how much is due to human impacts. Unless these two components are separated, I don't think we could really define some useful policy framework, which is basis of any hydrological research. 

The frontier in hydrological science is completing the transition from closed system science using alpha-numeric analysis (the hydro knowledge paradigm) ~ to complex open system science using true imagery merged with sound software and real time telemetry (hydro cultural intelligence paradigm) ~ including trans-disciplinary learning by doing research and development teams ... eg participatory watershed iGiS ...