Interesting that you should ask this. I have been pondering the subject myself. My conclusion is that our computer abilities exceed our physical hydrogeology skills. We have wonderful models that have ridiculous parameters for real-world materials, yet by manipulating these parameters some semblance of the real picture emerges. What if we actually built models with real parameters and manipulated the model code to produce a model?
Secondly, hydroGEOlogy implicitly involves the earth. HydroLUNology, HydoARESology, etc. - these are the subjects that new hydro___ologists will explore. What fun!!!!
Possibly the topics might consider hyporheic interchange and functions, location and management associated with groundwater recharge zones, groundwater water rights, uses and policies, affects of sea level rise on salt water intrusion into coastal aquifers, effects of fracking and other disturbances. LiDAR, GPR, GIS, well testing and monitoring are just a few of the tools that continue to be applied to expand influence of field data. Why are not more wells instrumented and monitored as to their hydrologic responses?
Good point, William. Another thing that comes into my mind is the use of isotopes (radioactive and stable) as tools for hydraulic connections, tracers, recharge zones, etc.