I often refer to hydrology as a place-based science
There are very many processes that can come into play in the hydrological cycle of which only a subset is relevant at any given location. There can be synergistic, neutral or antagonistic process interactions within any given subset. The net result is that every landscape requires local scale knowledge of what is actually happening in the transformation of precipitation into runoff.
We often don’t do a very good job of closing the water balance in these local systems because any residual is swallowed up in the uncertainties in our estimates of groundwater flux and evapo-transpiration. If we consider hydrology at a global scale then it can be worked on as a closed system, which should make it easier to close the water balance.
A recent article in Nature: ‘Source found for missing water in sea-level rise’ by Amanda Mascarelli is very thought provoking. This article is a summary of the findings of Pokhrel et al., 2012 and in Wada et al., 2012 about the discrepancy between the observed changes in sea level relative to the amount that is explainable by warming and glacier melt.
This discrepancy can almost entirely be explained by increases in groundwater extraction.
In other words, water is pumped from the ground and applied to the landscape from which it evaporates, runs off, or seeps back down to the groundwater store. Both evaporation and runoff are likely to wind up in the ocean.
The amount of water involved represents 0.00077 m per year of sea level change. Basically, this tells me that even very small trends in time-series that we are observing may be linked to very large-scale processes. This means that we need to be looking at how we collect and manage our environmental data in a way that is robust to the investigation of subtle trends.
The volume of water is about 280 km3, which may not seem like a heck of a lot on a global scale. However, this is one heck of a lot of, at least potentially, potable water that is being removed from supply in a freshwater scarce world.
This would also represent enough mass to explain why gravity measurements from space can detect groundwater depletion in areas like California and Northern India.
It may be that findings like this will result in larger scale efforts for artificial recharge of groundwater. It may seem like a good idea to offset extractive use during the dry season with artificial recharge during the wet season. Surface spreading techniques include: Bench terracing, contour bunds, gully plugs, check dams and percolation ponds; Flooding; ditches and furrows; recharge basins; and stream modification/augmentation. Sub-surface techniques include: injection wells; gravity head recharge wells; and recharge pits and shafts. In other words, there are many techniques for storing an excess of surface water in the ground.
In theory this is all good, solving the problems of having too much water during floods and too little during droughts. However, I would get really nervous about any attempt at doing this without very rigorous monitoring of both surface water and groundwater quality and quantity. Unfortunately, I can well imagine that there will be only enough money to develop these projects but not enough to ensure there is monitoring to ensure that the solutions are not compounding our existing problems.
Another aspect of this article that I find interesting is that the authors somehow managed to find enough data to conduct their studies. This is most incredible. Whenever I have tried to do data search discovery and access at a global scale I always end up with much less than I want and what I do have is highly biased to landscapes monitored by the USGS who are one of the few who really embrace data sharing. This research is an example of how data can help to develop an understanding of global scale problems. Exposing the data that is badly needed to manage our global water resource will take a much greater commitment to data sharing from all data providers.
Pokhrel, Y.N., N. Hanasaki, P. J-F. Yeh, T.J. Yamada, S. Kanae, and T. Oki. 2012 Model estimates of sea level change due to anthropogenic impacts on terrestrial water storage. Nature Geoscience. Advance online publication.
Wada,Y. L.P.H. van Beek, F.C.S. Weiland, B.F. Chao, Y-H. Wu and M.F.P. Bierkens. 2012. Past and future contribution of global groundwater depletion to sea-level rise. Geophysical Research Letters v. 9. L09402, 6pp. doi 1029/2012GL051230.