07 February 2011

New maps of surface permeability will aid hydrologic modeling

This story of newly published results came from press releases on 24 January 2011 from the ScienceDaily and PhysOrg.com science news networks.  A study by researchers at the University of British Columbia with co-authors around the world has just been published in the AGU journal Geophysical Research Letters describing a nested dataset of surface soil permeability to a depth of 100 m.  The spatial resolution works out to approximately 115 km for the global land-surface dataset and just under 10 km for the North American dataset.  This new product, along with recent datasets describing enhanced mapping of both surface watershed and subsurface aquifer maps, will be a boon to hydrologists and water resource practitioners seeking to describe better the processes of groundwater recharge, a hot topic as so much of the world's groundwater is now mined beyond the point of sustainability.
This new dataset, based on surface land cover and soil types and their underlying geology, follows quickly on the release of the most detailed dataset on land cover yet compiled, reported by ScienceDaily in March 2008.

As the releases state for this new subsurface permeability dataset:
"Using recent world-wide lithology (rock type) results from researchers at the University of Hamburg and Utrecht University in the Netherlands, Gleeson was able to map permeability across the globe to depths of approximately 100 metres. Typical permeability maps have only dealt with the top one to two metres of soil, and only across smaller areas.
"'Climate models generally do not include groundwater or the sediments and rocks below shallow soils,' says Gleeson. 'Using our permeability data and maps we can now evaluate sustainable groundwater resources as well as the impact of groundwater on past, current and future climate at the global scale.'
"A better understanding of large scale permeability of rock and sediment is critical for water resource management--groundwater represents approximately 99 per cent of the fresh, unfrozen water on earth. Groundwater also feeds surface water bodies and moistens the root zone of terrestrial plants."
Why is this so important?  Groundwater constitutes about 99% of the fresh, unfrozen water on the Earth's land surface.  It is often accessible, sometimes easily so, but still more costly to extract and deliver than surface water in rivers, lakes and reservoirs.  And even more to the point, surface water and groundwater are connected.  Yes, you read that right - despite whatever your state laws and property rights might say, or whatever the EPA rules are, or whatever some engineers decide is the easiest way to handle a project, the ground- and surface waters are two parts of the same system.  I belabor the point, which may seem obvious to scientists, because legislators and those who decide on the provenance of land and mineral and water rights just don't seem to see it in most cases. 

In the nascent and growing culture of modeling for prediction in water management, there is a significant need for expansive data resources on which to base modeling formulations and validate the results.  Because the subsurface is the poorest-observed branch of the water cycle, there is an acute need for better descriptions of the subsurface environment.  Because groundwater makes up such a large portion of human water use across much of the globe, and especially in the developing world, there is an acute need for accurate characterization of shallow groundwater behavior for both empirical reasons and for inclusion in water-cycle models.  Until we can accurately assess the regional dependence of a population on its surface and groundwater resources, we won't be able to characterize effectively either of these societal interactions with the natural and increasingly modified water cycle.  Without such attention, the numerical models that could be so powerful in our assessment of supply and sustainability will remain left out of their proper role in application to guidance on water usage and policy development.  This new map, with such an intentional methodology and detail put forth by the researchers, is a huge step in the right direction.

The formal citation of the study article is:
Gleeson, T., L. Smith, N. Moosdorf, J. Hartmann, H.H. Dürr, A.H. Manning, L.P.H. van Beek, and A.M. Jellinek, 2011: "Mapping permeability over the surface of the Earth." Geophysical Research Letters, v. 38, no. 2. DOI: 10.1029/2010GL045565
The authors hail from the University of British Columbia, the USGS, the University of Hamburg, and Utrecht University.  Support for the study was provided by the Natural Sciences and Engineering Research Council of Canada, the German Research Foundation, Utrecht University, and the USGS.

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