U.S. Geological Survey core science systems strategy: characterizing, synthesizing, and understanding the critical zone through a modular science framework - Bristol, R. Sky; Euliss Jr., Ned; Booth, Nathaniel L.; Burkardt, Nina; Diffendorfer, Jay; Gesch, Dean; McCallum, Brian; Miller, David M.; Morman, Suzette; Poore, Barbara; Signell, Richard; Viger, Roland J. (Circular, 2013)