M. KING HUBBERT AND THE RISE OF QUANTITATIVE HYDROGEOLOGY

M. King Hubbert was the single greatest influence on the development of the quantitative study of groundwater hydrology. His penetrating thought experiments recast the theory of groundwater flow in physically meaningful terms, rescuing the field from a morass of internal inconsistencies. Yet he published only a few papers in the field, most notably his 1940 masterpiece in Journal of Geology that ran 160 pages.

Lacking training and experience in hydrogeology, Hubbert would later write that his qualifications consisted of “intellectual curiosity and ignorance”. Expert in electrical resistivity surveys, he seized upon the analogy between electrical conduction and groundwater flow to re-interpret the results of Henry Darcy’s laboratory experiments, published in 1856, that gave rise to Darcy’s law in its empirical form.

Hubbert realized that groundwater flows in response to the gradient in a scalar quantity, just as electrical current arises from a voltage gradient. To take hydraulic head as this quantity, he noted, is akin to confusing the height to which mercury rises in a thermometer with temperature. He showed the drive for flow to be the gradient in hydraulic potential, the mechanical energy content per unit mass of groundwater. He recognized that a fluid’s hydraulic potential exists at every point in space, whether the fluid is present or not. This abstraction allowed him to approach the problem of seawater intrusion, and later, by an inverted analogy, petroleum entrapment.

The theory of flow thus rigorously conceptualized, a number of points of imprecise thinking in the hydrologic literature became clear: use of the pressure gradient as the drive for flow, failure to measure head with respect to a consistent datum, use of head gradient in problems of varying fluid density, and use of a velocity potential to predict flow patterns in heterogeneous and isotropic media. Not one to suffer fools gladly, Hubbert railed against the imprecise thinking he found in the writings of the foremost hydrologists and petroleum engineers of his day, creating lifelong professional animosities. But Columbia University, having denied this cantankerous and prickly genius tenure more than four decades earlier, invited him back to campus in 1981 to receive the Vetlesen award, the highest honor for an earth scientist.