July 22, 2004

50 Years of Unsaturated-Zone Research in Menlo Park

David A. Stonestrom

When the U.S. Geological Survey (USGS) established a research center in Menlo Park, subsurface hydrology knowledge of the unsaturated zone, that portion of the hydrologic cycle between the land surface and the regional water table, was notably deficient. By 1954, it was recognized that ground-water availability, flooding, and other water-management issues depended on unsaturated-zone processes such as infiltration, drainage, and redistribution. These processes were seen as especially important in western states, where unsaturated zones reach hundreds of meters thick. Agronomists had studied unsaturated zones, but focused attention on the portions occupied by roots. Ground-water hydrologists studied saturated zones, extending downward from the water table. Between the bottom of the root zone and the water table was terra incognita, where observations and theory were lacking.

In 1963 the USGS brought a pioneer of infiltration theory, Jacob Rubin, to Menlo Park to direct studies on water movement in the unsaturated zone. By 1966 there were 16 research projects run from Menlo Park and Sacramento studying “arid-land hydrology, urbanization effects, soil moisture, infiltration studies, ground-water recharge, and subsidence.” All involved unsaturated flow, described by an equation lacking general analytical solutions. This contrasted with the situation for saturated flow, for which analytical solutions were in use by the late 1800s. Electrical-analog models were available for saturated groundwater systems by the 1960s. Neither approach could model unsaturated flow.

Unsaturated flow could only be modeled by numerical approximations that were impractical to compute by hand. Numerical methods were developed as early as 1952. Menlo Park joined the digital revolution in 1963, developing numerical models for infiltration and other unsaturated-zone processes. Numerical modeling of saturated ground-water flow systems followed. USGS personnel from Menlo Park developed classes at the National Training Center on numerical methods and unsaturated flow. In this way the digital revolution spread through hydrology, at home and abroad.

List of numerical-modeling and unsaturated-zone courses from 1978-1979 with Jacob Rubin.

Numerical-modeling and unsaturated-zone courses, from 1978-1979 training bulletin, with Jacob Rubin.

Along with numerical studies, laboratory and field studies applied unsaturated-flow theory to hydrologic problems. Inventions in Menlo Park supported this work. An early invention packed columns of soils for laboratory experiments. The device, patented in 1974, enabled fundamental studies of unsaturated flow and chemical transport. Another device, patented in 1987, used a centrifuge to accelerate unsaturated flow. This device extended Darcy’s law to low water contents. A gas-flow meter patented in 1990 extended Darcy’s law to air flow in the unsaturated zone. Data acquired with these devices helped develop predictive models for unsaturated hydraulic properties.


Column packer patent.	Centrifuge method & device patent.	Gas flow meter patent.

Other devices targeted field applications. Research improved on time-domain reflectometry (TDR), an electrical technique for determining water content. Menlo Park scientists extended TDR to the measurement of electrical conductivity, opening the way to chemical studies. A manuscript explaining principles and application of TDR was widely circulated inside and outside the USGS. Menlo Park scientists developed the first multiplexed TDR system for field deployment. Menlo research helped instrument manufacturers develop off-the shelf products, and TDR became preferred for water-content determination.


TDR electronics housing.	TDR signal trace.	TDR probe.

Menlo Park researchers developed tracer techniques to study unsaturated zones. One group of techniques used naturally occurring heat to determine water movement. Another used isotopic and chemical tracers to quantify contaminant migration, ground-water recharge, and response of unsaturated zones to climate change. An applied tracer experiment revealed rapid, kilometer-scale transport in unsaturated basalt terrain, unexpected on the basis of core-scale measurements.

Research in Menlo Park showed that unsaturated-zone flow affects saturated-zone properties determined by aquifer tests, and that these influences may also depend on observational scale.

Geophysical and remote sensing techniques have extended observational scales. New techniques have helped establish connections among geology, hydrology, and biology, at multiple scales through time. Similarly, new techniques for characterizing solids and solutes have helped explore the connections between unsaturated-zone weathering and ground-water quality.


Geophysical profiling across dry channel.	Electrical profiling across dry channel.

Genomics and structural biochemistry have opened a new frontier for understanding the role of living organisms in unsaturated-zone processes. Areas of investigation include the biological controls on fluid compositions in pristine and contaminated areas. Studies by Menlo Park researchers into degradation of spilled oil from a burst pipeline have ventured into this frontier. Ongoing multidisciplinary studies, facilitated by new technologies, will continue to improve understanding of flow and transport in unsaturated zones.

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