The most direct and most generally reliable measurements of unsaturated hydraulic conductivity are from steady-state flow methods. These methods are seldom applied, however. In the simple gravity-driven implementations they have serious drawbacks: limitation to the wettest soil conditions, and slowness -- sometimes requiring months for a single K measurement. The Steady-State Centrifuge (SSC) method extends the range to lower steady-state K measurements by at least three orders of magnitude, and allows at least six points of the unsaturated K relation with water content to be characterized for a pair of samples in about five days. This method originated in our laboratory (U.S. patent no. 4,679,422) in the early 1980s and has been further developed by the Unsaturated-Zone Flow Project since that time. The SSC data of the introductory paper (Nimmo and others, 1987) and two later papers (Nimmo and Akstin, 1988; Nimmo and others, 1994) have made possible the development of hydraulic conductivity models specifically for conditions of low water content. Other developments include improved SSC apparatus for fine media and lower K values (Nimmo and others, 1992), and extension to saturated K, where this method is especially useful for tight media (Nimmo and Mello, 1991).
Our work on the SSC made possible a commercialized version of the apparatus (right) (Conca and Wright, 1998), now used in universities and other institutions for unsaturated zone measurements. This apparatus has water input from metering pumps (see photo above) connected to a rotating fluid contactor, which results in faster K measurements over a wider range of water contents. Below is a photograph of a sample retainer which is held within the sample holder. The sample holder is then inserted into the rotor and held by the sample bucket. Flow is delivered to the sample by a metered pump and outflow is collected in the effluent cup. We have developed new procedures and have identified operating options and conditions needed to produce valid results with this new apparatus (Lewis and Nimmo, 1998). We use this system routinely in current research. The SSC method is the basis for recharge studies using hydraulic properties of the deep unsaturated zone, and for a variety of investigations at low water contents.
The original motivation for developing the SSC method and still one of its most prominent applications is for the Darcian method for high-quality estimation of unsaturated water flux, in some cases interpretable as a recharge rate. This method applies Darcy's law at a point of sampling. It requires (1) an accurate unsaturated hydraulic conductivity (K) measurement at the known field water content of a core sample, and (2) knowledge of the driving force for water flow at the point of sampling. We obtain the necessary K measurements using the SSC method (Nimmo and others, 1994), the combined procedure referred to as the Darcian-SSC method. For a recharge rate estimation indicative of the long-term average at a point, samples should come from deep enough in the unsaturated zone that gravity is the only significant driving force. The description of research at our Palouse site, the first fully successful implementation of the combined Darcian-SSC method, gives additional details about the method in general.
| National Research Program | USGS | Water Resources | UZ Flow Homepage |