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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs We have found it useful to use both terms, Total water holding capacity (which includes the water still in the soil at the wilting point) and Plant-available water holding capacity. These are different from the concept of field capacity (which also can be just plant-available, or including wilting level), which allows the water to drain gravitationally. The total capacity is reached when all the pores are full and the water table is at the surface. Typically, the field capacity is 2/3 of the total capacity. Please see: Robock, Alan, Konstantin Ya. Vinnikov, C. Adam Schlosser, Nina A. Speranskaya, and Yongkang Xue, 1995: Use of midlatitude soil moisture and meteorological observations to validate soil moisture simulations with biosphere and bucket models. J. Climate, 8, 15-35. http://climate.envsci.rutgers.edu/robock/robock_pub9.html which discusses these concepts and Robock, Alan, C. Adam Schlosser, Konstantin Ya. Vinnikov, Nina A. Speranskaya, and Jared K. Entin, 1998: Evaluation of AMIP soil moisture simulations. Global and Planetary Change, 19, 181-208. particularly Figure 6: http://climate.envsci.rutgers.edu/robock/amipsm/smfig6.html which shows plant-avaiable soil moisture and field capacity, including regions wher the soil moisture routinely exceeds the field capacity. Alan Robock Professor Alan Robock Department of Environmental Sciences Phone: (732) 932-9478 Rutgers - The State University of New Jersey Fax: (732) 932-8644 14 College Farm Road E-mail: robock@envsci.rutgers.edu New Brunswick, NJ 08901-8551 http://www.envsci.rutgers.edu/~robock On Mon, 21 Feb 2000 owner-sowacs@aqua.ccwr.ac.za wrote: > NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za > with the body of the message containing the line: > unsubscribe sowacs > > I know that this is "old ground", but I need help in clarifying > definitions. The Irrigation Association is currently refining a glossary > of irrigation terminology. The glossary includes soil water terms. > > My question concerns the best definition to use with the term "water > holding capacity." In my mind, water holding capacity is defined as > the amount of water that the soil can "hold" above oven-dry. It is > expressed in terms of mm/m (in the US as in/ft). > > WHC contrasts with the term "available soil water" in that available > soil water is defined as the difference between field capacity and > wilting point. > > I realize and appreciate that water holding capacity is an old and > perhaps outdated term, as well as are field capacity and wilting point. > These latter two have been redefined as dynamic rather than static > values and have sometimes been given the names drained upper limit > and crop extractable limit, etc. > > My main question is whether the above definition for WHC is valid > and widely held, or if it should be modified. > > WHC is contrasted with AM as indicated by their calculation: > > In the classical approach: > AM = FC - WP > WHC = FC > > I believe that it is useful to have a term as WHC that describes the > total amount of measurable water that we can expect to see retained > in a soil profile. This is useful for comparing against methods that > measure total water such as neutron meter, TDR, etc. In addition, > many old soil surveys contain soil water holding information that is > often equivalent to the above definition of WHC. Therefore, it is useful > to keep a term around that is compatible with the old data. > > Thanks. > > Rick Allen > Univ. Idaho > > > >