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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 Dear Tom: I will try to address you questions. Question: I am curious how I might easily convert between the two without doing some gravimetric testing to establish the soil true capacity. Answer: As you pointed out in your text both devices measure two different things; at the same time they can be compared but not directely. As you know the Diviner2000 measures the soil water content; however, Watermarks determine the soil water potential (that is why you read centibars). As you know, there is a relationship between soil water content (Diviner2000) and soil water potential (Watermarks) this relationship is translated in what is known as soil water release curve (SWRC) . In other words, (assuming no hysterisis effect) for each water content there is a corresponding matric potential. Thus, if you know the matric potential you can determine the corresponding water content and vis-versa. Unless you have a soil water release curve for the soil in question I do not think you can go from one measurement to the other. If you are lucky you might find that the soil type you are using has already a soil water release curve data available in the state soil survey. For example there are soil characterization data, including soil water release curve, for almost every soil type that exists in the State of Florida. You can check with you country agent for that and you might be able to find out. Caution: this is just an approximation exercise to be more specific you need to establish the SWRC for the same site; in addition, you have to calibrate both devices for the same soil too to be able to have accurate results. Please feel free to contact me if you need further help. sincerely, Yours. A. Fares, Ph.D., Hydrologist/Soil Physist Sentek, Pty, Ltd 77 Magill Road, Stepney, Adelaide, South Australia 5069 Australia > -----Original Message----- > From: owner-sowacs@aqua.ccwr.ac.za [SMTP:owner-sowacs@aqua.ccwr.ac.za] > Sent: Tuesday, April 04, 2000 4:29 AM > To: AFares@Sentek.Com.Au > Subject: Volumetric vs Gravimetric > > 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 have done some moisture monitoring with Watermarks in the past and now > have use of a Diviner 2000 from Sentek. I wish to be able to compare > previous years data to current data. One problem I see off the bat is that > the Watermarks read in centibars while the D2K reads in mm of water in a > profile. I am curious how I might easily convert between the two without > doing some gravimetric testing to establish the soil true capacity. Even > then, without testing through many matric potentials to establish a proper > curve, how could I be sure that each unit was responding to soil moisture > in a similar fashion?? > Tom Thomson Northwest Agricultural Consulting > 1275 Oak Villa Road tomt@teleport.com > Dallas, Oregon 97338 Phone/FAX 503-623-0468 > > "The only difference between a problem and a solution is that > everyone understands the solution." Charles Kettering > > [Why not compare the two sensors directly in a test bed and allow it to > dry > across the range? Bruce]