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Dear Bruce and All: I am glad that more people, including myself, want to learn, get familiar and share their own results when studying the axial and radial zone of influence of the soil water content sensors at the interface of different media (soil, air, water). After 50 years of calibration of neutron scattering probes it is still refreshing to read from the "Soil water assessment by the neutron method" edited by E. L. Greacen (20 contributors), published by CSIRO-Adelaide Australia in 1981, about the zone of influence in the soils with different values of water content, bulk densities and at the air-water and air-soil interface. Williams et all., describing limitations of the neutron method in the chapter "Application in agriculture, forestry and environmental science" concluded: "In addition the radius of the sphere of influence is dependent on water content and this again is a constraint in the studies which require detailed information on the form of rapidly changing water content profile.", and " It is important to choose the method of measurement of water content which operates at the same scale as the analysis adopted in the investigation". It is hard to find in recent published papers the methodologies used for calibrating of neutron probes close to air-soil interface or their usefulness, when compared to the new sensors based on the measurement of the dielectric constant of the soil-water (chemicals)-air mixture surrounding them. In my report to the USDA-ARS-Water Management Research Laboratory, Fresno, California (1993) entitled "Radioactive vs. non radioactive calibration methods for real-time measurement of soil-water-plant-atmosphere interrelationships under high evaporative demands" the results of axial zone of influence at the air-water interface showed a decreasing influence of air which stabilizes at a distance from the sensor's center of measurement to the water surface of: approximately 22.5 cm for both the neutron probe (Troxler 4301) and for the capacitance probe (Troxler Sentry 200 AP) in the same PVC access tube; and about 6.5 cm for the capacitance sensor (Sentek-EnviroScan) in its own access tube. Literature on the direct measurement of axial and radial zone of influence of TDR and capacitance probes at the soil, air and water interface is not abundant, methodologies are sometimes similar or different, but they show the same pattern of a non linear response. In the new book "Geomeasurements by pulsing TDR cables and probes" by O'Connor and Dowding, published by CRC Press in 1999, a subchapter was dedicated to the "Electric field distribution and sampling volume around parallel rods". Based, mainly on the work done by Topp and Davis (1985), and by Baker and Lascano (1989), the authors of the book are showing that: "As the size of parallel rod probe (length and spacing) is increased, the resolution of the TDR system decreases" and "Thus compromises must be made depending on the specific measurement required". The authors reffered to Topp and Davis (1985) "who found that parallel-rod transmission lines with center-to-center spacing of about 50 mm are a practical compromise" and their work showed that the volume of soil measured by TDR was essentially a cylinder whose axis lies midway between the rods and whose diameter is 1.4 times the spacing between the rods. This gave a cross section of 3800 mm2 for rods spaced 50 mm apart". Also was cited the work of Baker and Lascano (1989), who used stainless steel rods 3.2 mm in diameter, 300 mm long, spaced 50 mm apart in a matrix of glass tubes, and showed that: "the sensitivity was found to be largely confined to a region with a cross-sectional area of approximately 1000 mm2 (20 mm by 65 mm) surrounding the waveguides, although a limited sensitivity extends much further, enclosing 3600 mm2 (60 mm by 80 mm) which was consistent with Topp and Davis (1985)". Also, "there was no discernible variation in sensitivity longitudinally, i.e., along the length of the waveguide". Literature on the axial and radial sensitivity of capacitance probes is not abundant and some of it was cited in our article (Dr. Starr and I) "Real-time soil water dynamics using multisensor capacitance probes: Laboratory calibration", published in the SSSAJ, 61:1576-1585 (1997). In order to draw his own conclusions from our results someone should read the paper entirely, including methods, materials and formulas. The axial zone of influence of the EnviroScan capacitance sensors (designed by Sentek Pty Ltd. to oscillate in excess of 100 MHz inside access tube in free air) at the air-water, air-soil interface was determined by using frequency units (Fa for air), (Fw for water), (Fs for soil). The zone of influence from the center of measurement to the soil ar water surface was approximately the same (50 mm), which means a total axial zone of influence of 100 mm. In order to study the radial zone of influence our paper describes in detail how we cut soil columns (cakes) circularly, >from the outside toward to the access tube columns, increasing the proportion of air in the zone of influence of sensors. We used a scaled frequency ratio (SF/SFmax.) formula to interpret the data with SF= (Fa-Fs)/(Fa-Fw) and SFmax=(Fa-Fsmax)/(Fa-Fw). Finally the SF/SFmax depends on the increasing values of Fs (by cutting the soil column, the air-soil proportion is increased in the total zone of influence of sensors). At the limit the values of Fs become closer, and finally equal to Fa which makes the numerator (Fa-Fs) equal to zero and the ratio SF/SFmax also zero. It is fair to say that is hard to keep a very thin column of soil around the access tube. This is why in Fig. 7. the SF/SFmax values continually decrease approaching zero. Our first results were conducted at only two volumetric water content values, shown in Fig. 7. Cutting the soil column circularly, from a thickness of 18 cm toward the access tube, Fig 7 is showing a continuous decrease of the SF/SFmax ratio, which means that the EnviroScan capacitance sensor is sensitive enough to air-soil ratios even at 10 cm from the access tube (SF/SFmax=0.99), and increasingly sensitive to air-soil interface, as the soil column becomes closer to the access tube. At 4 cm from the access tube the SF/SFmax =0.95. This trend is in agreement with the results of Kuraz (1982), who was working with a cylindrical capacitance sensor, but at initial frequencies of 60 MHz. By inserting the sensor in a set of glass vessels (with diameters from 7.7 to 18.7 cm) and filled with sand at different gravimetric water content, he concluded that: "On the base of these results it can be stated that with the developed apparatus, soil moisture may be measured in an annulus of 60 mm or less in depth, 30 mm in inner radius (the outer radius of access tube), and 130 mm in outer radius", and " At a distance of 20 mm from the access tube, 90% of the electric field was attenuated. Soil at a distance of more than 100 mm from the probe did not affect the results of measurements". Using EnviroScan capacitance sensors, if we consider the soil thickness column of 1,2,3 and 4 cm around the access tube (having the ratio of SF/SFmax<0.95), then the cross-sectional areas of 2095, 4819, 8170, and 12152 mm2 could be calculated and compared to the above TDR data. Our paper stresses the importance of careful conducted laboratory calibrations before going to the field. It is my opinion that TDR and capacitance sensors, calibrated and used properly in the real-time soil water dynamics studies over large areas, could offer an unique opportunity to develop our knowledge about soil-water-plant-atmosphere phenomena. Nobody can read the future of the soil water content probes in a crystal ball. It is everybody's own hard work, respect for the work of others, and lots of reading which will built innovation and trust between scientists, educators, consultants, manufacturers, and customers, all of them as equal partners. Best regards, Ioan C. Paltineanu, Ph. D. - President PALTIN International Inc. 6309 Sandy St. Laurel MD 20707 Phone & Fax: (301) 725-0604 Email: icpaltin@bellatlantic.net