Time domain reflectometry (TDR) method is increasingly have used for estimation of soil water content. With TDR, soil dielectric constant (K) is measured and related to soil volumetric water content (VWC) using a calibration equation. The main objective of this study was to compare of the TDR measurements and gravimetrically determined VWC and determine the relationship between soil water content obtained from these two methods are based on mathematical equations (Linear, quadratic and cubic) for Clay, Loamy and Sandy soil texture in 10 moisture range. The studied experimental models in this research were Topp et al (1980), Roth et al (1992) and Jacobsen and Schjonning (1993). Results showed that mean soil VWC measurements of TDR method was 3% less than gravimetrically measurements for clay soil texture and mean soil VWCs measurement of TDR method were 0.3% and 2.5% greater than gravimetrically measurements for Loamy and Sandy soil texture. The results indicate cubic model is the best model and the weakest ones were belonged to Roth et al (1992) for Clay, Loamy and Sandy soil texture. It is concluded that with increasing soil clay content, the accuracy of measurement by TDR was decreased and the maximum and minimum measurement accuracy relate to sandy and clay soil texture respectively. Also it is concluded that the accuracy of measurement by TDR was greater than Topp et al (1980), Roth et al (1992) and Jacobsen and Schjonning (1993) and less than Linear, quadratic and cubic mathematical equations.
