عنوان مقاله [English]
Soil moisture as an important factor in agricultural management which is measured or estimated using direct and indirect methods. Time domain reflectometry (TDR) method is One of the newest and fastest ways to measure the soil dielectric constant, indirectly which is based on the measurement of soil dielectric constant regarding the velocity of the electromagnetic waves propagation. In this paper, several experimental relations between dielectric constants and volumetric water content have been presented, but all soil properties is not considered in these methods. The main purpose of this research is the investigation of the effect of soil texture on the accuracy of volumetric water content measurement by use of TDR system. The experiments have been carried out on 5 types of texture (sandy, sandy loam, loam, sandy clay loam and clay loam) in three repetitions under quite random conditions. In addition to assessment of soil moisture using TDR, soil volumetric water content has been measured in direct (weight) method. Also, Estimated Values of soil moisture by the TDR system were compared with topp et al (1980) and mixed models. The results showed that the volumetric water content values obtained from TDR system decreased with The heavier soil texture, As the percent relative error (RE) in textures sandy, sandy loam, loam, sandy clay loam and clay loam have been Respectively equal to 4.07, 8.9, 8.03, 9.13 and 11.4. comparision of estimated moisture using models showed that in all textures, topp et al (1980) models had the highest Correlation coefficient and also The accuracy of topp models is more of mixed models. Finally, Relations have been calibrated with a high correlation (R2) between weight and TDR moisture measurements were presented.