This paper gives a systematic framework for evaluating the time domain reflectometry (TDR) response of soil. TDR measures the soil composite dielectric constant (Ka); thus any factor that has an influence on Ka measurement can affect soil moisture determination. On the basis of known dielectric constants of air, soil, water, and ice, as well as the volumetric fraction of each phase in a bulk soil, functional relationships between volumetric water content (&thgr;v, cm3/cm-3) and Ka are derived in the form of &thgr;v=aKa0.5+b. In general, the TDR functions derived from this study are in good agreement with the universal equation developed through experiments by Topp et al. <1980> and other available TDR calibration data. This paper demonstrates that (1) soil solid and porosity have little effect on dielectric constant measurement; (2) temperature has a minor influence except for very wet soils; and (3) surface area is an important factor affecting the water content measurement. ¿ 1999 American Geophysical Union