Fig. 10. Contour lines of resonant frequency-probe length are given as a function of permittivity and suggest bounds for using short probes (i.e., also bounded by maximal TDR frequency and by increasing attenuation). Numbers in parenthesis are computed soil electrical conductivities estimated using Eq. [11] (_w = 110 dS^–1) and the permittivity-water content relation (Eq. [12]) given by Topp et al. (1980). Reduced permittivity (water content) leads to a commensurate reduction in _b suggesting that there may be regions of overlap where TTA and SFF using short probes are applicable.