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001220 VIII.4 In somewhat different context, a "saturated watershed" has been taken to denote minimum potential infiltration and maximum potential runoff; this particular expression has been misused grossly. In mountain and foothill watersheds, all the soils probably are never saturated simultaneously. No less an authority than R. E. Horton states that, even with the heaviest prolonged rainstorms, the water content of soil generally becomes very little more than field capacity, Probably this is true of permeable, coarse textured soils on slopes and ridge 'crests. In the same watersheds, however, the soil adjacent to channels may become nearly saturated. The phenomena of soil-water flow, under conditions of saturation and partial saturation, are far more complex than those of floW in open channels. They may prove amenable to experimental analysis using new radioactive-tracer techniques, but even these techniques are by no means unquestionable, Certain tracers are adsorbed strongly by mineral particles or organic matter in the soil. Acceptable dosage of the environment and permissible dosage of personnel may greatly lessen effectiveness; Beyond these and other limitations'of procedure, the . flow paths in the soil may be so diverse that mean flow time is unique to the particular experiment. In other words, results cannot be trans- posed to ':,other areas or extrapolated to other soil-water conditions. Even so, tracer techniques promise to tell us something about soil-water movement in aspects of hydrology that have been expressed nebUlously as "interflow", "quick seepage", "storm flow", "soil-water contribution to ground-water recession", and so on. Methods of measuring soil water Periodic sampling of the soil Of those commonly used for determining the water content of soil, the gravimetric method is considered the most accurate. The standard procedure is to remove soil samples in the field from desired depths with soil tubes or augers, and to place them inmiediately in sealed containers which then are weighed, opened, contents dried at 10So C, and reweighed. The loss in weight represents soil water,. commonly computed as a percentage of the weight of dry soil. The gravimetric method has several disadvantages, including I (1) beoause each sample is removed from the ground, subsequent change in water content of the soil cannot be determined by re-sampling at precisely the same point; (2) the sample is relatively small in volume and, because soils in the field usually are decidely non-homogeneous, may not represent mean water content of the soil at the designated depth over all the area of interest; (3) usually at least 24 hours elapses before analytical results are available; and (4) considerable equipment and labor are involved.