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001219 VIII.3 Soil and subsoil may be distinctly stratified, in units that differ substantially in retentive capacity and in permeability. Capillary discontinuity at the interface between two unlike strata commonly impedes downward movement so that, immediately above the interface, the amount of retained water may be substantially greater than "field capacity" (spec:ific . retention) of either of the two strata. In extreme cases, local thin zones may become saturated even though all the soil materials are appreciably permeable. These perched zones of saturation may be transient or lasting. Whether or no a saturated zone exists, a pressure gradient may impart a horizontal component to the movement of soil water. For this reason among many, water may become dispersed non-uniformly through the soil mass. Temperature gradients cause soil water to move, horizontally or vertically, and lead to non-uniform dispersal. Over forested and brush-covered terrain, soil water commonly is dispersed non uniformly because precipitation that reaches the land surface is disproportionately large at drip line and stem line of indi- vidual trees and shrubs. For example, stemflow of trees may be as much es 10 percent of gross precipitation; that of shrubs, as much as IS percent. Whhtever the type of vegetal cover, root channels commonly afford preferred sites for infiltration and lead to non-uniform dispersal of soil water. In somewhat similar fashion, micro-relief of the land surface may concentrate much of the precipitation in scattered minute pools, from which a disproportionately large part of infiltration takes place. These effects may be exaggerated in the case of "hydrophobic" soils which resist wetting and from which precipitation may be rejected as overland now even though the soil is quite dry. Having been wetted slowly, however, these same soils can accept infiltration readily, both in total amount and uniformly from one point to another. Given such soil, the common assumption that "the wetter the soil the less the infiltration rate" may be invalid, Investigations of water balance and water yield commonly have taken water-content of the soil as an index of watershed wetness affecting infiltration and runoff, especially flood runoff, It has been found, however, that C0l1lD10nlY the "soil-wetness effect" can be estimated more consistently from records of antecedent precipitation than it can be derived from conventional deternrinations of soil-water content. This despite the fact that soil-water content is the significant variable.