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o rn21 7 VIII .1 VIII. SOn. WATER General concepts and "constants" The simple case The term "soil water", as used herein, re1'ers to a.ll the moisture contained in the soil and sub-soil, between the land sur1'ace and the water table. In other words, it re1'ers to all water in the lIone 01' aeration, including that within the root 1I0ne 01' plants and that in the capillar.y 1'ringe which is immediate~ above the water table. It excludes ground water--that is, water in the zone 01' saturation, belqw the water table. ' Of all the water that circulates in the "hydrologic cycle," a large :fraction is in the zone 01' aeration transient~--in other words, 01' all the circulating water, a large 1'raction is soil water at one time or another. Much of this transient water is extractable by evaporation or hy the transpiration of growing plants; the water so extracted returns . ! -,' to the atmosphere. 01' the water not extracted by evaporation or trans- piration, a part is retained by the soil against the downward pull 01' gravity; the part so retained depends chie1'~ on the assortment 01' grain sizes, the hUl1\us content, and the temperatu:;e 01' the soil.' A residual part of the transient soil water may move downward and ultimate~ may reach the water table. In arid and semi-arid regions, the amount 01' this residual--the ground-water accretion--usually is small or zero. However, it may be relative~ large in a hUl1\id region, especially i1' the soil is uncommonly permeable and if vegetal cover on the land sur1'ace is sparse or absent. In one example, somewhat more than 8 inches 01' water "drained" through a 20-1'oot pro1'ile of sandy soil whose surface was devoid of vegetation, during the 1'irst month a1'ter thorough wetting by prolonged rainfall. This oversimplified concept 01' soil-water occurrenCe and essentially vertical movement applies primari~ to relatively homogeneous soil and nearly level terrain. So also do the following soil-water "constants" which, lik~ many other terms in the physical sciences', denote character- istics that var.y: "Field capacity" or "specific ret~ntion" is th;lt quantity of water which, owing to capillarity, a particular soil holds against the pull 01' gravity after that soil has been wetted thorough~ and has drained from one to three days. This ,quantity of retained water is a useful but approximate characteristic of the soil; however, it is not an absolute characteristic. Beyond the three days that commonly are presUl1\ed to establish 1'ield capaci ty, gravity drainage may continue to remove water from a soil at a steadily diminishing rate over a period of weeks, months, or probably years. Thus, after prolonged drainage the quantity 01' water retained in the soil may be appreciably less than after three days.