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<br />orn21S <br /> <br />VIII. 2 <br /> <br />The "permanent 'ldlting point" of a soil is that quantity <br />of water which is not available to sustain growth by most plant <br />species. However, some species or some individual plants of a <br />particular species will die when water content of the soil <br />exceeds wilting point; some species and plants 'ld11 survive long <br />periods while water content is less than 'ldlting point. <br /> <br />Even after it has been depleted to the wilting point <br />("lIilting coefficient" or "'ldlting percentage"), a flu'ther frac- <br />tion of the water content in a soil may be removed by evaporation., <br />In tw-n, when evaporation ceases, there remliinsin the .soila <br />small quantity of "hydroseopic water" that can be removlld o~ by <br />drying at a temperatw-e greater than the boiling point 'of water. <br />The quantity of this hygroscopiC water may vary some'What; it <br />represents an equilibrium between the soil and the temperatw-e <br />and humidity of air to which the soil is exposed. It is termed <br />the "hygroscopic coefficient" when the equilibrium is established <br />with water-satw-ated air at standard temperatw-e. <br /> <br />"Available water"--that is, water that plants extract for <br />their growth--oommonlyis taken as the difference between field <br />capacity and permanent wilting percentage. Again, this is a <br />useful approximation o~, because neither field capacity nor <br />wilting percentage is absolute. Usua11y it is a minimum approxi- <br />mation because (1) a substant1s1 part of the water used by a plant <br />during its life cycle may be transpired immediately after rain or <br />irrigation, while water content of the soil exceeds field capacity; <br />and (2) under drought conditions some plants may, as has been <br />stated, continue to transpire after water content of the soil has <br />been depleted to wilting percentage. <br /> <br />Non-homogeneous conditions in the field <br /> <br />In fitting these over-simplified concepts to field situations the <br />hydrologist must deal with non~homogeneous conditions of many sorts. <br />These may be especially complex in hilly or mountainous; forested or <br />brush-covered terrain. In any current investigation, the scope of data <br />colleotion should encompass the non-homogeneities; data from the liter- <br />ature should be evaluated against identified or inferred non-homogeneities <br />that may be relevant. Limitations of space here permit only a few <br />suggestive examples. <br /> <br />