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<br />001225 <br /> <br />VIII 09 <br /> <br />Uses of data <br /> <br />. <br /> <br />A common use of soil-water data assumes that the quantity of water <br />extracted from a mass of soil over an interval of time (as by trans- <br />piration of plants) is equal to the change in soil-water storage during <br />the interval. Actual1;y', water extracted equals change in soil-water <br />storage plus precipitation less runoff provided that: <br /> <br />1. Initial soil-water storage plus interim rainfall is <br />less than field capacity (specific retention); and <br /> <br />2. Water table and its capillary friIige are belOW the <br />reach of all roots and of evaporation. <br /> <br />Some workers have presumed that rate of evapotranspiration, or rate <br />of plant growth, correlates with and varies according to water content <br />of the soil. However, the very wide range in the presumed correlations <br />casts doubt on all of them. One extreme is that the availability of soil <br />water to plants :Is invariable between field capacity and wilting percentage. <br />The other extreme is that availability varies inversely as soil-water <br />content less wilting percentage--that is, inversely as the residual content <br />that is extractable by plants. Suffice it to say that no momentary value <br />of soil-water content is a universally acceptable index for rate of evapo- <br />transpiration or of plant growth. Differences among soilS and plants make <br />for widely unlike responses. Some part of the unlike responses is inherent <br />in our imperfect understanding of soil-water dynamics--accumulation, move- <br />ment and depletion--and in variability of the so-called soil-water constants. <br /> <br />Certain data on soil-water content in excess of field capacity have <br />been interpreted in terms of temporary storage and transmission of water <br />governed by soil permeability. Transmission rates cited in the literature <br />commonly are not widely applicable because diverse hydraulic heads are <br />involved, representativeness of the samples is questionable, or there has <br />been no clear distinction between the unlike phenomena of saturated flow <br />and unsaturated flow. <br /> <br />, <br />, <br />, <br /> <br />) <br /> <br />