<br />00309G
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<br />SECCION IV - GEOHlDROLOGiA
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<br />The e!fect of humidity is the reverese of that of temperature. Experiments
<br />in the Safford Valley of the Gila River of the southwestern United States show
<br />that, as the relative humidity increases, the use of water by the plants decreases,
<br />and as the relative humidity decreases the use increases (Gatewood, Robinson,
<br />Colby, Hem, and others, 1950, p. 143).
<br />The effect of wind movement is to increase the use of water, owing to removal
<br />of the air or high humidity next to the plant leaves and replacement with air
<br />of lower humidity from the adjacent desert areas, For the 'other elements of
<br />climate, it has been found that. the plants' use of ground water is greatest when
<br />the growing season and daylyght hours are long and the rainfall canty. In
<br />arid regions generally, during the growing season, the temperature is' high,
<br />the humidity low, the wind movement substantial, and the rainfall scanty. This
<br />combination of climatic conditions is conductive to a high rate of water use
<br />by phreatophytes, and, hence, to a substantial depletion of the groundwater
<br />reservoir.
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<br />The density of phreatophyte growth is not uniform but may differ widely
<br />from place to place, the plants raging from a dense jungle, like forest to a few
<br />scattred individuals. The height or' size of the plant and the amount of. foli-
<br />age affect the quantity or ground water transpired. The variations in density
<br />and size of the plants may be evaluated in terms of areal density and vertical
<br />density, and expressed as a product of the two, volume density. Areal denshy
<br />relates to the density of growth of the plants with respect to the maximum
<br />possible, Vertical density relates to the depth or thickness of the foliage with
<br />respect to the maximum possible, The conditions of growth may be described
<br />in terms of percentage of density by assigning a value of 100 percent to the
<br />maximum possible, and zero to essentially no growth. A density of 100 percent
<br />indicates growth so thick that the addition of a new growth .would caU3e an
<br />equivalent amount of old growth to he choked out and to die,
<br />It has been found that the use of water by phreatophytes is proportional
<br />to the quantity of the transpiring material (Gatewood, Robinson, Colby, Hem,
<br />and others, 1950, p. 27), As volume density is a measure the transpiring mat-
<br />erial, the draft on the ground water varies as the volume density. The draft
<br />is greatest where the growth is dense and the plants large, and least where the
<br />growth is small and scattered.
<br />Data on the annual rate of consumption of water by the different specie.
<br />of phreatophytes are scanty, The data that are available indicate that the con-
<br />sumption varies widely between species, even when growing in the same locality
<br />and under essentially the same conditions. Data have been obtained for the
<br />use of water by 8 common species of phreatophytes at different localities in
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