|
<br />003\)83
<br />
<br />78
<br />
<br />CONGRESO GEOL6GICO INTERNACIONAL
<br />
<br />derived from the Greck and means Hdry plant." Cacti are excellent examples
<br />of xerophytes,
<br />Phreatophytes are plants that habitnally obtain their water supply from the
<br />water table either directly or thorugh the capillary fringe (Meinzer, 1923, p,
<br />55). The term phreatophyte also is derived from the Greek and means "well
<br />plant." The form is apt, for by sending their roots to the water table they actual-
<br />ly dig their own wells and are able to pump their own supply of water, The
<br />distinction between xerophytes and phreatophytes with respect to their sources
<br />of supply is shown in Figure 1. It is important that this distinction be kept in
<br />mind when considering the hydrology of arid regions. Although the area oc.
<br />cupied by xerophytes is much larger than the area occupied by phreatophytes,
<br />the latter have a greater effect than the former on the water that is available
<br />for man's use.
<br />The relative importance of the roles that these two classes of plants play in
<br />the hydrology of arid regions is shown by considering their position in the
<br />hydrologic cycle. Figure 2 depicts graphically a simple hydrologic cycle-that
<br />for an undeveloped closed basin in an arid region, In this endless cyele of
<br />water movement through the atmosphere to the land, and back to the atmosphere
<br />again, the wa.ter moves as liquid and as vapor through a maze of overland and
<br />undeground routes. The diagram shows: (I) the distinction between the sources
<br />01 supply for these twoelasses of plants-soil water for xerophytes aud ground
<br />water for phreatophytes; (2) the relative positions of the supply reservoirs;
<br />and (3) the paths of the movement of water to and from them,
<br />Consider first the soil-water reservoir, which is replenished largely by the
<br />seanty precipitation and to a lesser extent by occasional overland runoff. The
<br />water in this reservoir is not directly available to man, for he cannot extract
<br />it in liquid form. It may be available to kim indirectly in the products of plant
<br />growth, but in this form it does not quench his thirst or meet his other every-
<br />day needs. Water that eute'IS the soil in quantities that are in excess of its mois-
<br />ture-holding capacity eventually becames available to man through dawnward
<br />percolation as recharge to the ground-water reservoir. Generally in arid regions
<br />the excess i's quite small, owing in part to the sca.ntiness of the precipitation and
<br />in part, as shown by the diagram, to depletion of the soil-water reservoir by
<br />the transpiration of xerophytes.
<br />In contrast, the water in the ground-water reservoir is avaliable to man.
<br />The reservoir is recharged by the percolation of excess soil water and by seepage
<br />from streams and lakes. The water is available ..0 man either directly through
<br />spring discharge and seepage to streams and lakes or indirectly through wells,
<br />tunnels, or ditches. In some localities it is the only supply of water. This water
<br />is available to phreatophytes also, which, through their draft on ground water,
<br />
<br />"". ".
<br />".J'i'>
<br />c';".": ,
<br />
<br />,",ii,',
<br />X')?J
<br />
<br />. ~~'
<br />"L;(}.:::i:\::
<br />
<br />",">",)"',
<br />ie,.',;;:,
<br />,..... "}f"';-c.i.
<br />. :.0" ::."~/:
<br />,)' :;:-:>X
<br />," ;:::{t;d~:;r'
<br />. '~: :~~?;,~;,;
<br />.;;; ", :':\
<br />:y
<br />
<br />..........'<;t:
<br />.,.,........";,
<br />;:."
<br />}'c
<br />
<br />".;
<br />-,,'"
<br />;;:.
<br />
<br />s,:,.-',
<br />':;;.
<br />
<br />'> ~~~;
<br />ii
<br />;'
<br />,..,
<br />)::i;:
<br />
<br />,\,
<br />.""j"
<br />i;:i,c
<br />
<br />.1",
<br />'<:
<br />
<br />'..i.:"
<br />
<br />I
<br />
<br />.~
<br />;,
<br />j
<br />
<br />"
<br />
<br />,i>
<br />
<br />., .;,\~ii~;i.
<br />..; ?~\0(::':,;
<br />" ;~~~~;'t
<br />:.,' ~;j;!l ?:
<br />
<br />~~)~tt'1~J~~~ Gl:!~:;;:i
<br />
<br />:::'.$)';,,: ,""'; ~~'::i. ;~
<br />~,:{;:~:!;:0 i' :,
<br />;;
<br />
<br />.. ::~
<br />
<br />"
<br />
<br />~
<br />I,', 'i',
<br />
<br /><,...,<:
<br />
<br />,'j::}< . c: j~',;';..:,;:<
<br />:;:,~r:/ :;{<,
<br />";;.,:::,...,,;:,,,,'\,::..,>";:
<br />".:."...,,..::,:
<br />::}:...~;'..:c;>.?:,
<br />,.', ',.' ..,.,..,'. '.' '. ',., ."',"" >:/ ',' "":';-"'>/':/:;,:"',' <' '0.-".'" .' '. '."" . "',"",,;;;: ;'.,.":' ,." "'" ::;:,;f!i~;;:;\<' .:-,'
<br />.; <\;, > <{i:;:"'<'i'): ,; >,/. "<;:; ,.... :: ,'<:: :. .::.'.,......f;.'::' 'j" ""-':"""X:''''::''C'}i'f'L 'p\}'t;, 'C:\:':,,""'.,"'i:'-' ;<....
<br />\' :';"".;" '> .",;:...<n ,<,.:,.,:..;;,\,:,:,..<<"::;,.L.i:':;?~:';~'~<!~/;:;;';;;:-'};;\';.:;>~).> ..'..'.. .':'<:, <
<br />;>::,"',;':: ,.i.. .',,:"'.";',:;,,:''\,}:' ."\. \,:', ,;-,,:-';:,(';'" '\..' .:::'.;,'C'::,(: :<,"~';"'~"~i,:,~",j;::';:"';;";:"" ,:.,,:.\.><
<br />:,';:;:>,;q :.'.;>\,::';::P;....\: Cr: ";\""\'};,,.}/:. ,". ......;/:.-7".
<br />'i"" ;',c' t~ .::' ;i, ~':, j;;':;?':';"':':J':}"~.':"'::;: ,~;;', ;...5:>.,:,..,.'....\t1:.f.;{.,~T:::,:,:,;:i;:::;;>.':);'?\;':.;'.';~,i ":,:., .."':~.f. :;:'i:D,:;;:;::::'::r:}t';:
<br />
<br />~~~T' . ~~..'~?/~fi[itj ~ ..~~~~?;.; \;:;T:;;..;~..~~g.j!~;;~;. ".:i~j;
<br />
<br />
<br />
<br />... .c ...t.:
<br />
|