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<br />14 <br /> <br /> <br />SOIL MOISTURE <br /> <br />-r::~ +- ?'~~ <br />n-Y ~~ <br />FUC,!mnON J J ) <br /> <br />APO~~SPlRAtION <br />EVAPORATION <br /> <br />~ _~VERLM~ FLow! t <br />~- ~~;~r ~) <br />.ATER ~--' I I <br />TABLE -- - ~ <br /> <br />.... - y~ <br />-::-,-..c .:... ~ <br />~:::;;-rABL~ ~WD,-;;;;::~ <br />STR.E.A."l <br />CHA.'OOJ. <br /> <br />Figure 7. Simplified vater cycle. <br /> <br />Preeipi tation <br /> <br />, Water vapor enters the atmosphere by eVI,poration and leaves the <br />at_phere by precipitation as rain or snow. The amount of atmospheric <br />water vapor (0.0001 percent) varies considera.bly geographically and. <br />seasonally. It is greatest at and near the 'equator with an average <br />rainfall of about 44 cm. In middle latitudes (40-50 degrees) we see large <br />variations that depend on geography and weather patterns producing an <br />average rainfall of 10 to 20 cm. In the polu regions the average rainfall <br />ranges between 3 cm in winter and 8 cm in sUI.mer. Once in the air, water <br />vapor circulates locally or becomes part of :the general circulation of the <br />atmosphere. Because precipitation may occur close to the source of <br />evapOration or thousands of a1les away,' tha Iresidence t:Lme for atmospheric <br />water vapor may vary fro. a few hours to a f41w weeks. A general average is <br />nine, or ten days. However, additions to atmospheric wal:er all a result of, <br />eva,o~zat1on equal water losses due to precipitation o"er long periods 01 <br />tiM. Likewise the water gains and losses o"er land and sea remain <br />balanced. Bence, the total quantity of precipitation that falls on the <br />eattll 11 ,exactly equal to the quantit:y that 1"~aporate8 l!ro. 'its surhce. <br /> <br />On . ,lobal bash IDOre water eV6porates from the oe:eans, since they <br />cover .-larler aurface area, than 18 returned by prec1pHation, whi.:h means <br />the continents receive .ore precipitation than is lost t~ <br />