▁precip itation ▁from ▁the ▁atmosphere , ▁in ▁the ▁form ▁of ▁mist , ▁rain ▁and ▁snow , ▁as ▁part ▁of ▁the ▁water ▁cycle ▁over ▁e ons , ▁mill enn ia ▁and ▁in ▁the ▁present ▁day . ▁F resh water ▁constit utes ▁only ▁ 3 % ▁of ▁all ▁water ▁on ▁Earth , ▁and ▁of ▁that , ▁slightly ▁over ▁two ▁third s ▁is ▁stored ▁fro zen ▁in ▁gla ci ers ▁and ▁polar ▁ice ▁caps . ▁The ▁remaining ▁unf ro zen ▁fresh water ▁is ▁mainly ▁found ▁as ▁ground water , ▁with ▁only ▁a ▁small ▁fraction ▁present ▁in ▁the ▁air , ▁or ▁on ▁the ▁ground ▁surface . ▁Sur face ▁water ▁is ▁stored ▁in ▁wet lands ▁or ▁la kes ▁or ▁flows ▁in ▁a ▁stream ▁or ▁river , ▁and ▁is ▁the ▁most ▁commonly ▁util ized ▁resource ▁for ▁water . ▁In ▁places , ▁surface ▁water ▁can ▁be ▁stored ▁in ▁a ▁res er voir ▁behind ▁a ▁dam , ▁and ▁then ▁used ▁for ▁municipal ▁and ▁industrial ▁water ▁supply , ▁for ▁ir rig ation ▁and ▁to ▁generate ▁power ▁in ▁the ▁form ▁of ▁hydro elect ric ity . ▁Sub - sur face ▁ground water , ▁although ▁stored ▁in ▁the ▁p ore ▁space ▁of ▁soil ▁and ▁rock ; ▁it ▁is ▁util ized ▁most ▁as ▁water ▁flow ing ▁within ▁aqu if ers ▁below ▁the ▁water ▁table . ▁Gr ound water ▁can ▁exist ▁both ▁as ▁a ▁renew able ▁water ▁system ▁closely ▁associated ▁with ▁surface ▁water ▁and ▁as ▁a ▁separate , ▁deep ▁sub - sur face ▁water ▁system ▁in ▁an ▁aqu ifer . ▁This ▁latter ▁case ▁is ▁sometimes ▁called