▁an ▁often ▁che aper ▁and ▁easily ▁accessible ▁source ▁compared ▁to ▁the ▁river . ▁Water ▁imported ▁from ▁Northern ▁California ▁through ▁the ▁Central ▁Valley ▁Project ▁managed ▁to ▁st ave ▁off ▁increasing ▁rates ▁of ▁withdraw al ▁for ▁several ▁years , ▁until ▁the ▁ 1 9 7 7 ▁California ▁dr ought ▁heavily ▁decre ased ▁water ▁supplies ▁from ▁these ▁sources ▁for ▁a ▁while ▁and ▁caused ▁many ▁far mers ▁to ▁return ▁to ▁ground water ▁p ump ing . ▁ ▁Gr ound water ▁withdraw al ▁reached ▁its ▁peak ▁during ▁the ▁ 1 9 6 0 s ▁with ▁more ▁than ▁ ▁being ▁drawn ▁from ▁the ▁aqu ifer ▁each ▁year ▁– ▁over ▁twice ▁the ▁present ▁flow ▁of ▁the ▁San ▁Jo aqu in ▁River ▁– ▁which ▁account ed ▁for ▁ 6 9 . 6 % ▁of ▁all ▁the ▁ground water ▁p ump ed ▁within ▁the ▁Central ▁Valley , ▁and ▁nearly ▁ 1 4 ▁percent ▁of ▁all ▁the ▁ground water ▁with dra wn ▁in ▁the ▁United ▁States . ▁In ▁a ▁ 1 9 7 0 ▁study , ▁it ▁was ▁found ▁that ▁more ▁than ▁ ▁of ▁the ▁San ▁Jo aqu in ▁River ▁valley ▁had ▁been ▁affected ▁by ▁land ▁subs idence ▁in ▁excess ▁of ▁. ▁The ▁maximum ▁drop ▁in ▁elev ation ▁was ▁near ▁M end ota , ▁the ▁north ward ▁b end ▁of ▁the ▁San ▁Jo aqu in ▁River , ▁at ▁over ▁. ▁In ▁some ▁areas , ▁the ▁water ▁table ▁has ▁decl ined ▁more ▁than ▁ ▁vert ically , ▁forcing ▁far mers ▁to ▁sink ▁their ▁well s ▁as ▁deep ▁as ▁ ▁to ▁hit ▁more ▁abund ant