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", ~,' 3 ,) (\ ud I, i), Harold E. T" _ ,/IIS 189 An important LH.:tor in mallagelllcnt of grr)UTld-l,.4;ateT reservoirs-and one that docs not apply to the open water in surf;lcc reservoirs-is the variable hut prevailingly slow ratc of movement of 'water throllg-h the rock materials of the resen'oir. Houston, Texas. provides a good exarnple of the limitations imposed by the slow rate of movement of water underground: wells within the city arc several miles from the area where recharge occurs, and liTe now pumping from depths collsiderably below sea level. depending for replenish, ment upon Wjlter moving tells of miles at snail's pace. By contrast with surface reservoirs, gTound-wtlter reservoirs were formed and Riled by nature to overAowing before we began to use the water from them, and with a relatively small investment we can develop <lnd use water ~.dthout botll(:ring to learn anything about the reservoir th<lt is furnishing that water. As"a result, for many <lreas of intensive pumping from wells we do not have the basic d;1ta necess<lry for efficient reservoir l1lantlgcment. The initi;:tlly full reservoirs have <llso created customs and :lttituclcs prejudici<lI to good reservoir man<lgcment, for the full reservoirs hild the adv<lntages of sprinR" Aow, artesian pressure enoug-h [or Rowing- wells, water tables high enough for minimum pumping- lifts :lnd even for sl1birri~ation. and sizilble contrihutions to the base flow of streams. We liked tlll these features. \Ve have very few eX<lmples to date of effective ground-water m<lnag-cment. Pardy bcc<luse of inndequate data. partly because of the public attitude Inward fillling water tabJes, many regulatory me;:tsures h:lve heen directed rowilrd prevclltion of pumping ill exccss of the estimated <lllllual recharRc- nr if we1Js are already pumping more, to prevent :lny further inCH':1SeS in withdrawal. This restrictive "hold the line" oper~tion may be a neccss:lry sfnpgap, hilt it is not crfective W:l(er man~lg(,Jllellt. With incre:lsed requirements and resulting incre~sed withdraw;]]s hy wells, it can be expected th;"lt in many ,P;Tound-water reservoirs the tot:11 discharge (including net draft by wells plus any llatllTa! discharge that has not been eliminated) will equal the average recharge. I say it can be f~xpected bCGlUse it has already happened in many places.16 This quantity is the most we can expect to be sustained by nature. But nature does not re- charge reservoirs <It ;111 3YCr<lg;c r:lte-it is we who fig-ure the a\'era~es. III some JO.year periods we may receive 12 to 14 times the avera~e annual rainfall, and in others only 7 or 8. There is even more tll<lrked variation in strcilmAow. Ground-water rccharg-c. dependent upon either nr hoth of these, necessarily varies markedly [rom year to year, <InrI from :l wet cycle to a dry cycle of years. Efficient reservoir oper:ltj()l1s must reckon with these long-term v<lri:ltions in inflow. For optimum use the reservoir must be <lr<lwn down. over ~ome people's violent objections. and then the inventory mUst he built up in wetter years, regardless of other people's covetous eyes. If the reservoir were regarded. <lS a warehouse, :lny business- J1l<ln would agree that either an overstocked or an empty warehouse means "poor business." A different aspect of ,,,,'ater 1l13n:lgcment is the di~r()sal of water ll')cd nOli. consumptively in mtmicip:]lities, inrltlStries. ;l1lc1 irrig-:-ttiol1: sew:lg-e. in- dustria! w<lsles, :llld irrig-ation return (lows. Some of this may be rcch;lrgerl into ground.water reservoirs if it is not too contaminated for use, or if it is tre:lterl to remove pollutants. Other problems result when these nOI1- consumptively used waters enter the ground and cau.~e wate"rlogging or l'~H. E. Thomas. The: C011un1afioll of (;)"(lIIIl(f WokI'. l'\CW YOlk, ~r('Cr;lw-Hil1 Rook (;0.. pp, 35,160 (1951).