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Nc, qu trnh, cung cp v sdng

M u

Nc l mt trong nhng ngun ti nguyn c bn v quan trng nht ca

chng ta.

m bo rng chng ta duy tr tha ng v an tan ngun cung cp nc l

mt trong nhng mc tiu mi trng quan trng nht. Vic thiu gii php cho

vn nhim v tnh trng thiu nc gy ra vn mi trng nghim trng

cho hng t ngi nhiu khu vc trn th gii. Trong ti ny chng ta s

cp n nhng ch thy hc, ngun cung cp v s dng nc, qun l nc,

v nc v h sinh thi. Mc tiu ca ti ny l :

C s nh gi ng cho ngun nc trn tri t.

Hiu c ti sao nc l cht lng c bit nht trong mi trng ca

chng ta.

Hiu bit v nhng ngn d tr chnh trong chu trnh nc.

Tm hiu cc nhn t chnh iu ha dng chy b mt v lng trm tch.

hiu nhng khi nim c bn ca a cht hc nc di t, bao gm

c s di chuyn ca nc di t v nh lut Darcy.

hiu nhng loi hnh s dng nc chnh.

C th bn v 1 s cch bo tn ngun nc.

Tm hiu vi hu qu mi trng ca s pht trin s dng ti nguyn

nc, bao gm c vic xy p v knh o.

Bit tiu chun nhn bit khu vc m ly v hiu tm quan trng v mi

trng ca m ly v s mt i m ly.

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I. WATER AS A UNIQUE LIQUID.

Nc l mt cht lng khng th thay th, khng c n, chng ta khng th

tn ti c. Mi phn t nc cha hai nguyn t hydro v mt nguyn t oxi.Mi lin kt ho hc m gi cc phn t li vi nhau l lin kt ho tr, c ngha

rng mi nguyn t hydro chia s n in t ca n vi nguyn t oxi v nhng

nguyn t ny chia s nhng in t ngoi cng ca n vi nguyn t hydro.

S thc nc l mt cht phn cc lin quan n nhiu c tnh quan trng ca

n cng nh cch thc n phn ng trong mi trng. Chng hn, nhng phn t

nc c kh nng thu ht ln nhau, v vy chng sn sinh ra nhng lp mngmng hoc nhng tng phn t nc gia v xung quanh nhng ht quan trng

trong s chuyn ng ca nc trong i khng bo ha pha trn mc nc

ngm. Qu trnh ny l trong s mt s dnh kt. Nhng phn t nc c th b ht

li, u m ca phn t nc (oxy)

ht nhng ion dng nh natri,

canxi, magi v kali. Bi v nhng

ht st c khuynh hng tch np

cc ion m, chng thu ht nhiu

ion dng hn (hydro) v nhng

phn t nc s c hydrat ho.

Cui cng s phn cc ca nc l

nguyn nhn sinh ra sc cng mt

ngoi: nhng phn t nc thu ht

ln nhau cng nhiu hn so vi

nhng phn t ca khng kh. Sc

cng mt ngoi v cng quan trng

trong nhiu qu trnh sinh ha v

vt l ko theo nhng chuyn ng

nc xuyn qua nhng khe h.

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Nc thng c coi l dung mi c bn. N c kh nng ha tan nhiu cht

v khin n tr thnh mt b phn c bn v quan trng ca cu thnh s sng. V

nc t nhin c tnh acid yu, chng ho tan mt lng ln hn hp t mui n

gin n cc cht khong v . Nc c bit quan trng trong s phong ho hohc ca v cc cht khong, cng vi nhng qu trnh l ha v sinh ha, bt

u s to thnh t.

Nc l cht duy nht m th rn nh hn th lng, iu gii thch hin

tng bng tan. Nu nc th rn nng hn th lng th n s chm xung. Mc

d ci ny th an ton hn cho nhng con tu i du lch gn nhng ni bng tri,

nhng thuc tnh ca sinh quyn nhiu khi khc nhau: dng sng, i dng ngbng t y ln trn.

c tnh quan trng khc ca nc l tnh b ba ca n, nhit , p sut v ba

pha ca n: rn (nc ), lng (nc), kh (hi nc) c th tn ti cng nhau.

B ba ca nc xut hin t nhin trn hay gn b mt tri t.

iu c mi lin h quan trng trong vic chuyn nc t cc i dng

vo kh quyn v sinh quyn theo vng tun hon nc. Th gii s tr thnh mt

ni khc hon ton nu nh nc khng th bc hi t i dung vo kh quyn

ngay ti iu kin gn b mt (chu trnh nc dng li).

Nc tri t cho php lm du hiu ng v mi trng bi v nhit dung ring

cao ca n. Nhit dung ring l bch phn tng mt gam cht lng ln mt

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. Nhit dung ring ca 1 calo/gam c so snh vi nhng dung mi c nhit

dung ring khong 0.5 calo/gam. Nc c kh nng ln nht ht v gi nhit.

II.

DNG CHY B MT V LU LNG PHSA.

Dng chy b mt c tc ng quan trng trong vic xi mn v vn chuyn

cc vt cht. Nc mang nguyn liu trong trng thi ho tan hoc tri thi l

lng, v nc mt c th tc ng cun cc ht t v i. S lng v kch

thc ca cc ht l lng b mang i bi nc b mt ph thuc vo th tch v

su ca nc v vn tc dng chy. Dng chy cng nhanh th lng ht vt chtc chuyn i cng nhiu. Bi vy, nhng nhn t nh hng n dng chy

cng nh hng n s xi mn trm tch, s chuyn ch v lng ng.

Dng chy ca nc mt b chia ct bi nhng ng phn thu. Mt ng

phn thu hoc lu vc sng l mt vng trong bt k lng nc no ri

xung s chy xung trong cng mt dng hoc dng sng. (nh ngha ny trong

trng hp khng c lng nc no b tiu th bi sinh quyn, bay hi, lu gi,

hoc c chuyn ra khi ng phn thy bi cc dng chy bn di.) Cc lu

vc sng ln c th c chia thnh cc phn nh hn. Chng hn, lu vc sng

Missisipi chy qua khong 40% ca M nhng cha nhiu cc lu vc sng nhnh Ohio, Missouri, Red River v nhiu ni khc. Lu vc sng nh Ohio cn

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c th chia nh na. Chng ta cng c th ngh lu vc sng l mt vng t ln

bao gm nhng dng chy trong n cho n mt mng li dy c cc dng thot

nc. Nh vy, lu vc sng thng l mt vng t ni m mng li cc dng

thot nc t cc con sng hay dng chy cc knh trong lu vc.

Nhng nhn t nh hng dng chy:

Lng dng chy b mt v lng cc cht trm tch c mang i bi cc

dng chy thay i mt cch ng k t lu vc sng v t dng sng ny sang

ci khc. Kt qu ca s bin i a cht, l hc, sinh hc, v nhng c trng s

dng t ca cc lu vc v s bin i c bit ca nhng nhn t theo thi gian.

Thm ch ngi quan st tnh c nht c th nhn thy s khc bit trong s lng

cn mang bi cng dng sng trong nn lt

1. Nhng nhn t a cht

Nhng nhn t thit yu tc ng n dng chy b mt v s trm tch bao

gm loi t v , khong vt, thi tit v nhng c trng cu trc t v . t

c cu trc ht mn, nng v t st v nhng loi c phi ra vi t vt nt

gy v ni chung c th cho nhng dng nc nh b di chuyn xung di v

tr thnh b phn ca nhng dng chy di b mt. nhng dng chy t lng

ging thu trn vt cht c tc tng i nhanh. Ngc lai, t ct v si, c

vt nt tng i ln, d tan li hp thu mt lng nc ging thy ln v lm

gim dng chy b mt.

Cc nhn t l hc, cc nhn t l hc tc ng n lu lng v vn chuyn

cc cht trm tch bao gm hnh dng ca lu vc, a hnh v dc, hng ca

dng chy cho n vic to thnh l.

Hnh dng ca bn lu vc nh hng ng k bi nhng iu kin a cht.

Chng hn, mng li sng c th chia lu lng thnh nhiu nhnh ph s nhanh

hn nhng lu vc c h thng cc dng chy chnh nhng ngn hn, ngon

ngoo hn.

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Nhng con sng lu vc sng nhanh chng c lu lng nhanh chng tng

cao trong sut hoc sau lng ging thu c gi l dng chy xit (flashy) v

c th to ra l.

Nhn t ng dc v a hnh c quan h vi nhau: ng dc cao (s khc

bit cao gia nhng im cao nht v thp nht ca h thng thot nc ra sng

hoc dng sng hoc bt c dng a hnh no) dng chy cng ging nhau khi

dc, nghing v vng t dc ca dng chy cng ging. ng dc v a

hnh quan trng bi v chng nh hng khng ch n vn tc ca nc trong

dng chy m cn n t l nc thm qua t hoc v t l ca dng chy trn

mt t, c nhng th nh hng n tc ca dng chy b mt v di b mti vo nhnh sng.

S nh hng ca lu vc sng nh hng n t l dng chy, nh dng

chy, khong thi gian ca b mt chy rng, s lng v tht thot do s bay

hiSau cng l nhn t nh hng ca lu vc nh hng n lng hi nng

t mt tri cng nh hng gi ch o.

2. Nhn t kh hu.

Nhn t kh hu nh hng n s chy rng v s vn chuyn trm tch gm

dng lng xut hin, cng lng, khong thi gian lng vi vic nh gi cao

tng mc thay i kh hu hng nm v cc loi bo (d c lc xoy hay sm

st). Tm li, s tho ca th tch ln nc v trm tch th c kt hp vi

cng cao ca bo tc ng ln sn dc, a hnh khng vng chc bn di

gy nn nguy c gy xi mn cao.

3. Nhn t sinh hc.

Thc vt, ng vt, v sinh vt trong t tt c u nh hng n lng trm

tch. Thc vt c kh nng lm nh hng n cc dng chy theo mt vi cch :

Thc vt c th lm gim i s chy rng bng cch gi nc v thot rabi s bay hi. Lng nc b gi li bi thc vt cng ri vo khu t 1

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cch nh nhng hn v thm qua t. Da trn nhng th nghim r rng

ca lu vc sng c trng rng c th hin lm tng ln dng chy

ca nhnh sng mc d lm gim s thot hi nc (nc c s dng

bi cy v s thot hi ra bu kh quyn). Vic lm gim hay tht thot mc d c lm kh hu thay i, hay t s

dng s lm gim s chy rng v sn phm ca trm tch. y c cho

rng l nhng iu kin (ging nh con knh) trc s chy rng v khng

c bo hay nhng s kin khc xut hin. Tip theo 1 trn bo ma va

phi v qua s chy rng, ton th con knh b si lp y theo cc ng

dc do vic t chy ng dc. Tip theo bo ma v dng chy va phi

khc, cn lng trong con knh c vn chuyn ra ngoi h thng v con

knh nh n tng sau chy rng v trc cn ma u tin. Chuyn g

xy ra ? Ngn la loi b thc vt trn a hnh, v lm mt i nhng

vt liu (cn lng) m tch ly trn dc ni, nhng gi bi thc vt

trc ngn la, di chuyn dc xung theo hng nhnh sng ca con knh.

Qui trnh ca s vn chuyn kh ca s mt mt vt liu ny c gi l

dry ravel. Khi cn ma u tin ri xung trn vng sn dc b t chy,

s chy rng cao v 1 lng cn phong ph di chuyn xung di sn i

n nhnh sng ca con knh.Dng nc khng chuyn i tt cnhng cn lng xung,v rt nhu cn s ng li trn knh. Quan trng

hn na l trn bo u tin s cun tri rt nhiu cn t dc i, ri khi

trn ma tip theo tn cng th li c t cn b cun i hn t dc i n

dng chy. Nc khng ngng chy t sn i b xi mn sinh ra nhiu

dng chy, lm sch tt c nhng vt cht ng li trc trn b knh.Tuy nhin, nh hng ca ca cht b chy s gy ra 1 s tc ng mnh

ln nhng cn t nhng dc b mn ra khi lu vc sng. y l 1 s phn

ng li thng thng tip theo cht chy. t ph bin hn, nhng mnh vn

ln cun theo c th to ra nu mt tp trung c y thi gian bt

u tri xung cun theo nhiu nhng mnh vn th.

Thc vt trn mt nc lm tng s i lp n dng chy,m lm chm li

ng i ca nc l.

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Thc vt trn mt nc lm chm li s xi b dng chy bi v r ca n

kt li v gi nhng mnh t nh ti 1 ni.

nhng lu vc sng c trng rng, nhng mnh v hu c (than cy v

nhng mnh g vn) c th nh hng su sc n hnh dng dng sng vnhng qui trnh. Lu vc nc nhng ngn ni dc, nhiu vng nc t

nhin quan trng cho ni sng ca c c th c to ra bi nhng mnh

vn hu c ln.

ng vt nh hng n nhng nhnh sng bng cch di di thc vt hoc o

bi. Vic chn th nhng ng vt c v ln c th ph hy mi trng nc mt,

gy ra cc vn xi mn t dc 2 bn b sng. ng vt o bi thng quacng tc phng chng lt cc con c th bt u nhng vn xi mn m cui

cng dn ti vic cc con khng hot ng nh mong mun.

Lng trm tch c mang i do sng nh mt phn ca qu trnh t nhin

trong phm vi chu trnh vi nhng iu kin a cht, thi tit, vt l, sinh

dng, v nhng iu kin khc. Do , mt vi con sng mang tnh n nh v

khc r rt trong v tnh trng ca n. V d, trung bnh sng Lo TrungQuc mang theo hn gn 200 ln ti trng l lng so vi sng Nile Ai Cp.

M, sng Mississippi cng khng c nh sng Missouri v sng Colorado.

Mi quan h chung gia kch c ca lu vc sng v ti trng trm tch gi ra

l, khi lu vc c m rng, lu lng ph sa trn mi n v din tch b gim

i. Kt qu ca mi quan h ny t vic m rng ca tr lng trm tch v s

lng ng vi s m rng lu vc, thc t l lu vc nh hn th dc hn, v sgim st rt c kh nng xy ra ca tng lu vc bao ph bi mt mt cn ma

m lm m rng kch c lu vc.

III. NC NGM

Ngun ch yu ca nc ngm l lng ma ngm qua b mt vo v chy

qua phn trn ca i thm nc. i thm nc bao gm tt c cc cht pha trnmc nc ngm. Nc c thm nhp qua t b mt chy xung qua i thm

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nc, him khi c bo ha. Cch y khng lu, i thm nc vn c gi l

i khng bo ha, nhng by gi chng ta bit rng tn ti 1 vi vng bo ha

ngay lc nc chy qua. i thm nc c ngha quan trng bi v cht nhim

ngm qua b mt cn c lc qua i thm nc trc khi chng vo i boha pha di mc nc ngm. Theo cch , trong mi trng lp di b mt

i thm nc l mt vng cnh bo sm v s nhim tim tng ti nguyn nc

ngm .

Nc c lc qua i thm nc c th vo h thng nc ngm, hay i bo

ha, ni dng bo ha xut hin. Vng b mt pha trn i ny c gi l mc

nc ngm. i mao dn ngay pha trn mc nc ngm l mt vnh ai bin dy ni nc b rt ra bi hot ng mao dn, do lc hp dn gia nc v b mt

ca vt liu tri t v n sc cng b mt (lc ht ln nhau ca cc phn t

nc).

Thm vo l s kt ta, nhng ngun khc ca nc ngm bao gm c nc

thm thu t nc mt, gm c h v sng, nc ma c gi li hay lm y

ao, cho ti tiu nng nghip v h thng x l nc thi.

S di chuyn ca nc vo i thm ht v qua lp vt liu tri t l mt

phn khng th thiu ca c qu trnh nc v qu trnh . V d, nc c th

phn hy khong t vt liu, nc mt c th chuyn ha trm tch, nhit, kh v

vi sinh vt.

1. Tng ngm nc (aquifer).

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Mt vng ca tri t c kh nng cung cp nc ngm mc thch hp t1 ngun nc c gi l tng ngm nc. Si, ct, t, sa thch, cng nh

granite v bin cht vi xp cao l tng ngm nc tt nu nc ngm tn

ti. Mt vng khc s gi nc nhng khng truyn nhanh c bm ln t

1 mch nc gi l lp t thm. Nhng lp t thm thng to thnh lp ph hu

hn vi ch t nc. t st, phin st, magma hay bin cht vi xp hay

vt nt nh thng to thnh nhng lp t thm.

Mt tng ngm nc c coi l tng ngm nc khng gii hn khi n khng

c lp ph hu hn gii hn b mt pha trn ca i bo ha ti mc nc ngm.

Nu lp ph hu hn hin din th lp t thm gi l tng ngm nc hu hn

v nc pha di n di p sut, to thnh trng thi c p. Nhng iu kin ny

tng t trn nh hng ca n n 1 thp nc, m c th em li p sut nc

cho cc gia nh. Nc h thng c p dng ln im cao nht ca i b sung

(ni ma thm nhp qua b mt n h thng nc ngm) to thnh 1 ging phun.

mt ngha tng qut hn, s b sung nc ngm l bt c qu trnh m b

sung lng nc ti tng ngm nc v c th b ngm t nhin hay do con ngi

nh r r. S tho nc ngm l bt c qu trnh no m chuyn nc ngm ra

khi mt tng ngm nc. C th k n c s tho nc t nhin t sui hin

din ni nc chy trn 1 tng ngm nc phn ct b mt tri t. S tho nc

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ngm cng xut hin khi nc c bm ln t ging. C tng ngm nc hu

hn v khng gii hn u c th xut hin trong cng 1 vng.

Khi nc c bm ln khi t ging, 1 phu gim p c to thnh ti mt

nc ngm hay gradient p lc. Mt phu gim p ln c th lm bin i s di

chuyn ca nc ngm trong mt khu vc. S bm phun tro ca mt tng ngm

nc l nguyn nhn lm mc nc gim i lin tc theo thi gian, lm cho cn

phi khoan nhng ging su hn. Nhng s iu chnh ny thng rt tn km, v

n c th hot ng hoc khng, ph thuc vo nhng iu kin thy hc. V d,

tip tc o su thm, nc t nhng ging ny c bm t h thng nhng khe

nt m c xu hng gn hay gim v s lng v kch thc vi s tng cng su. Hn na, cht lng ca nc ngm c th b gim i nu n c ht ra t

ngun nc su hn, cha nhiu hn nhng khong ha tan.

2. S di chuyn ca nc ngm

C tc ln hng dch chuyn nc ngm u ph thuc vo dc ca

mc nc ngm v c tnh ca nhng cht khong hin din. dc thy lc

trong trng hp n gin nht cho mt tng ngm nc khng gii hn xp x

vi ch dc ca mt nc ngm. Kh nng nhng vt liu ring bit cho php

nc chy qua c gi l h s thm nc ca n. Din t mi quan h ca

dc thy lc v h s thm nc n dng chy nc ngm cho php chng ta lm

sng t nhiu vn lin quan n nc ngm.

H s thm nc ca mt vt cht trn tri t l mt hm s ca c c tnh

ca vt cht (nh l ng knh ht, kch thc ca l rng, mi lin h gia cc

l rng vi nhau) v c tnh ca cht lng chy qua n (nh nht v t trng).

T l ca khong trng ca t hay c gi l xp ca n. Ch rng mt

vi vt cht c xp cao nhng h s thm nc rt thp, v d nh t st. Bi

v nhn v nh ca phn t ca n, nhng vt m rt nh v gi nc rt tt.

Gii hn thm cng c dng lm thc o cho kh nng truyn cht lng

ca mt vt cht tri t, nhng ch gii hn trong c tnh ca vt cht (khng

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phi c tnh ca cht lng). Khi ni v nc ngm chng ta s thy c h s thm

nc v thm u m t nhng c tnh thy hc ca nhng vt liu tri t.

IV.

CUNG CP NCNc bao ph 71% din tch ca qu t trong c 97% l nc mn, cn li

l nc ngt. Nc gi cho kh hu tng i n nh v pha long cc yu t gy

nhim mi trng, n cn l thnh phn cu to chnh yu trong c th sinh vt,

chim t 50%-97% trng lng ca c th, chng hn nh ngi nc chim

70% trng lng c th v Sa bin nc chim ti 97%.

Trong 3% lng nc ngt c trn qu t th c khong hn 3/4 lng nc

m con ngi khng s dng c v n nm qu su trong lng t, b ng

bng, dng hi trong kh quyn v dng tuyt trn lc i... ch c 0,5% nc

ngt hin din trong sng, sui, ao, h m con ngi v ang s dng. Tuy

nhin, nu ta tr phn nc b nhim ra th ch c khong 0,003% l nc ngt

sch m con ngi c th s dng c v nu tnh ra trung bnh mi ngi c

cung cp 879.000 lt nc ngt s dng (Miller, 1988).

1. Tr lng nc.

Tr lng nc ph thuc dng vo, dng ra v lng nc lu tr trong mt

h thng. Tng lng nc i qua M mi ngy xp x 152000 triu m 3 trng

thi lng. Tng lng nc ny xp x 10% lng ma, tuyt trn th gii. Gn

2/3 thc y vic bay hi nhanh chng hay hp th bi sinh vt. Cn li 1/3 tc l

khong 5510 triu m3 chy trn b mt hoc thm vo t i vo cc mch nc

ngm, chy ra bin hay di chuyn qua cc ng bin gii. Chng c dng

tiu th hay bay hi t cc h cp nc t nhin. Nh cc hin tng t nhin

thay i thc y cc thin tai xy ra nh l lt, hn hn. Ch mt phn nc c

th pht trin cho lng s dng ln. Nh vy ch khong 2656 triu m3 mi ngy.

S phn vng mc c quy m l rt quan trng ti vic thc y chu k xy

ra hng nm.

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Mt s vn tim tng trong vic cung cp nc c d don cc khu vc

ni m lng ma hng nm v lng dng chy thp nh min Ty Nam nc

M v khu vc Great Plains.

Tht khng may, vic lu gi nc cc dng chy li khng kh thi v lng

nc mt i do bc hi cc h nhn to ln, s hn ch s lng cc v tr thch

hp xy h, v cn cc mc ch s sng nc khc nh vic vn chuyn nc

trn sng v i sng hoang d. Kt qu l, s thiu ht nc cung cp xut hin

nhng khu vc c lng ma thp v t dng chy qua. Nhng cuc tranh lun

quyt lit l cn thit m bo 1 s cung cp nc hp l.

2. Cung ng nc ngm:

Nc ngm l ngun cung cp nc ung cho gn mt na dn s Hoa K.

May mn thay, tr lng nc ngm y kh ln,theo tnh ton c khong 20%

tng lng nc c khai thc M. Trong 0.8 km2 c khong 125.000-240.000

km3. Nhn chung, 125.000km3 l gn bng lng nc ca sng Mississippi trong

sut 200 nm qua. Khng may l, do hao ph ca s khai thc nc, lng nc

c sn s dng t hn nhiu so vi tng lng nc ngm.

Bo v ngun nc ngm l vn ang c c th gii quan tm bi v a

s ngi dn s dng nc ngm cho mc ch sinh hot. Thi gian hnh thnh

mt tng nc ngm l t hng trm n hng ngn nm. V vy khi n b nhim

th rt kh m phc hi c. C th ni nc ngm l ngun ti nguyn khng

th phc hi. Tng ngm nc c vai tr rt quan trng bi v 30% dng chy

Hoa K c cung cp bi nc ngm. Cc dng chy ny chy l ra mt t hoc

l do nc thm qua t dc theo cc dng sng. chnh l cc dng chy c

bn, cung cp nc cho hu ht cc dng chy c lu lng nh hoc cc dng

sui vo ma kh. V vy duy tr lu lng ca dng chy th phi gi cho mc

nc ngm cao.

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nhiu khu vc, ngi ta o ging ly nc ngm. Trong nhng trng

hp nh th nc ngm b khai thc v c xem l ti nguyn khng

phc hi c. H mc nc ngm l mt vn ht sc nghim trng

cc khu vc nh: Texas - Oklama High Plain, California, Azona, Nevada,New Mexico, v nhiu khu vc c lp Louistana, Mississippi, Arkansas,

Bc i Ty Dng.

Texas Oklahoma - High Plain lng nc ngm b mt gn bng lu lng

ca sng Colorado. Tng nc ngm Ogallala c hnh thnh t tng ct v si

di mt khu vc rng khong 400.000 km2 ko di t nam Dakota n Texas.

Mc d tr lng nc ngm rt ln nhng lng nc khai thc gp 20 ln

lng b cp. Tnh trng h mc nc ngm xy ra trong nhng nm gn y,

v nhng cnh ng s b kh hn nu nh ngun nc b cn kit.

Hin nay c tr lng nc ngm b gim khong 5%, nhng mc nc b h

khong 30-60m mt s khu vc ca Kansas, Oklahoma, New Mexico v Texas.

Khi mc nc ngm gim th mc nc ging cng gim. nh vy phi tn nhiu

nng lng hn khi bm nc.

mt s khu vc, vic bm nc ngm s lm thay i t ai ni . V d

nhng con sng Tucson, Arizona, trc y c rt nhiu loi sinh vt sinh sng

nh: c hi, hi ly, v cc ng vt khc. Hin nay i thc vt ven b b cht

v con sng gn nh kh cn quanh nm. V nhng qu trnh ny cng lm gia

tng l lt Tucson. Mt i i thc vt ven b lm cho b sng d b st l, xi

mn. L lt xy ra lm h hi rt nhiu cng trnh xy dng, c s h tng nh:

ng x, cu cngnhng i thc vt ven b cn c mc nc ngm cao

cung cp nc cho s pht trin. Tht khng may, vic khai thc nc ngm

Tucson nh hng xu n s pht trin ca i thc vt ny.

V. S DNG NC

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Trckhi tho lun v vn s dng nc chng ta phi phn bit r gia s

dng ni dng v ngoi dng. S dng ngoi dng l ly nc ra khi ngun.

V d nh nc dng trong vic ti tiu, chn nui, nhit in, cc qu trnh cng

nghip, v trong cp nc cng cng vic s dng nc trong sinh hot l mthnh thc ca s dng ngoi dng. Trong trng hp nc khng quay li

dng chy ngay lp tc sau khi s dng. l lng nc bay hi, lng nc

c hp th vo cc sn phm hoc cc v ma, v lng nc tiu th bi ng

vt v con ngi. S dng ni dng lin quan n nc c s dng nhng

khng ly ra khi ngun. V d nh vic dng nc sng trong hng hi, thy

in, nc dng lm ni cho c, ng vt hoang d, trong gii tr.

S dng ni dng thng gy tranh ci v i vi mi mc ch s dng th

cn nhng tiu chun khc nhau trnh gy nh hng xu. V d nhu cu cht

lng nc v dng chy ca c v cc sinh vt hoang d khc vi tiu chun

nc khai thc thy in. Hot ng thy in yu cu v s thay i ca dng

chy. Tng t nh vy, i vi mc ch vn chuyn, hng hi th nhng iu

kin v dng chy c th i lp vi cc mc ch trn.

Lm cch no vn chuyn nc t sng sui n ni khc m khng lm

nh hng n dng chy l mt mi quan tm ln hin nay. l vn ln

Ty Nam i Ty Dng, ni c cc loi c nh: c hi, c thu ang b suy gim

v s lng do con ngi gy ra nh: ph rng, xy p, ngn dng chy v s di

c cng lm thay i dng chy.

1. Vn chuyn nc n ngi tiu dng

cc quc gia vn minh hin i, qung ng vn chuyn nc rt di t ni

c lng nc di do n ni khan him nc. California, cc con sng min

Bc cung cp nc cho cc thnh ph cc bang pha Nam. Hai phn ba sng

sui phn b pha Bc San Francisco, nhng pha Nam San Francisco ni khan

him nc li s dng n 2/3 lng nc. Trong nhng nm gn y,cc cng

trnh knh o c xy dng nh cc d n nh California Water, California

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Valley vn chuyn c mt lng nc rt ln t pha Nam n pha Bc ca

bang. Tuy nhin, vic ny lm nh hng n h sinh thi mt s sng ca

pha Nam California v s tc ng lm thay i dng chy.

Mt vn ang c tranh lun gia thnh ph Los Angles v nhng ngi

dn Owens Valley nm pha ng ca Serie Neveda. Los Angleles phi tri

qua mt thi k kh hn vo cui th k 19, sau tm c mt ngun nc

Owens Valley. Thnh ph ny bn nc cho Los Angles. n nm 1913, h

thng ng ng dn nc hon thnh. T , ngun nc ngm ny c

bm v vn chuyn qua h thng ng dn nc. Kt qu l mt lng ln nc

ngm v nc mt Owns Valley b bn i v ni y b thiu nc trm trng,gn nh sa mc ha. Vo u nhng nm 1900 c nhng khng ngh rt mnh

m ln ta n. Vn ny ch mi c gii quyt l Los Angles ch c mua

mt lng nc trong mc gii hn v phi c d n gim thiu tc ng n

mi trng.

Mt con p ln c xy dng Croton trn thng ngun New York nm

1900, nhng do dn s tng nn li xut hin m hnh nh sau: s dng nc, nhim, nhim mn, cn kit ngun nc, dn n vic xy dng mt h thng p

mi vng xa hn. Th trn ca Brooklyn v Qeen nm pha ty ca o Long

c b nhim nc ngm vo khong u th k 19 v h phi nhp nc t

cc vng cao hn. Cc vng pha ng ca o Long th khng phi nhp nc v

h c ban hnh o lut v s dng v bo v ngun cung cp nc ngm.

Tuy nhin, ngun nc ni y cng tng xy ra hin tng nhim, nhimmn, v cn kit ngun nc.

Mt iu quan trng l New York v Los Angeles khng phi l hai thnh ph

duy nht m c nhiu thnh ph khc cng lm tnh trnhg thiu nc cung cp

cho dn c. Mt iu c th ngh l gi s dng nc s c mt ci gii hn

trn. V gi nc thng mc thp v c ti tr bi cc chng trnh ca

chnh ph. Con ngi cc khu vc c vn v mi trng nn hnh ng

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bo v mi trng, gim thiu cc vn v cung cp nc. Nhng ngun nc

cha n mc b cn kit ngay lp tc. Tuy nhin, cc khu vc cn ang c chin

lc pht trin khuyn khch bo v mi trng. Cc nh sn xut nn gii thiu

cc loi my git, v cc dng gia dng dng t nc hn.

2. Khuynh hng trong vic s dng nc (trend in water use)

Khuynh hng s dng nc M c nhng chnh sch rt sng sut l

bao gm c s quan tm v s cn thit trong qun l ngun nc.

Xu hng s dng ni dng v ngoi dng t nm 1950 n 1990 M :

Vic s dng ni dng nc b mt vt xa nhiu so vi vic s dng

ngoi dng. L do l c 1 s lng rt ln nc c s dng sinh ra

in, phc v cho i li v gii tr trn sng sui v m bo lng c v

ni c tr ca cc loi ng vt hoang d.

C vic s dng ni dng v ngoi dng u tng n khong 1975, th bt

u gim. dn s M khong 151 triu ngi vo nm 1950 v khong 216

triu ngi nm 1975. Dn s vn khng ngng tng ln k t nm 1975 v

t con s 252 triu ngi nm 1990. Nh vy, trong sut thi gian

nc mt i s cn bng ca n, dn s vn khng ngng tng. iu ny

yu cu s qun l v bo tn nc tt hn trong 15 nm .

Xu hng trong vic phn loi s dng nc t nm 1950 n nm 1990

M:

Nhu cu ti tiu v ngnh cng nghip nhit in l c nhu cu s dng

nc cao nht.

Nhu cu v nc ca cng ng con ngi ( th v khu vc nng thn)

vn tng u n trong thi gian ny, xu hng ny c th gii thch t vic

tng dn s ca quc gia.

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Vic s dng nc trong ngnh nng nghip ti tiu th cn bng trong

nm 1975 v bt u gim mt mc khng ng k t . C th l

gii t h qu ca vic bo qun nc.

Tr khong thi gian t 1975 n 1980, nc s dng cho nhit in btu cn bng (1950 n 1970) s dng tng u n.

Ngnh cng nghip t nm 1980 s dng nc thp hn ng k, nh c k

thut mi, yu cu t s dng nc hn, tng hiu qu trng trt v tng

vic ti s dng nc.

C nhng du hiu tch cc l vic cng ng nhn thc tt hn v ti

nguyn nc v nhu cu bo tn n. Kt qu l nhiu tiu bang, nc c yucu gim s dng. Du hiu tch cc khc cho thy vic ci to nc thi th c

xc tin mnh m, gia tng t khong 200 triu gallon 1 ngy nm 1955 ln n

750 triu gallon 1 ngy nm 1990. ng k hn, xu hng tr nn nhanh hn ti

ch, ti s dng nc thi gia tng khong 30% t nm 1980 n 1990.

3. Tnh hnh s dng nc trn th gii v Vit Nam

a) Tnh hnh s dng nc trn th gii

Khi con ngi bt u trng trt v chn nui th ng rung dn dn pht

trin min ng bng mu m, k bn lu vc cc con sng ln. Lc u c dn

cn t v nc th y p trn cc sng h, ng rung, cho d c gp thi gian

kh hn ko di th cng ch cn chuyn c khng xa lm l tm c ni mi

tt p hn. V vy, nc c xem l ngun ti nguyn v tn v c nh th qua

mt thi gian di, vn nc cha c g l quan trng.

Tnh hnh thay i nhanh chng khi cuc cch mng cng nghip xut hin v

cng ngy cng pht trin nh v bo. Hp dn bi nn cng nghip mi ra i,

tng dng ngi t nng thn x vo cc thnh ph v khuynh hng ny vn

cn tip tc cho n ngy nay. th tr thnh nhng ni tp trung dn c qu

ng c, tnh trng ny tc ng trc tip n vn v nc cng ngy cng tr

nn nan gii.

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Nhu cu nc cng ngy cng tng theo pht trin ca nn cng nghip,

nng nghip v s nng cao mc sng ca con ngi. Theo s c tnh, bnh qun

trn ton th gii c chng khong 40% lng nc cung cp c s dng cho

cng nghip, 50% cho nng nghip v 10%cho sinh hot. Tuy nhin, nhu cunc s dng li thay i ty thuc vo s pht trin ca mi quc gia.

Th d: Hoa K, khong 44% nc c s dng cho cng nghip, 47% s

dng cho nng nghip v 9% cho sinh hot v gii tr (Chiras, 1991). Trung

Quc th 7% nc c dng cho cng nghip, 87% cho cng nghip, 6% s dng

cho sinh hot v gii tr. (Chiras, 1991).

Nhu cu v nc trong cng nghip: S pht trin cng ngy cng cao ca

nn cng nghip trn ton th gii cng lm tng nhu cu v nc, c bit i

vi mt s ngnh sn xut nh ch bin thc phm, du m, giy, luyn kim, ha

cht..., ch 5 ngnh sn xut ny tiu th ngt 90% tng lng nc s dng

cho cng nghip. Th d: cn 1.700 lt nc sn xut mt thng bia chng 120

lt, cn 3.000 lt nc lc mt thng du m chng 160 lt, cn 300.000 lt nc

sn xut 1 tn giy hoc 1,5 tn thp, cn 2.000.000 lt nc sn xut 1 tnnha tng hp. Theo pht trin ca nn cng nghip hin nay trn th gii c

th d on n nm 2000 nhu cu nc s dng cho cng nghip tng 1.900

km3/nm c ngha l tng hn 60 ln so vi nm 1900. Phn nc tiu hao khng

hon li do sn xut cng nghip chim khong t 1 - 2% tng lng nc tiu

hao khng hon li v lng nc cn li sau khi s dng c quay v sng

h di dng nc thi cha y nhng cht gy nhim ( Cao Lim, Trn c

Vin - 1990 ).

Nhu cu v nc trong nng nghip: S pht trin trong sn xut nng

nghip nh s thm canh tng v v m rng din tch t canh tc cng i hi

mt lng nc ngy cng cao. Theo M.I.Lvovits (1974), trong tng lai do thm

canh nng nghip m dng chy c nm ca cc con sng trn ton th gii c th

gim i khong 700 km3/nm. Phn ln nhu cu v nc c tha mn nh ma

vng c kh hu m, nhng cng thng c b sung bi nc sng hoc nc

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ngm bng bin php thy li nht l vo ma kh. Ngi ta c tnh c mi

quan h gia lng nc s dng vi lng sn phm thu c trong qu trnh

canh tc nh sau: sn xut 1 tn la m cn n 1.500 tn nc, 1 tn go cn

n 4.000 tn nc v 1 tn bng vi cn n 10.000 tn nc. S d cn s lngln nc nh vy ch yu l do s i hi ca qu trnh thot hi nc ca cy, s

bc hi nc ca lp nc mt trn ng rung, s trc di ca nc xung cc lp

t bn di v phn nh tch t li trong cc sn phm nng nghip. D bo nhu

cu v nc trong nng nghip n nm 2000 s ln ti 3.400 km3/nm, chim

58% tng nhu cu v nc trn ton th gii.

Nhu cu v nc sinh hot v gii tr: Theo s c tnh th cc c dn sinhsng kiu nguyn thy ch cn 5-10 lt nc/ ngi/ ngy. Ngy nay, do s pht

trin ca x hi loi ngi ngy cng cao nn nhu cu v nc sinh hot v gii tr

ngy cng cng tng theo nht l cc th trn v cc th ln, nc sinh hot

tng gp hng chc n hng trm ln nhiu hn. Theo s c tnh th n nm

2000, nhu cu v nc sinh hot v gii tr s tng gn 20 ln so vi nm 1900, tc

l chim 7% tng nhu cu nc trn th gii (Cao Lim, Trn c Vin - 1990).

Ngoi ra, cn rt nhiu nhu cu khc v nc trong cc hot ng khc ca con

ngi nh giao thng vn ti, gii tr ngoi tri nh ua thuyn, trt vn, bi

li ... Nhu cu ny cng ngy cng tng theo s pht trin ca x hi.

b) Tnh hnh s dng nc Vit Nam

* Nc mt

Vit Nam nm trong vng nhit i m c lng ma tng i ln trung bnh

t 1.800mm - 2.000mm, nhng li phn b khng ng u m tp trung ch yu

vo ma ma t thng 4-5 n thng 10, ring vng duyn hi Trung b th ma

ma bt u v kt thc chm hn vi ba thng.

S phn b khng ng u lng ma v dao ng phc tp theo thi gian l

nguyn nhn gy nn nn l lt v hn hn tht thng gy nhiu thit hi ln n

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ma mng v ti sn nh hng n nn kinh t quc gia, ngoi ra cn gy nhiu

tr ngi cho vic tr thy, khai thc dng sng.

Theo s c tnh th lng nc ma hng nm trn ton lnh th khong 640

km3, to ra mt lng dng chy ca cc sng h khong 313 km3. Nu tnh c

lng nc t bn ngoi chy vo lnh th nc ta qua hai con sng ln l sng

Cu long ( 550 km3 ) v sng Hng ( 50 km3 ) th tng lng nc ma nhn c

hng nm khong 1.240 km3 v lng nc m cc con sng ra bin hng nm

khong 900 km3. Nh vy so vi nhiu nc, Vit nam c ngun nc ngt kh

di do lng nc bnh qun cho mi u ngi t ti 17.000 m3/ ngi/ nm.

Do nn kinh t nc ta cha pht trin nn nhu cu v lng nc s dng chacao, hin nay mi ch khai thc c 500 m 3/ngi/nm ngha l ch khai thc

c 3% lng nc c t nhin cung cp v ch yu l ch khai thc lp nc

mt ca cc dng sng v phn ln tp trung cho sn xut nng nghip(Cao Lim-

Trn c Vin, 1990)

* Nc ngm

Nc tng tr trong lng t cng l mt b phn quan trng ca ngun ti

nguyn nc Vit Nam. Mc d nc ngm c khai thc s dng cho sinh

hot c t lu i nay; tuy nhin vic iu tra nghin c ngun ti nguyn ny

mt cch ton din v c h thng ch mi c tin hnh trong chng chc nm

gn y. Hin nay phong tro o ging khai thc nc ngm c thc hin

nhiu ni nht l vng nng thn bng cc phng tin th cng, cn s khai

thc bng cc phng tin hin i cng c tin hnh nhng cn rt hn chch nhm phc v cho sn xut v sinh hot cc trung tm cng nghip v khu

dn c ln m thi.

* Nc khong v nc nng

Theo thng k cha y th Vit Nam c khong 350 ngun nc khong

v nc nng, trong nhm cha Carbonic tp trung nam Trung b, ng Nam

b v nam Ty nguyn; nhm cha Sulfur Hydro Ty Bc v min ni Trung

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b; nhm cha Silic trung v nam Trung b; nhm cha St ng bng Bc

b; nhm cha Brom, Iod v Bor c trong cc trm tch min vng H Ni v ven

bin vng Qung Ninh; nhm cha Fluor nam Trung b....Phn ln nc

khong cng l ngun nc nng, gm 63 im m vi nhit t 30o - 40o C;70 im nng va vi nhit t 41o - 60o C v 36 im rt nng vi nhit t

60o - 100oc; hu ht l mch ngm ch c 2 mch l thin thuc loi m gp

trung Trung b v ng Nam b. T nhng s liu trn cho thy rng ti nguyn

nc khong v nc nng ca Vit Nam rt a dng v kiu loi v phong ph c

tc dng cha bnh, ng thi c tc dng gii kht v nhiu cng dng khc.

Trong nhng nm gn y nhu cu nc s dng cho cng nghip v sinh hotkhng ngng tng ln theo pht trin ca cng nghip, s gia tng dn s, mc

sng ca ngi dn khng ngng c nng cao v s pht trin ca cc th.

Nc s dng cho nng nghip cng tng ln do vic m rng din tch t

canh tc v s thm canh tng v. Theo s c tnh ca cc nh chuyn mn th t

nay n nm 2000 a din tch ti cho nng nghip ln 6,5 triu ha th tng

lng nc cn khong 60km3

, cho chn nui khong 10 -15 km3

, nhu cu vnc cho 80 triu dn khong 8 km3; tnh chung nhu cu v nc s tng ln

khong t 90 -100 km3. Nh vy n nm 2000 lng nc cn cho s pht trin

t xp x khong 30% lng nc c cung cp trn ton lnh th. iu c

bit l nhu cu ny phn ln tp trung vo ma kh trong khi mc nc trong cc

sng ngi xung thp nn c ni nc s khng dng, iu ny cho thy nu

khng qun l v phn phi tt s xy ra tnh trng thiu nc gay gt nh hin

nay.

VI. CC VN LIN QUAN N TI NGUYN NC

1. Hn hn

Theo cc nh nghin cu th kh nng cung cp nc ngt hin nay l mt vn

nghim trng trn ton th gii. C t nht 80 nc vng sa mc v bn samc (chim khong 40% dn s th gii) thuc hai lc i Chu v Phi Chu

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thng xuyn b hn hn v mt ma nn thng xuyn khng cung cp lng

thc nui sng dn ca h.

Trong nhng thp nin 1970 thm ha hn hn e da trn khong 24,4 triu

ngi v hng nm git cht hn 23.000 ngi, hu qu ny vn cn ko di

n 1980. Nm 1985 hn 154 triu ngi thuc 21 quc gia Phi Chu ri vo

nn i do hn hn, thm vo s gia tng dn s qu mc v chin tranh lan

rng, mt khc cn do vic qun l v s dng ngun ti nguyn v pht trin

nng nghip km hiu qu. cc nc ny, ngi dn ngho phi mt nhiu thi

gian i tm nc thng l nhng dng sng v sui b nhim v c

c nc nhng ngi ph n v tr em phi i b t 16 km - 25 km mt ngyv ch mang c mt bnh y nc trn ng tr v (Miller, 1988).

2. Ngp lt

Ngc li, nhng quc gia khc c v lng ma tng i ln th mt

lng ln nc ma nhn c ch trong mt thi gian ngn trong nm. Chng

hn nh n , 90% lng nc ma tp trung vo gia thng 6 n thng 9

thng gy nn ngp lt.

Trong nhng thp nin 1970, thm ha lt li e da trn 15, 4 triu ngi

v hng nm git cht trung bnh 4.700 ngi, lm thit hi trung bnh 15 t USD,

hu qu ny vn cn ko di n nm 1980. Nguyn nhn dn n lt li l do con

ngi ph rng, t rng ly t canh tc, khai thc qung m, m rng

th... Mc d lt li c xem l mt thin tai gy cht ngi v lm thit hi hoa

mu, ti sn ca ngi dn nhng sau cc trn lt, do s lng ng ca ph sa lm

tng thm mu m cho t. ngn nga v lm gim s tn ph ca lt li

nhng quc gia ny, nhiu bin php c thc hin nh xn kinh thot nc, xy

p v h cha nc, trng cy gy rng trn cc i trc, gi li rng u

ngun.

3. S ng nc

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nhng vng c a hnh thp hoc ni c mc nc ngm qu cao lm cho

mt t lun b ph kn bi mt lp nc t ng lu ngy to nn trng thi ng

nc, t b ng nc nn lun ym kh.

Trn nhng vng t b ng nc thng c nhng thc vt thy sinh c

trng nh mt s cc loi rong to, nn, lc rt pht trin v th nn t ni di

do mn , m v cc acid hu c v th lm cho t v nc b chua, t ngho

ln nhng li giu nhng cht c nh H2S, CH4, Fe 2+. Do nhng tnh cht vt l

v ha hc ca nc v t ca vng b ng nc khng tt cho s trng trt

cng nh s dng nc cho cng nghip v sinh hot

4. S ng nc

Theo nhp pht trin ca nn cng nghip, nng nghip v s nng cao mc

sng ca con ngi th nhu cu v nc s dng ngy mt tng. Vn v nc

ngy cng tr nn nghim trng, c bit l nc mt ngy cng thoi ha v mc

nhim nc ngy cng tng. Theo t chc y t th gii (WHO -1980) c

tnh rng cc quc gia km pht trin th 70% dn chng cc vng ven thnh

ph v 25% dn c cc th khng c nc sch s dng.

Vit Nam, do nn cng nghip mi pht trin, s th v cc khu cng

nghip cn t v cc im tp trung dn c cha nhiu nn lng nc dng cho

cng nghip v sinh hot cn qu t so vi tr lng trong t nhin. Tuy vy, s

nhim bn ngun nc bt u xut hin do vic s dng thuc tr su trong

nng nghip; lng nc thi ra mi trng ca cc nh my luyn kim, nhit

in, ha cht, thc phm ; cng vi lng nc thi do sinh hot... tr thnh

mt vn cp bch cn phi c quan tm.

VII. QUN L NC

Qun l ngun ti nguyn nc l mt vn phc tp v tr nn rt kh khn

trong tng lai khi m nhu cu s dng nc ngy cng tng. Trong khi iu ny

tr nn rt c bit Ty Nam nc M v cc vng t, khu vc kh cn trn th

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gii, New York v Atlanta, trong s nhng thnh ph ca nc M, cng ang i

mt vi nhng vn cung cp nc. Nhiu s la chn m ra cho nhng ai

mun gim cc vn v nc bng nhng phng thc thay th khc, nhng

cch thc qun l cung cp nc tt hn, hay kim sot s gia tng.

TNG LAI CA LNH VC QUN L NC

Cc thnh ph cn nc bt u xem nc nh mt mt hng c th mua v

bn trn th trng, nh du hay kh t. Cc thnh ph sn sng tr tin v trnh

quy nh v nc hin ti, sau phn phi v nh gi. Nu gi c nng ln va

tm, nc mi c th s dng t nhng ngun gc khc nhau. V d cc khu

vc ti tiu (qun l nc cho mt khu vc nng nghip) c th hp ng vi

mt thnh ph cung cp nc cho khu vc ni thnh. H c th lm vy m

khng lm gim lng nc cung cp cho v ma bng cch tho thun lm gim

lng nc b mt i do bc hi hay mt nc khi tho ra cc knh o. Cc khu

vc ti tiu khng c vn cho cc phng thc bo tn nc t tin, nhng

s tin c tr bi cc thnh ph c th cp vn cho nhiu d n. iu r rng

cho thy nc cng tr nn t gi hn trong tng lai. Nu gi c cng thch hp,th cng c nhiu phng thc i mi. Nghim tc nhn ra rng tng ko cc

ni bng cn nguyn cc vng bng gi ti cc khu vc ven bin ni nc sch

rt cp thit.

Luna Leopold (mt chuyn gia trong ngnh hc ti nguyn sng nc) ch

ra mt lun im mi trong ngnh qun l nc l cn phi da trn kin thc

a cht, a l, v cc nhn t thay i kh hu cng nh nn ti kinh t, x hi vchnh tr. ng cho cho rng qun l nc khng th thnh cng khi cha nm

c ci cn bn hay nm bt hi ht quan im kinh t v lp trng chnh tr.

Tuy nhin, l cch vn s dng nc c tip cn. Thut ng s dng

nc c t hp bi v chng ta t khi thc s qun l nc.

Nc b mt v nc ngm u l ti t nhin theo thi gian. Trong nhng

nm ma nhiu, nc b mt c lm y, v ngun nc ngm gn b mt cng

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c b sung. V trong nhng nm ny, hi vng cng tc phng chng lt, bo, h

thng cu cng c th chng chu c vi lng nc d di. Mi h thng c

thit k chng chu c vi lu lng chy nht nh (v d, lng chy trong

20 nm), nu vt qu gii hn c th gy nguy him hay l lt.

Nhn chung, Leopold kt lun rng chng ta cn phi chun b nhiu hn cho

cc trn l nng n hn l vic thit ht nc. Cn trong nhng nm kh hn,

chng ta cn c nhng k hoch lm gim nhng kh khn, v d, lng nc

di b mt ca nhng khu vc khc nhau pha Ty nc M li qu su

u t, cp vn hay s dng nc gn b. Lng nc ny b phn lp t chu

trnh nc v v th khng ph thuc vo s np li ca t nhin. Lng nc lnc s dng khi nhu cu s dng cao, v d nh khi thc vt cn nc n c th

vn r m su xung t tm ngun nc. Hn na, thc vt c th lm sch

nc. Ti s dng nc u n cho cc nhu cu cn thit c th tn nhiu chi ph

v b phn i v nhiu l do, nhng vic thit k chu trnh x l nc hin nay lc

tnh trng khn cp th c th c m rng v pht trin.

Nc ngm c b sung t nhin vo nhng nm ma nhiu, chng ta c thpht trin cy ci v s dng nc b mt khi c th v khng ngi s dng nc

ngm vo nhng nm kh hn. Ni cch khc, nc ngm c th bm ra khi b b

sung lm y qu mc s dng vo ma kh nhng c th c b sung 1 cch

t nhin vo nhng nm ma nhiu hay do con ngi tc ng (do ti tiu, bm

nc tha ln b mt t). Phng thc qun l nc ny c th lm tha v thiu

ht nc mt cch t nhin v c th c p dng.

QUN L SNG: SNG COLORADO

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Thng ngun sng Colorado v cao nguyn Wind River Wyoming, cch

2.300 km qung ng sng chy ra bin, qua 7 ni: Wyoming, Colorado, Utah,

New Mexico, v California Mexico. Ngay c khi vng trng rt nc rt ln,

vy quanh nhiu khu vc ca Ty Nam nc M, lu lng hng nm vo khong

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3% ca sng Missisippi v di 10% ca Columbia. V vy, khch thc sng

Colorado l lu lng tng hp v cha tr thnh 1 trong nhng dng sng n

nh nht, nhiu tranh ci v vn nc nht th gii. Cuc tranh ci v sng

Colorado ca cc trung tm ln v pht trin khu vc nng nghip ca California,Colorado, New Mexico v Arizona. S cn nc cc khu vc kh ny l kt qu

ca s lm dng, phung ph v lm suy gim cht lng nc. Tho thun gia

cc tiu bang a ra cc cch gii quyt ca to n, hip c quc t i vi

nhng ngi s dng nc lu di ca sng. Nhng lut l k tha v nhng quyt

nh ca ta n c th thay i ty thuc kiu hnh v cch s dng nc tip tc

lm nh hng n i sng v k sinh nhai ca hng triu ngi Mexico v M.

Nc sng Colorado b chim gi theo nhng ngi s dng khc nhau, bao

gm 7 bang v Mexico. Vic chim gi ny xut hin t nhiu nm thng qua

thng lng, hip c quc t, tho thun lin bang v quyt nh ca to n ti

cao, gi chung l lut ca sng. Hai trong s nhng nhng vn bn sm quan

trng trong lut ny l hip c sng Colorado nm922, chia quyn gii hn nc

trong gii hn trn, di ca lu vc v tho thun vi Mexico nm 1944, tho

thun lng nc chia hng nm trong khong 1,85 km3 (1,5 triu feet Anh chiu

di, 1 foot Anh su ct nc trn, khong 325.829 gallons) ca nc sng

Colorado cho Mexico. Gn y l quyt nh ca tan n ti cao M, 1963 bao

gm Arizona v California. Nhng phn nn ca Arizona t 1922 v xung t vi

California lin quan n vn s dng nc. Quyt nh ca to n l pha Nam

California phi nhng khong 0,74 km3 (600.000 feet Anh) ca sng Colorado

khi d n trung tm Arizona hon tt. Cui cng, nm 1974 quyt nh qun l

vng sng Colorado do b nhim mn c ph chun bi quc hi Hoa K. y

quyn cho vic qun l, kim sot mn ca sng Colorado,bao gm vic trng

nhng cy c kh nng kh mui hon thnh n gia nhng nm 1990.

Lu vc sng Colorado vn cha n nh, lu lng hng nm khong 20 km3

lc trc, by gi ch khong 16,6 km3. L trn sng Colorado trn v khi tuyt

tan, ma ng di v lng nc ln, ma h ngn vi nhng con bo nhit i,

tng lng nc hng nm v vy m ri vo tnh trng khng hong. Trn sng

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c nhiu p v h cha nc nhn to. 19 con p cao trn sng lu gi khong

186,3 km3 (70 triu feet nc). 80% c lu gi 2 h cha nc nhn to, trn

hm ni Hoover v Glen. Nu vic qun l hiu qu, n s l ni lu gi nc d

tr trong vi nm . Tuy nhin, nu hn hn xy ra, vic phn phi nc tr nnkh khn. Sng Colorado l mt trong nhng con sng ln ca quc gia. Vic

cung cp nc cho ngi s dng s cng tr nn kh khn v nan gii.

Vic xy dng cc con p, h cha nc v i hng sng Colorado thnh

cng v l im nhn cho vic cung cp nc. Tuy nhin, khng phi lun lun

nh vy. V d, bin Salton thung lng Imperial hin ti c hinh thnh t nm

1905 1906 khi sng Colorado b lm chch hng khng c ch ch v phaNam thung lng Imperial. Cng thi im sng Colorado cha c p v vic

kho st lu lng sng lnh th Mexico tht bi v trn l nm 1905 -1906.

Sau sng c kho st nm 1907. Bin Salton lc c hnh thnh vi mc

nc cao hn by gi. Nc Salton c gi li khi nc chy trn v c

dng cho vic ti tiu v lc mui cho t nng nghip. Khi lng nc chy

trn dng li hay gim bt, bin Salton nhanh chng b rt nc v bc hi nhanh

chng. V h ny tr thnh a im gii tr (cu c) nn tng lai ca n gy

nhiu tranh ci. Nu nc h tr nhin nhiu mui hn by gi, h sinh thi v

ngh nh bt, nui trng thy sn s gp nhiu tr ngi. y khng phi l h

duy nht ng bng Salton, nhng h khc thung lng Emperial sut thi gian

a cht lu di tr nn kh hn,

Ngay c khi lng nc cung cp nguyn vn nh trc y, vn trn lu

vc sng Colorado v vn qun l cht lng nc s c cp. C khi kim

loi nng v cc cht phng x c thu gom trong lu vc v cc h cha th

mui l nguyn nhn chnh ca rc ri. Nng mui khong 550 ppm cao hn

tiu chun cho php cho ngi tiu dng, hn 750 ppm s lm nng nghip nh

tr. Lng mui t nhin ca sng Colorado thng ngun ch khong 50 ppm.

Khi sng chy ra bin, nhng dng nc mui chy trn vo sng, di iu kin

t nhin th nng mui ca Colorado t khong 250 380 ppm. Tuy nhin,

vic chy ngc dng v s bc hi nc s lm tng nng mui ca h lu

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sng Colorado vo khong 1.500 ppm, v ti thi im hin ti l 2.700 ppm.

Cht lng nc qu ngho, khi nng dn Mexico cho nc chy qua cnh ng

ca h, lng mui nhiu hn cho php gy nguy hi cho cy trng v t ca h.

M v Mexico ng vo nm 1973 rng M s chia phn nc cho Mexico vinng mui khng cao hn 115 ppm p Imperial, cch bin gii mt khong

ngn v pha thng ngun. Nng mui y khong 800 ppm. lm c

iu ny, cng trnh trng nhng loi cy kh mui ln c thc hin gn ng

bin gii gi tr vi trm triu v hn 10 triu mi nm duy tr v hot ng.

y l kinh ph vn u t nhm n lc kim sot nng mui ca sng. Ch c

thi gian mi c th tr li c hiu qu ca c gng ny.

Vn qun l nc khu vc sng Colorado rt phc tp, nhng n cho thy

nhng vn ln nh vic i mt vi tnh trng kh hn Ty Nam nc M

vo nhng nm ti.

1. Gia tng s cung ng nc s dng

Mc d khng th lm gia tng lng nc ngt trn tri t nhng con ngi

c th iu phi li ngun nc ngt s dng. C hai vn c t ra trong

vic qun tr ngun ti nguyn nc ngt: mt l gia tng s cung ng nc tiu

dng, hai l gim s s dng v hao ph ngun nc. Vn v vic qun tr

ngun ti nguyn nc rt khc nhau hai khi cc quc gia km pht trin

(LDCs)v cc quc gia pht trin (MDCs): cc quc gia km pht trin d c

hoc khng c nc th h cng khng c tin d xy dng cc p, h

d tr nc v h thng phn phi nc, v vy nn dn c u tp trung sinhsng nhng ni c ngun nc. Ngc li cc quc gia pht trin, ngi

dn tp trung sng nhng ni c kh hu thun li mc d nhng ni ny khng

nc i na, nhng h vn c ngun nc s dng nh h thng dn

nc phn phi n tn n hoc h cn c th tp trung sng nhng vng

ngp lt th h cng c h thng gi nc hoc thot nc khi cn thit (Miller,

1988).

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a) p, h cha nc nhn to v knh o

Nhng tranh lun ca chng ta v vic cung cp nc nh phn phi cho

nhng vng pht trin Nng nghip, ni thnh i hi ngun cung cp nc gn.

hon thnh h thng lu gi nc v phn phi nc bng cch xy dng cc

knh o v cc ng dn t h cha nc khi cn n. Nhng d n pht trin

nc m khng quan tm n vn mi trng to s phn i vi ngi dn

xung quanh, ng thi ph hu v lm suy thoi h ng thc vt hoang d v a

dng sinh hc.

P V CC H CHA NC

Cc con p v cc h nc thng c thit k vi cu trc c nhng chc

nng ring bit. l, nhng ngi xut xy dng p v cc h cha nc ch

ra rng cc h cha nc c th c s dng kim sot l v m bo vic

cung cp nc. y l iu quan trng dung ho nhng cch s dng nc

khc nhau vn gp nhiu kh khn. V d, nhu cu nc trong nng nghip c th

cao trong ma h, kt qu l rt nc trong h v to lp bn phm vi rng. y

vn l a im gii tr th v khi mc nc h thp nhng nhng lp bn lm mt

i m quan ca n, v nhng tc ng ca vic nhu cu nc ln c th tc ng

xu n t nhin hoang d (c bit l c) bi vic cn tr hay gii hn thi gian

chu k trng n. Cui cng, p v cc h cha nc nhn to gip duy tr h

thng sng di cu trc lu gi nc, bi v p khng th hon ton bo v

chng ta khi nhng trn lt ln.

Mu thun y l cng ngy nhu cu xy dng p v cc h cha nc cng

nhiu, nhng phi m bo cnh quan xung quanh. Nu loi hnh s dng nc

trong nng nghip v ni thnh khng thay i th vic cung cp nc cho th

vn c u tin trn ht.

Ni no trn sng c cng trnh p, h cha nc nhn to th h thng sng

s thay i mi mi. Lu lng nc v cht trm tch b thay i, cng nh tnh

cht l sinh hc ca mi trng v t bn di p.

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KNH O

Nc t h cha nc pha trn cn c dng a xung bn di bng

nhng cch t nhin nh sui, lch hay cc knh o v ng dn nc.

Knh o, d c gii hn hay d tho nc, thng d thu ht nhiu mi nguy

hi cho con ngi v ng vt. Ni n chy qua khu vc ni thnh , lm ngp lt

khu vc v e do. Khi n c dng d tho nc, knh o lm mt nc xung

b mt t. Cho d n c th c cho l mt dng b sung nc cho h thng

nc ngm, nht l nhng vng c cht lng nc ngm ngho nn. Trong

trng hp ny, knh o d tho nc lm r nc xung t v lm mt nc.

Vic xy dng cc knh o, c bit l cc nc ang pht trin, thng to

nhiu cc vn mi trng. V d, khi con p High Dam Aswan, Ai Cp c

hon thnh nm 1964, knh o cn chuyn nc n cc khu nng nghip. Knh

o tr nn ng s vi cn bnh sn mng . Cn bnh lun lun l mt vn

Ai Cp, nhng dng nc l ca sng Nile dng ln chm chp hng nm ra

sch. Lng nc ln ti trong knh by gi tr thnh ngi nh cho cc loi sinh

vt. Nhng cn bnh lm suy nhc c ph bin mt vng Ai Cp v hu nh

ton b dn s ca vng u chu nh hng. Knh o ca ngi Ai Cp cng

l ni ca mui, mt vi trong s mang n bnh st rt.

Ngy nay, h thng h cha v knh o c qui hoch trong 1 lot cc loi

mi trng khc nhau. M, li ch mi trng bo m rng vic xem xt s

kin mi trng quan trng no s din ra. N khng lun ng cho nhiu quc

gia ang pht trin, ni , li ch mi trng khng c u tin nhiu nh l

vic sn xut lng thc. Cc knh o rng v di c th lm bin i vng t

v mi trng sinh vt bng cch gy ra mt h thng nc mi v khc bit v

chn ngang s di tr ca ng vt hoang d. iu khng c ngha l khng th

pht trin ti nguyn nc nhng nc ny, nhng n nhn mnh rng li ch

ca s cn thit v mi trng trong mc h sinh thi khi qui hoch v pht

trin ngun ti nguyn nc. T nht, chng ta c th gi thng tin cho cc nc

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ang pht trin v nhng thnh cng v tht bi trong khi qui hoch cc d n v

nc, m chng c th mang li li ch t nhng kinh nghim ca chng ta.

Pht trin ti nguyn nc v li ch mi trng khng nht thit phi tng thch

vi nhau. Tuy nhin, tha thun s t c nu cht lng mi trng c mbo.

b) Khai thc nc ngm

Ngun nc ngm chim 95% l nc ngt cung ng trn th gii. S khai

thc ngun nc ngm c tin hnh t lu cc quc gia pht trin. Hoa K,

khong 50% nc ung cho dn c (96% vng ven v 20% th), 40% lng

nc dng ti tiu u c ly t nc ngm.

Vic gia tng s dng nc ngm hin nay t ra nhiu vn cn quan tm:

- S cn kit ngun nc ngm: do s khai thc ly i nhanh hn s trc di

ca nc lm cho ngun nc ngm tr nn cn kit. Th d s cn kit ngun

nc ngm xy ra California, min Bc Trung Quc, Mexico v n

...l do khai thc ti tiu.

- S ln sp: Khi lp nc ngm cn b ly i nhanh to nn khong trng

trong cc lp ngm nc l nguyn nhn gy nn s ln sp. Hin tng ny

xy ra vo nm 1981 California tn ph nh ca, nh my, ng dn nc,

ng xe in ...

- S nhim mn: S khai thc nc ngm cc vng ven b bin to nnkhong trng trong cc lp ngm nc, lm cho nc bin trn vo chim ly

khong trng gy nn s nhim mn ngun nc. S nhim mn ngun nc

xy ra nhng vng ven b bin ca Israel, Syria.

-S nhim ngun nc: Khi khai thc nc ngm s dng cho ti tiu, cho

sn xut cng nghip v cho sinh hot, lng nc thi c th len li theo cc

ng ng dn lm nhim ngun nc ngm. S nhim nc ngm xy ra nhiu nc pht trin v c Hoa K, ngun ti nguyn nc ngm b nhim

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bi hot ng nng nghip v k ngh, nc ngm b nhim mun phc hi li

phi mt hng trm thm ch n hng ngn nm.

c) S kh mui

Nc bin c mn khong 3,5% (tng ng 40gram mui trong mt m 3

nc, mun s dng c chng ta phi kh mui, v qu trnh ny rt tn km.

Tiu chun nc sau khi x l l mn khong 0.05%. bng phng php ny

chnh ph M cung cp c khong 20.000-30.000 m3 nc mi ngy, v

ngi tiu dng phi tr vi gi gp 10 ln so vi bnh thng. Bi v vic kh

mui cn nhiu nng lng, nn ga nc s tng theo nng lng. phng php

ny ch c s dng khi khng s dng c ngun nc no khc.

S kh mn l phng php ly lng mui ha tan trong nc bin hay nc

l to ra nc ngt l s kh mn. Hai phng php thng dng l s

chng ct v thm thu ngc (reverse osmosis): s chng ct bao gm vic un

si nc mui, hi nc bc ln c dn theo ng dn n ni lm lnh, hi

nc ngng ng li thnh nc ngt; s thm thu ngc l cho nc mui i

qua mng mng, mng ny c l rt nh ch cho cc phn t nc i qua cn mui

th c gi li. C hai phng php trn u c chi ph qu cao, khng c gi tr

kinh t. Ngoi hai phng php trn, ngi ta cn s dng phng php cho dng

in i qua dung dch nc mn, dng in l kt ta mui v phn nc cn

li l nc ngt.

Bi v s gia tng dn s, lng nc khng p ng lm xut hin nhng

thi k kh hn trong thi gian gn y, mt s bang M nh: Santa Barbara,

California a ra phng n kh mui nh thc o cho s cp bch ca tnh

trng kh hn trong tng lai.

Cc quc gia vng Trung ng ni ring vn phi dng phng php kh

mn. Nc mt v nc ngm khu vc Trung ng, l nc l vi mn

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khong 0.5 %(bng 1/5 so vi nc bin). R rng vic kh mn y s t tn

km hn, v cng c nhiu phng n cho cc khu vc ny.

d) Ma nhn to

mt s nc, c bit l Hoa K thc nghim nhiu nm to ma nhn

to nhng vng thiu nc ti vi nhng ha cht to ma. Phng php

ny l bn vo trong nhng m my cc cht nh Iode bc hoc t la cho bc

ln cao. Nhng ha cht ny hoc cc bi than c tc dng nh l ci nhn lm kt

t dn nhng ht nc nh ri ln dn ln, nng v ri xung t to nn

nhng cn ma nhn to. Nm 1977, ma nhn to thnh cng 23 tiu bang

Hoa k v cung cp 7% lng ma trong c nc.

Vic to ma nhn to khng thnh cng nhng vng kh v nhng vng

ny khng my. Hn na, vic s dng Iode bc to ma lm cho t v

nc b nhim Iode bc, ngoi ra s tch t Iode bc trong t bo ca thc vt v

khuch i dn qua c th ca ng vt thng qua chui v li thc n v cui

cng gy nhim c cho con ngi gy nn nhiu tranh ci ( Miller, 1988).

2. Bo tn nc:

Tm quan trng ca vic bo tn nc

Nc rt cn thit cho mi c th sng v cng cn cho a s cc hot ng

ca con ngi. Nhu cu v nc c con ngi s dng ngy cng tng theo

nhp pht trin ca nn cng nghip, cng nghip v nng cao mc sng cacon ngi. Trong iu kin sn xut ngy cng pht trin v dn s ngy cng

tng, cc hnh thi v quy m tc ng ca con ngi trong sn xut tng ln lm

nh hng n ngun ti nguyn ny:

- S pht trin mnh m ca nn cng nghip hin nay cng i hi s cung cp

mt lng nc rt ln cho cc hot ng ca nh my.

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- S pht trin nn nng nghip theo hng thm canh tng v cn s dng nc

ngy cng nhiu hn nn cng lm kit qu ngun ti nguyn nc ngt nht l ti

mt s vng kh hn trn th gii.

- S gia tng dn s ng thi vi s nng cao i sng x hi cng i hi lng

nc s dng ngy cng nhiu cho nhu cu sinh hot ca con ngi.

- Tt c cc hot ng mt mt lm tiu hao ngun nc trong t nhin, mt

khc cn thi ra ngoi mi trng nhng cht thi lm nhim bn ngun nc gy

nhiu nh hng tai hi trc mt v lu di.

Ngy nay s nhim bn ngun nc cng ngy cng tr nn nghim trng.

Trong tnh hnh ny, vn bo m nhu cu nc cho i sng x hi tr

thnh mc tiu phn u trong pht trin kinh t v bo v ti nguyn. Loi ti

nguyn ny l c gii hn, khng phi l v tn, khng cn l th ca tri cho,

mc sc m s dng na. V th, vic bo tn nc trn th gii l iu cp thit.

C th lm g s dng nc hiu qu hn v gim bt vic s dng nc?

Pht trin ti tiu nng nghip c th gim bt s lng ph khong 20-30%. Vic

pht trin bo tn nc bao gm ngn cch v che ph cc con knh gim bt

s r r v bay hi; kim tra v lp danh mc nc thot khi knh rch, hp nht

vic s dng nc b mt v nc ngm; vic ti tiu ban m; ci tin h thng

ti tiu (ti di dng ma ro hay nh git); chun b t c m cao.

Lng nc s dng cho cy trng trong trang tri ch khong bng 6% tng

lng nc rt xung. Tuy nhin, bi v vic s dng nc ny mang tnh cht tp

trung nn t ra mt vn ln trong khu vc. Vic tiu hao nc ny c th

c bn c gim mt lng nh vi dng nh tm hiu qu hn v t vt c

nh pha di, ti nc vo m v h thng ti nh git.

Vic con ngi nhn thc tm quan trng ca lng nc cung cp nh th

no quyt nh lng nc c s dng. V d, ngi dn sng Tucson,

Arizona, l khu vc sa mc (vi cc loi cy sng thch nghi vi ni y nh

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xng rng). Lng nc cung cp cho Tucson c ly t ngun nc ngm,

ang c khai thc (m vic s dng nc nhanh hn nhiu vic b sung nc t

thin nhin); nc c s dng vo khong 605 lt (160 gallon) mi ngi mi

ngy. Khng xa l ngi dn sng Phoenix, Arizona, s dng khong 983 lt(260 gallon) mi ngi mi ngy cho cc bi c ln, cy du tm, v Phoenix b

coi l mt c o v lng nc s dng.

T l nc cng khc nhau. Ngi dn Tucson phi tr cho lng nc 75%

hn so vi ngi dn sng Phoenix, ni m lng nc cung cp b rt xung t

Salt river nhanh hn t nc ngm. Lng nc Tucson gy ra nhiu tranh ci,

v nhiu nh doanh nghip nhn thy nc l mt cng c ng gi. iu ny cngha l, v nc Ty Nam nc M v nhng khu vc khc c kh nng cung

cp thp trong tng lai, chng ta c th lm mt ci g cho vn sa mc

ho Tucson, c bit l nhng vng nh Los Angeles v San Diego.

S vn ng ca nc thnh cc dng chy to ra in c th b lm gim n

25 30% do s dng cc thp nc lm mt c thit k dng t nc hoc

khng cn dng nc. Sn xut v cng nghip c th hn ch lng nc b rtbng cch tng cc cch s l bng thc vt v chu trnh nc hay bng cch pht

trin nhng cng c mi v quy trnh cn t nc hn. Bi v nhng cuc tranh

lun v ch nc thay i rt nhanh chng, mong rng s c nhiu i mi

lm gim lng nc mt i do cc nhu cu s dng khc nhau, trong khi bit

rng nhu cu s dng nc ngy cng tng.

Mt s phng n nhm bo tn nc

a) Gim s tiu hao nc do thy li

Do s phn phi lng ma mi ni khc nhau ty theo iu kin kh hu v

a hnh, c nhng vng th nc rt di do cng c nhng vng rt thiu nc.

Nc c s dng canh tc nng nghip phn ln c ly ra t ngun nc mt

i khi cng phi ly t nc ngm.

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Trong nng nghip, c nc cho canh tc ngi ta thng xy dng h

thng thy li dn nc t ni c nhiu nc n ni thiu nc. Ngun nc

ny c dn theo h thng knh mng c o theo quy hoch ca thy li, s

s dng h thng knh mng dng nc lm tht thot i mt lng nc ngk. Theo s c tnh c khong 50% ngun nc ny b mt i do s bc hi ca

lp nc trn mt cc knh mng v do s trc di xung cc lp t y cc

knh mng.

gim bt s hao ph , mt s nh nng s dng hu hiu h thng

ri nc. Theo h thng ny th nc c bm vo phn trung tm cu h

thng v t y nc theo mt h thng ng dn chng cht n ti cho cytrng vng kh hn, vi h thng ny, ngi ta lm gim s hao ph t 30% -

50% so vi khi s dng h thng thy li. H thng ri nc kiu ny rt ph

bin Israel t nhng nm 60, cc ng dn chng cht v ch gii phng mt th

tch nc rt nh ha tan phn bn cho r cy hp th, tit gim n mc ti

thiu s bc hi nc v trc di nc xung cc lp t bn di. Hoa K, h

thng ny cng c s dng vng California v Florida (Miller, 1988).

b) Gim hao ph nc trong hot ng cng nghip

Theo s c tnh th ch ring 5 ngnh cng nghip ln nh cng nghip thc

phm, du ho, giy, luyn kim v ha cht s dng ht 90% tng lng nc

c s dng cho ton b cc ngnh cng nghip. Lng nc sau khi c s

dng c a tr li vo sng h cha y nhng cht gy nhim.

lm gim bt nhim ngun nc v ng thi lm gim s hao ph nc

trong hot ng cng nghip, cc phng hng cn c tin hnh nh tit gim

ti a nc s dng trong cng nghip, k c vic chuyn nhiu x nghip qua quy

trnh cng ngh kh; a cc trm nhit in v cc x nghip cng nghip vo

chu trnh cung cp nc khp kn sao cho nc c s dng cho x nghip ny

sau khi c lm sch ri s dng cho x nghip khc.

c) Gim hoang ph nc gia dng

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S hao ph nc gia dng xy ra l do s r r ca cc ng ng dn, vi

nc cc nh v sinh, nh tm trong mi gia nh v cng cn do s tiu xi

hoang ph ca con ngi. Theo s c tnh s hao ph ny chim khong t 20%-

35% nc c cung cp cho sinh hot. Hin nay, hin tng hao ph ngun nc gim cc thnh ph do ngi ta s dng bin php ci t ng h nc, do

mc s dng nc trong sinh hot gim i 1/3 v mi ngi c gng tit

kim nc tit kim tin.

VIII. NC V H SINH THI

Cc h sinh thi chnh ca th gii tin ha cng vi cc iu kin vt l m nbao gm kh hu, cht dinh dng, ngun vo, cc loi t v thy hc. Thay i

cc nhn t nh hng n h snh thi, c bit, s thay i do con ngi gy

ra nh hng rng ri. Khp th gii ngy nay, vi 1 t trng hp ngoi l, con

ngi lm gim gi tr sinh thi t nhin ca vi qui m a phng v ton cu.

iu kin thy hc, c bit qu trnh v cht lng nc mt l yu t hn ch

cho tnh trng ca nhiu h sinh thi. iu c bit ng khu vc m ly.

Vic m rng s dng ngun ti nguyn nc c tc ng ln vo h sinh thi.

Cc cng trnh p nc ln, v d vic xy dng p nc High Dam Aswan,

Ai Cp ly i ph sa pha ng a Trung Hi t sng Nile lm gim i 1/3

vic to ra cc sinh vt ph du. Kt qu l c mi, c vn, tm hm, vic nh bt

tm gim st a Trung Hi, v cc ngun ny ph thuc vo ngun thc n l

cc phiu sinh vt ph du.

Lu vc sng Amazon Nam M c lu lng trung bnh chim 20% lng

nc ngt so vi cc con sng ca th gii. Lng nc ra bin ln n mc

cc thy th c th ly c nc ngt cch t lin 150km. Dng sng c nhiu

nhnh, vi nhnh c kch c vt tri nh sng Mississpi, n l ngun ti nguyn

to ln c th pht trin thy nng, v Brazil hi vng pht trin n. p Tucurui

trn sng Tocantin lu vc sng Amazon c mong ch c th cung cp 8000

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megawatts in, nng lng tng ng vi 8 nh my ht nhn. Tng nng

lng lu vc sng c th t 100 000 megawatts.

K hoch h cha Tucurui s lm ngp nc 2000km2 rng ma nhit i

cha khai ph. Vn ny v d n tng t chc chn s tc ng n lu vc

sng Amazon, n dng chy h lu nh hng n thc vt v ng vt hoang

d. c bit ng nu thuc dit c l cn thit kim sot c di sng nc, v

d nh lan d hng nc, n cung cp mi trng thun li cho cc bnh st rt,

bnh sn l mng. Thuc dit c s dng trong qu kh c cht 2,4-D, n c cha

Cht c mu da cam, mt loi ha cht lin quan n bnh ung th v sinh ra

qui thai. Cc ng vt c v di nc n thc vt v cht c thm nhp vo.

Cc h nc mi c to vng nhit i lm ny sinh cc vn km theo

bao gm s phn hy k kh ca cc vt liu hu c trong nc su v s lp y

ca cn lng trong h cha nc. Phn hy thc vc cung cp acid, n c th gm

mn thit b, cn lng lm gim bt gi tr ca h cha trong vc d tr nc v

trong in nng. iu ng lo ngi l vic khai thc g u ngun v thng lu

ca p Tucurui vn tip din v ng quan tm l h cha nc s gia tng ximn t v vic sn xut cht lng ng quan tm, lm gim gi tr ca h.

Rng ma nhit i bao gm hn 50% loi trn Tri t. Hu nh mt s loi

cha c nghin cu, mt s c th cha cc cht ha hc gip chng li bnh

tt, c th lm cy lng thc, hoc c nhng gi tr thc t khc. Th nghim

qui m ln thay i thy hc c bn 1 s vng nn cn thn trnh thit hi

h sinh thi mt cch khng cn thit.

IX. TNG KT

Vng tun hon ca nc bao gm s di chuyn, s lu tr, s chuyn i ca

nc t 1 phn ca vng tun hon sang cc dng khc. S di chuyn ca nc

trong t s chy trn trn b mt v di b mt t - l 1 phn ca vng tun

hon c lin quan trc tip nht n con ngi. Nhn chung, nc l mt trongnhng ngun ti nguyn phong ph nht c th ti to c. Tuy nhin, hn 99%

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lng nc trn tri t khng th s dng hoc khng ph hp vi nhu cu s

dng ca con ngi do mn v v tr ca n.

Thuc tnh c nht ca nc lm cho n tr nn khng th thiu c cho i

sng ca con ngi. Nhiu thuc tnh trong s c c l do s sp xp ca cc

phn t. Nc l 1 dung mi ph bin, n l thnh phn c bn ca tt c cc c

th sinh vt v ng vai tr quan trng trong vic quyt nh thnh phn ca t.

Nc dng cc mng mng bao quanh phn t t v nhng mng mng ny

quan trng trong d di chuyn ca nc trn mt nc ngm.

Dng chy ca nc trn mt t c chia theo lu vc hoc ng phn

thy. Dng chy v lng trm tch thay i rt nhiu nhng khu vc khc nhau.

S thay i ny chu nh hng ca yu t a cht, a vn, kh hu, v nhn t

sinh hc. Nc c 3 cch chy chnh: chy trn b mt, chy di b mt t

nng, v nc chy ngm. Hiu c ba kiu vn hnh ny c ngha quan trng

trong vic hiu bng cch no t s dng thay i li c th nh hng n vic

chy trn v sn phm trm tch.

Nc ngm i bo ha pha di mc nc ngm. Ngun chnh ca n

lng ma thm qua i b cp i xung i thm ht. Mt vng ca tri t c

kh nng cung cp nc ngm mc thch hp t 1 ngun nc. S hin din

ca lp hu hn pha trn tng ngm nc c th y nc t tng ngm nc ln

b mt. C hng v tc ca s di chuyn nc ngm ph thuc vo dc

thy lc (trong trng hp n gin nht xp x vi ch dc ca mc nc ngm).

nh gi kh nng cung cp nc ca mt vng, tr lng nc c pht

trin xc nh tnh bin thin v kh nng s dng ca nc. Kh nng cung

nc th c gii hn, ngay c nhng vng c lng ma cao v lng chy rng

ln. Trong nhiu vng, nc ngm c khai thc vt qu lng nc c b

sung ,v trong mt vi vng vic ny lm thay i vnh vin tnh cht ca t. S

kh mui nc bin tip tc c s dng khi nhng ngun nc khc khng th

s dng c.

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Vic xy dng p trn sng gip cho vic m bo cng sut phn phi nc,

cho php ti nhng vng hn hn trc y, nhng vic ny mang li cc du

hiu v s thay i h sinh thi. Thm vo , ti theo hng ngc dng sng

lm tng ln mt cch ng k mn ca nc chy xui dng. Xy dng ,h cha nc, v h thng knh o gy nn vn lin quan n mi trng

v cc vn v sc khe, c bit l nhng nc ang pht trin. V vy qun

l ngun ti nguyn nc cn mt trit l mi gm cc nhn t a cht, a l, kh

hu v nhng cch tn dng sng to khc. S m t thng kh nh ngha bi

v 1 vi m ly ch m t trong khong thi gian rt ngn ca nm. Tuy nhin,

s c mt ca nc d trong khong thi gian rt ngn u n, lm tng c im

t ca m ly v c bit l lm thc vt pht trin.

Wetlands ( Cc kiu m ly)

Swamps (m ly do cy ci hoc cy bi chim u th)

Marshed (Loi m ly thng xuyn hay lin tc b ngp nc)

Bogs (dng m ly c tch lu mt s lng ln than bn)

Prairie potholes (loi m ly ngp nc nh nh h)

Vernal pools (nhng ni thp nng thnh thong gi nc)

Nt c trng chung v qu trnh hot ng ca m ly c nh ngha l

n b ngp bi nc hoc vng t bo ha nc vi su vi centimet t nht l

vi ngy trong nm. Thnh phn chnh thng xc nh s c mt hoc s vng

mt ca m ly l tnh trng m t, cc loi thc vt, loi t

m ly v cc h sinh thi km theo c nhng c trng mi trng quan

trng :

Nhng m ly ven bin nh m ly ngp mn cung cp vng m cho

vng t lin t xi mn ven bin cng vi bo v nhng cn sng ln.

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Nhiu m ly l nhng nh my lc t nhin ca thin nhin. Cc thc vt

trong m ly c th gi li cn b v nhng c t mt cch hiu qu.

Nhng m ly nc ngt l ming bt bin lc t nhin. Trong sut ma

l chng gi nc, gp phn gim bt dng l h ngun. Nc c gi lithot ra chm sau cn l.

Cc m ly thng l nhng vng t ln,ni m nhiu dng cht v ha

cht l chu trnh t nhin cung cp ni cho mt lng ln cc ng vt

hoang d v thc vt khc nhau.

Cc m ly nc ngt thng l vng t ca ti lng nc ngm cho

tng thy cp. Mt vi ci trong s chng - chng hn nh m ly (spring

fed marsh) - cc im ra ca nc ngm.

DCMT moi

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