Hoi Dia ly Vift Nam Dai hoc Thai Nguyen NGHlfiN CLTU P H A N CAP XOI MON TltM NANG LUXl VlTC S 6 N G GAM
( P H A N L A N H T H 6 VIET NAM)
Nguyen Quyet Chien", Dd Van Thanh' Abstract
Research the potential soil erosion in Gam river basin (tbe territory of Vi^nam)
Potential erosion value of the Gam river basin is calculated based on universal soil loss model (USLE) of Wischmeier and Schmid. The results for the potential erosion across the entire basin and level 2 basins by 5 levels of potential erosion show: the area of potential erosion at very low level and low level accounted for 63.7%. at high level and very high level accounted for 16.6% of the total area of the basin.
Between the river basins level 2 is quite clear differentiation of the level of potential erosion, in which, river basin level 2, river basin level 4 and river basin level 3 have the highest soil erosion risk.
1. Md DAU
Sdng Gim l i phy luu cap 1 ldn nhit cua he thong sdng Ld-Gam. Dogn ddng chinh song Gam tren lanh tho Vigt Nam dii 297 km, vdi dien tich luu vyc la 9.168 km^ chilm 63,23% tong dien tich toan luu vyc sdng (LVS) Gam Theo ranh gidi hanh chinh, LVS Gim nim tren dia phgn eua 17 huyen thudc 4 tinh Ii H i Giang, Tuyen Quang, Cao Bang va Bic Kan Do ciu triic ^ a chit - dia hinh, Idp phu tho nhuong cua luu vyc phan hda da dgng, phirc tap trong dilu kien lucmg mua Idn va phan miia da lira tang nguy ca xdi mdn dat, dae bigt la d nhiing khu vyc cd miic dg che phu thuc vgt thap.
Xoi mdn tiim nang (XMTN) dupc xic djnh trong dieu kien ldp phu ihyc vgt bj huy bd, dya tien cic yiu t l ty nhien tirong dli In dinh nhu dja hinh, mua va tho nhudng vi chua tinh din vai tro cac bign phip canh tic dit. Phan cap XMTN LVS Gam phan anh sy phan hda lanh tho theo miic dp nguy co thoai hda dit, tao co sd khoa hpc eho viec xic djnh cic loai hinh su dyng dit tren toan luu vyc.
2. PHUONG PHAP D A N H G I A TIEM N A N G XOI MON D A T THEO LUtJ VUC 2.L Lua chon phirong p h i p danh gia
Tren till gidi da cd nhilu phuang phip danh gii xdi mdn dit. Nhirng phuang phip chii yiu trong dinh gii xdi mdn dit hign nay [4], [7], [10] l i
- Phan Iogi, phan vimg lanh tho theo miic dp xdi mdn tren CP sd dinh gia tong hop inh hudng ciia cac dieu kien ty nhien din xdi mdn. Day l i phuang phip dupc ip dyng rang rai d Lign Xd (cu) v i Tnmg Quoc. Cic bin do phan vung theo dp nguy hiem riem nang xuat hign xdi mdn dupc xay dyng trgn eo sd tong hpp cac ban do phan cip cic nhan to tu nhien tham gia vio qui tiinh xdi mdn nhu dia hinh, khi hgu, Idp phii thyc vgt, trong do ^ a hinh va khi hgu la nhiing nhin to dupc chii y nhilu nhat.
- Phuang phip mo hinh hoa dya tren ca sd md hinh thuc nghigm nghien ciiu qua trinh xdi mdn (md hinh thyc nghigm), nhiing kit qui quan sit xdi mdn thyc te (rad hinh kinh nghigm) hogc dya tren sy hiiu biet vl cic quy lugt vgn dpng v i co ehl cua qua trinh xdi mdn da dupe Iy thuyet hda dudi dgng cic dinh luat hay phucmg trinh vat Iy. Day Ii phuong phip mang tinh djnh Iupng, sii -Jiag cae md hinh toin thuc nghiem hoac rad hinh Iy thuyit dl thi hien qui trinh xdi mdn Phuang
p h ^ md hinh hda cd the cho phep iing dyng cic cdng nghe hien dgi vio nghien cuu tinh toan. Tuy nhign, do qui trinh xdi mdn dien ra rit da dang nen khi ip dyng cho cic lanh tho khac nhau can chii y toi nhiing dgc thu dja ly ciia timg dia phuang bing cich sii dyng cic thdng so ciia md hinh da (^pc kiem chiing cho dia bin nghien ciiu.
-J-Khoa Dja Iy - Bai hoc Su phgm Hi Njj Kpyeu Hpi nghi Khoa hoc Dia lp todn qudc ldn thu 7
Hoi Dia ly Vigt Nam Dai hgc Thai IN Do kha nang iing dung rang rai cdng cy CIS vdi cic uu thi ciia nhung ky thuat md qua trinh nghien ciiu nen tic gia da su dung phuang phip md hinh trong nghien ciiu da XMTN luu vyc.
2.2. Lua chon mo hinh danh gia a. Ca sd lua chon mo hinh
Md hinh dinh gii dupc sii dyng can dim bip eic yeu cau ehii yeu sau*
- Tinh kha thi, bao gdm c i viec khi thi ve nguon tu lieu v i phuang phip thyc hign - Tinh chinh xic v i phii hop ve thdng tin" ket qui tinh toin ciia md hinh phii cd dp chi cao, diy dii va phu hgp vdi muc tieu, nhiera vu nghien ciiu dgt ra.
- The hien dupc sy phan hoa khdng gian phu hpp vdi dieu kign chia cit dja hinh ndi ri(
phan hda cic dilu kign ty nhien ndi chung cua lanh tho nghign ciiu.
Trong nghien ciiu nay. tic gia da su dyng md hinh mit dat pho dyng USLE (Univers Loss Equation) ciia Wischmeier va Schmid dg danh gii XMTN LVS Gam, sau khi da Iogi bi C v i P. Viec lya chon md hinh nay xuat phit tir nhiing Iy do sau day:
Thii nhat, viec lya chpn md hinh USLE cho phep dip iing dupe yeu eiu nghien cim ph XMTN theo luu vyc, trong do cac nhan to anh hudng tdi xdi mdn dupc dl cap rapt each riei _ trong mdt raoi tuong quan chat che.
Thii hai, nhieu tac gia nghign ciiu xdi mdn tren the gidi va trong nude da khang djnh USLE la md hinh cdtheap dung thanh cong cho xdi radn luu vyc [5], [6], [9], [11], [12], [13], [14],
Thir ba, cac tham so cua mo hinh USLE cd the dupc sira dpi de thich hpp vdi nhiing hoan cinh cu thi vl ty lg khdng gian, dilu kign khi hgu eiing nhu cic dieu kign dja - vat Iy khac. Tuy nhien, cd the xuat hign cic sai lech do ip dung may mdc cac cdng thiic cua USLE. Chinh vi viy, de cd the sir dyng phuang trinh USLE, tic gii da chii y den lap luin v i cdng tiuic ciia cac tie gii khac di cdng bo vdi cic luu vuc cd dilu kien tuang ty va dgc bigt Ii trong dieu kign cua Vigt Nam. Cac he so dupc quan tam nhilu trong viec thay doi la LS, R v i K.
Thir tu, vdi each tiep cgn he thong theo tirng thdng so inh hudng xdi mdn, md hinh USLE co thi dupc tinh toan bang cdng eu GIS.
b. Md hinh danh gia
Phuong trinh mat dat tong quit USLE dupe Wischmeier va Schmid hoin thign vio nam 1978 tir kit qui quan trie va thong ke Idn tir thye te. Phuong trinh USLE dupc thiet lap tren co sd dinh Iupng cic nhan to gay ra xdi mdn, Lupng xdi mdn dit la mpt him ciia nhieu bien so; tien h ^ tach bigt tiing biln so va bieu diln nd bing so; khoi Iupng dat xdi mdn bing tich cua tat c i cac bien so dd Phuang trinh mit dit tong quit cd dang: A=RxKxL>(SxCxP, Trong dd:
A: Iupng dat mat tnmg binh bing nam chuyin tdi chan sudn (kg/m^.nam).
R: hg so xdi mdn do mua (thang do dp xdi mdn dupc lap tren co sd EDO) (KJ.mm/m^h.nam).
K: he so khing xdi cua dat (dupe xac djnh bing Iupng dit mit di cho mpt don vi xdi mon cua mua trong dilu kign chieu dai sudn la 22,4m, dp die 9%, trong luong theo chiiu tir tren xuong sudn doc) (kg.h/KJ.nun).
L: Hg so chieu dai sudn doc, ty Ie dit mit di cua thua dit can tich toan so vdi lupng dat mat di.cua.thife,^_Qhuan,,
S: Hg so dp doc (ty Ig dit mat di cua thua dat can tich toan so vdi lupng mit dat cua tiiiia dat chuan cd do doc 9%).
*"i ^^^^° ^^y *^"S liogc Idp phu (khdng thu nguyen) ty Ig Iupng dat mit cua thiia dat SP VOI Iupng dat mit di cua thua dit ehuan (bd hoi each nam).
^ P: He so canh tic bio ve dat (ty Ig luong dat rait di ciia thua dat can tinh toin so vdi Iupng dat mat di cua tinia dit khdng thyc hign bign phip canh tic bio vg dat (thua dit chuin, trong luong theo chiiu tii tren xuong sudn doe)
Kpyiu Hpi nghi Khoa hgc Dia lp todn qudc ldn ^f
Hgi Dia ly Vi|t Nam Dgi hoc Thai Nguyen Trong ndi dung nghien ciiu, he so C va P duac loai bd de tiianh lap bin do XMTN luu vuc.
Khi dd, md hinh USLE cd dang: A-RxK^LxS Chi tiet ve phucmg p h ^ vi quy trinh ciing nhu ket qua tinh toan cic dai luong trong phuang trinh nay se duae phan tich cy the d myc 2 va 3 cua bai bao.
3 P H A N C A P XOI MON TIEM N A N G CHO T O A N LUXJ V U C SONG GAM 3.1. Thanh lap cic ban do thanh phin
Ban do XMTN eua LVS Gam dupc xay dyng theo md hinh USLE ciia Wischmeier va Schmid dya tren cic ban dd thanh phin sau
fl. Bdn dd hi sd xoi mon do mua R
Bin do hg so xdi mdn do mua ciia LVS Gam dupc thinh lap trgn co sd lupng mua tmng binh nam cua Nguyen Trong H i [1] theo cdng thuc: R = 0,548257P - 59,9 bing chiic nang Raster Calculator trong phin mem ArcGIS 9.3, Trong dd:
R Ii he sd xdi mdn mua tmng binh n t o (J/m^)
P Ii lupng mua tmng binh hing nam (mm/nam). Bin do P dupc ndi suy tii bin do lupng mua trung binh nam ciing vdi cic so lieu do tai 09 trgm trong LVS Gam v i khu vyc lien kl bing phan mem ArcGIS 9.3 vdi thuat toin tuyin tinh.
b. Bdn dd hi sd khdng xoi c^a ddt K
Ban do hg so khang xdi cua dit dupc thanh lap tren co sd ban do tho nhudng. He so niy phu thupc vio dae diem di m? v i eic thinh phan co Iy cua dit (kich thudc hgt dat, kit ciu dat va kha nang thim thiu, tuang quan giira cic thinh phin trong dit). Cic tic gii Wischmeier v i Smith da dua ra cdng thirc tinh hg so K nhu sau:
lOOK = 2,I.10-4M1,14(12-OS) + 3,25(A-2) + 2,5(D-3), Trong dd:
Khg so xdi mdn ciia dat, don vjIaT/acre,1000foot,tonfinch.acre-I h-1
M: trgng lupng c ^ hgt (trpng Iupng theo dudng kinh c ^ hgt) M duac xac djnh: (%) M = (%Iimon + % cat mjn) (100% - %set).
OS: ham lupng chat huu cp trong dat, do bing phin tram. D: hg so phy thudc.khi nang tieu tham cua dat. A: hg so phy thudc vao hinh dgng, sap xep va loai kit ciu dat. Cic tic gii tren cung dua ra toan do dya vio edng thirc 4 dk tra hg so K.
FAO cung da cdng bo bing phan c ^ hg so xdi mdn dit [8].
d Vigt Nam, cic tic gii Nguyen Trpng Ha [1], Phgm Hung [2], Nguyen Quang My [3] trong nhung nghign ciiu cua minh cung da tinh toin v i dua ra hg so K cho rapt so Iogi dat ehu yiu d Vigt Nam. Ke thira cac ket qui trgn, hg so khing xdi K cua cic logi dat trong LVS Gam dupc xac Igp cho timg logi dit eu thi (bing 1):
Bdng 1. Hi sd khdng xdi mdn cua cdc logi ddt LVS Gdm TT
1 2 3 4
—5—
e
7 8 9 10
T£n dii Viet Nam Dat feialit d6 nau tren macma bung tinh va bazic (Flc) D^t feialit d6 nau trfin da vol (Fv)
ak feraht d6 v ^ g tiSn dd s^ vd bien chSt (Fs) D^ feiaUt vdng d6 trgn macma axit (Fa) -Sat feialit-vdng nhat tieo da cat (Fq)
Batd6ctu(D)
Dit mim (16 nau tr£n macma tning tinh va bazic (FHk) Bit mim dd nSu atn di voi (FHv)
Dit nmn di vdng trtn dd set vd biln chit (FHs) Dit miin vdng nhat Hgn da cat (Fq)
H | S 6 K 0,23 0,22 0,23 0,28 0,39 0,01 0,20 0,22 0,15 0,25 Kpyhi Hpi nghi Khoa hgc Dia lp todn qudc ldn thd 7
H6i Dia ly Viet Nam Dai hpc Thai Nguygn TT
11 12
Ten dit Vift Nam Ddt miin vdng xam CO noi bi potzon hoa (Ha) Ddt mim vang nhat lien dd cat
H e s i K 0,20 0,25 Ngudn. I 'lin Quy hogch vd Thiet ke Ndng nghiep c. Md hinh sd dp cao DEM (Digital Elevation Model)
Mo hinh sd dp cao DEM ciia LVS Gam dupc xay dyng tir du lieu dudng binh dd va diem do cao trong luu vyc vdi khoing cao deu la 20m bing phan mem ArcGIS 9.3 theo phuang phip npi suy trong sd nguac IDW (Inverse Distance Weighting). Dp phan giii khdng gian l i 30m, Thyc chat cua md hinh DEM la rapt dang du lieu khdng gian sd (dang raster) thi hign be mat ciia dja hinh rao phdng tir mpt ham so xic dinh bdi tap hop cac gii trj dp cap tren rapt khdng gian lien tuc. Diy la dii lieu sd dp cao ca bin de cd the tinh cac md hinh dp ddc (Slope), md hinh chieu dii sudn doc (Length of slope), md hinh hudng doc (Aspect), thudng dupc sii dung nhieu trong cac phep toan phan tich khdng gian.
d. Bdn dd hi so chieu ddi sudn dde L
He so chieu dai sudn doc L la yeu td trie Iupng hinh thii dja hinh va cd inh hudng Idn den xdi mdn dit trong khu vyc. Thdng thudng hg so L dupc tinh theo cdng thiic sau (Wischmeier va Smith, 1978): L = (xy22,I3)m Trong do: L - hg sd chieu dai sudn, x- chieu dii sudn doe (m); m - he so mu tuy thupc vio dp doc (m=0,2 khi dp doc <1%; m =0,3 khi dp doc tir l%-3%; m=0,4 khi dd doc tir 3%-5%; m=0,5 khi dp doc >=5%).
He so chieu dii sudn doe L ciia LVS Gam duac tinh tu md hinh so dp cao bing iing dyng LSfactor.amI cua tac gia Jaeek Blaszczynski d trung tam National Applied Resource Sciences Center (NARSC) [14]. Day Ii iing dung chgy trong he thong phin mim ArcWork Station 9 3. Gia tri he sd chiiu dii sudn Idn nhat trong LVS Gam i i ban 52m
e. Ban do hg so dp doc S
Ban do hg so dp ddc S cua LVS Gam dupc tinh thep cdng tiuic cua Wischmeier va Smith:
S=(0,43 + 0,30S + 0,043sI2)/6,6I3.6 bing iing dung LSfactor.aml, chay trong mdi tmdng ciia phan mim ArcWork Station 9.3. Trong dd si la dd doc cua sudn, don vj Ii %.
3.2 Md hmh XMTN dit A cua LVS Gam
Md hinh XMTN dat ciia LVS Gam dupc thinh l^ thep edng thiic ciia Wischmeier va Schmid: A=RxKxLxS. Trong dd, trj so XMTN A dupc tinh tir cie yiu to ty nhign bao gom: hg so xdi mdn do mua R. hg so khang xdi ciia dit K, hg so chiiu dii sudn doc L v i hg so dp die S.
Tic gii da tiln hanh phan c ^ miic dp XMTN ciia LVS Gam dya vio trj so XMTN A vi day cung la CO sd dl thyc hien phan logi lanh tho ciia luu vyc. Tren pham vi LVS Gam dupc chia thinh 5 cip XMTN. Sd dT lya chpn 5 c ^ XMTN vi qua khio sit dien tich cua tri so XMTN A cho thay:
trj so A tmng trong khoang tii 10-90, cao nhit la 155 v i thip nhit Ii 0. So cap ciing khdng qua nhieu hoac qui it se khdng till hien rd trgn lanh tho hogc lim sy phin hda tren lanh thi qui ehi tiet (bing 2),
BangZCdc cdp XMTN LVS Gdm cip tiem nang xoi mon
Cip 1: dJ-XMTN ratthip cip 2; do XMTN thip Cap 3: do XMTN tnmg binh Cip 4: do XMTN cao Cip 5: do XMTN lit cao
Tii so A
<15 15-30 31-45 46-60
>60
Trong LVS Gam co toi 117.276,5 ha rung djc dyng, bao gom cac Vuon Qu6c gia, khu Bio ien nhien va rung van hoa lich sii, tuong ling vdi 12,7% tong dien tich luu vyc. Neu khong Kyyiu Hoi nghf Khoa hoc Dia ly toim qudc ldn M^
Hoi Dia ly Viet Nam Dai hoc Thai Nguyen tinh dign tich cic khu rimg die dung nay, dien tich c ^ XMTN ciia toan luu vuc d cip 1 (cip rat thip) la 341,865,8 ha, chiem khoing 37,3% dign tich cua luu vyc, phan bd chu yeu d cic vung tiiip phia nam cua luu vuc v i dpc thep cic thung liing ddng chmh sdng Gam. Dien tich XMTN cap 2 (diap) Ii 242.212,0 ha, chiem 26,4% dign tich luu vyc. Dien tich XMTN cip 3 (tmng binh) Ii 62.803,4 ha, chiem 6,9% dien tich toin luu vuc Dign tich XMTN cap 4 (cao) cd dien tieh nhd nhat 52 508,8 ha, chilm 5,7% tong dien tich luu vuc. Dien tich XMTN cip 5 (rit cao) la 100.135,6 ha, chilm gin 11 % tong dign tich LVS Gam (bang 3)
Bdng 3. Diin tich cdc cdp XMTN loan LVS Gdm cip XMTN
Donvi Dien tich
Cipl (ha) 341 865.8
(%)
37,3 Cap 2 (ha) 242.212,0
(%)
26,4 cip 3 (ha) 62 803,4
(%)
6,9 Cip 4 (ha) 52 508,8
(%)
5,7
Cap 5 (ha) 100 135,6
(%)
10,9 Ngudn: Phdn tieh vd thdng ke lu bdn dd luu vuc vd cdp XMTN Tinh chung trgn toan luu vyc, tong dien tich XMTN cap 1 va cip 2 da len tdi 584.077,8 ha, chiem 63,7% dign tich tu nhien ciia luu vuc. Tir c% I den c ^ 4, dign tich XMTN cd xu hudng giam dan vi nhd nhit Ii dign tich XMTN d cap 4, sau dd tang Ien d cip 5 vdi dien tich Idn Ihii 3 trong tong so dign tich d cac cap XMTN vdi tren 100.135lia.
Tong dign tich XMTN cap 4 v i cap 5 li 152 644,4 ha, chiera 16,6% dign tich cua luu vyc Dien tich cd muc dp XMTN hai c ^ cao nhit niy chu yiu d phin lanh thd phia bic vi ddng bac luu vyc, thudc cac tinh H i Giang v i Cao Bang.
Neu tinh tir cap XMTN tmng binh trd lgn (cip 3 den e ^ 5), tong dign tich ciia 3 cap niy la 215.447,8 ha, chiera 23,5% dien tich ty nhien ciia luu vyc va phan bo chu yiu d khu vyc dau nguon, thupc cae phu luu la song Nhiem, sdng Tong Gudng, sdng Nhi A, phia bic ciia ddng chinh song Gam. Trong dd, dign tich XMTN cip 4 v i cap 5 da chiem khoang tren 70% tong dign tich cua ca 3 c ^ XMTN tren.
4. P H A N C A P XMTN CHO CAC PHU LlTU (LUtI VUC CAP 2) 4.1. Hg thdng cic Imi vuc cap 2 ciia LVS Gim
Theo "Bio cio de tai quy hoach lam phin phdng hd diu nguon Vigt Nam" cua Tnmg tam tu vin tiidng tin lam nghigp (CFIC) - Vign Dilu tra quy hogch rimg - Bd Ndng nghiep va Phit trien nong tiidn (1998), lanh tiio Vigt Nam chia tiiinh 41 LVS cip I, 254 LVS cap 2 va 1.150 LVS cip 3. Toin bp LVS Gam phan lanh tho Vigt Nam dupc ehia thanh 05 LVS cap 2. De tien cho vigc theo doi vi xir ly dii lipu, cac LVS c ^ 2 dupe dinh theo raa luu vuc Theo dd, LVS Nho Que cd ma l i 1 va lin lupt cho din LVS Nang cd m i Ii 5 (bing 4),
Bdng 4. Hi thdng cdc luu vuc cdp 2 cita LVS Gdm
MS luu vuc 1 2 3 4 5
Ten luu vuc LVS Nho Que LVS Nhiem -Tong Gudng Luu vuc dong chmh song Gam XVSJffliiA
LVS NSng Ting
Di0n tich (ha) 100.305,2 122.487,3 364.876,5 114.992,7 214.140,4 916.802,1
%
10,9%
13,4%
39,8%
12,5%
23,4%
100%
Ngudn: Viin Dieu tra Quy hogch rimg LVS Nho Qui (luu vuc 1): cd dien tich 100.305,2 ha, chiira 10,9% tong dign tich LVS Gam.
Phgm vi phan bo cua luu vuc I nim chu yeu d huygn Meo Vgc (Hi Giang) va phan phia bic ciia c ^ huy$n Bio Lam, Bio Lgc (Cao Bing). Day la nhiing huygn miln niii cd dja hinh cao, doc va _^tia cat manh nhit nen thung lung song thudng nhd hpp va sudn doc.
Kpyeu Hpi nghi Khoa hgc Dia lp todn qudc ldn thd 7
Hoi Dia ly Viet Nam Dai hpc Thai Nguyen LVS Nhiem - Tdng Gudng (luu vyc 2): cd dign tich 122.487,3 ha, chilm 13,4% tong dign tich LVS Gam. Luu vyc 2 chu yeu phan bo d cic huygn Yen Minh, Ddng Van, Meo Vac (Hi Giang). Ciing giong nhu LVS I, day l i khu vuc viing eao bign gidi, cd dja hinh cao, doc vi chia cit hilm trd nhit. . ^
LVS ddng chinh sdng Gam (luu vye 3)- cd dign tich ldn nhat vdi 364.876,5 ha, chiem gan 40% dign tich LVS Gara. Pham vi phan be eiia luu vyc 3 nam d phia tay cua LVS Gam, tren dja phin cic huygn Bic Mg, mdt phin cua Vi Xuyen (Ha Giang), Lam Bmh. Chigm^Hoi, phia tay Na Hang va Yen Son (Tuyen Quang). Day ciing la luu vyc cd phan Idn dign tich ho thuy dign Tuyen Quang. So vdi cac LVS cap 2 khic, ddng chinh sdng Gam chay qua cic khu vyc ^ a hinh cd dp die va chia cit nhd ban ci, dae bigt l i phan hg luu thuy dign Tuyen Quang. Trong luu vuc cd 4 khu Bio ton tiiign nhien v i rimg van hda Ijch sii la Du Gia, Bic Mg v i Tit Ke - Ban Bung v i Kim Binh
LVS Nhi A (luu vyc 4): cd dien tich 114.992,7 ha, chiem 12,5% tong dign tich LVS Gam, phan bo chu yiu trgn cac huygn Bio Lgc, phia nam cua Bio Lam v i rapt phin nhd cua Nguyen Binh (Cao Bing). Dia hinh luu vyc cd su phin hoi giira khu vyc phia tay O^ia t h ^ , dp dPC nhd) va phia ddng (dia hinh cae, chia cit ragnh v i dp dpc Idn)
LVS Nang (lira vyc 5): cd dign tich kha ldn vdi 214.140,4 ha, chiem 23,4% tong dien tich LVS Gam. Luu vyc 5 nim chii yiu d cac huygn Pic Nam, Ba B I , mot phin Ngan Son, Chp Din (Bic Kan) v i phin phia tay cua huygn Na Hang (Tuyen Quang). Ngoii ra, mpt dign tich nhd ciia huyen Nguygn Binh (Cao Bing) ciing nim trong pham vi luu vyc. So vdi luu vuc ddng ehinh song Gim, day la khu vyc cao v i doc hon nhung nhd hon cic luu vuc cip 2 khic ciia sdng Gam. Trong khu vyc ciing cd phan Idn Vudn Quie gia Ba B I v i rapt phin khu Bao ton thien nhien Phia Oac.
Cie LVS cap 2 tren la cac don vj eo sd vl mgt lanh tiio de danh gii phan c ^ XMTN LVS Gam. Qua trinh niy thyc chit Ii vigc phan tich tong hpp cic dieu kien ty nhien theo luu vyc de xic dinh cac khu vyc cd miic dp XMTN cao nhat.
4.2. Phan cip tiem ning sdi mdn d i t cho cic luu vuc cip 2
Ket qua phan cip XMTN cho eac LVS cap 2 dupc t6ng hpp dua tren ket qui thing kg dign tich bin do eac cip XMTN vdi bin do ranh gidi LVS cap 2 bing chiic nang Zonal/Tabular Area trong phin mim ArcGIS 9.3 (bing 5).
Bang 5. Diin tich cdc cdp XMTN theo luu v\rc cdp 2 cua LVS Gdm Ma
LV 1 2 3 4 5
Ten lira V\K
LVS Nho Qui LVS Nhi^in -Tfing Gironi LV d6ng chinh sfing GSm LVSNhiA LVS Nang Tong
C a p l (ha) 41 490,6 51 047.4 137 604,5 46 692,2 65 031,3 341.865,8
(%)
41.4 41.7 37.7 40,6 30,4 37,3
Cap 2 (ha) 35 439.4 30 111,8 81 131,7 32 273,0 63 256,2 242 212.0
{%) 35.3 24,6 22.2 28.1 29.5 26,4
cip 3 (ha) 8 497,4 7 425.0 23 SI 8,0 8 293,5 14 764,5 62 S03,4
(%)
8,5 6,1 6,5 7,2 6.9 6,9
cip 4 (ha) 6 759.8 7 291.7 21 349.3 6 661,0 10446,9 52 508,8
(%)
6.7 6,0 S.9 5,8 4.9 5.7
Cap 5 (ha) 8 118.1 22 764.2 44043,7 17 206,1 8 003.4 100 135.6
(%)
8,1 18,6 12,1 15,0 3.7 10,9
Bathnig dae dpg (ha)
00 3,8471 56.9294 3.86! ! 52.6381 117,276 5
Ngudn: Phdn lich vd thdng ki Id bdn dd luu v\rc vd cdp XMTH Tir bang kit qui tren cho thiy, theo cic luu vyc c ^ 2 ciia sdng Gam, cap XMTN cao va i^t cao (cip 4 va 5) tgp tiung nhilu d LVS 2 (24.6%), LVS 4 (20,8%), LVS 3 (18,0%). LVS 1 (14,8%) va nhd nhit d LVS 5 (8,6%). Mgc dii LVS 3 (ddng chinh sdng Gam) cd dien tieh XMTN cap 4 vi5 kha Idn (18.0%) nhung do di|n tich LVS Idn nhit nen miic dp XMTN tren mpt don vj di|n tich kha nhd so vdi cac LVS cip 2 cdn lai. Tong dign tich XMTN c ^ 4 v i 5 chilm tdi 15.6% di|n tich toan luu vyc.
Kp yiu Hpi nghi Khoa hgc Dia lp todn qudc ldn thi 1
H6i Dia \p Viet Nam Dai hpc Thai Nguyen Dien tich XMTN d eip 2 vi 3 khdng chgnh Igch nhilu giiia cic luu vyc c ^ 2, trong dd tip trung nhilu hon c i d LVS I vdi 43,8% tong dien tich XMTN cua cac c% d LVS niy.
Dien tich XMTN cip I cd dien tieh Idn nhit d LVS 3, LVS 5 va LVS 2. Nhin chung, dien tich XMTN cap I tiiudng dao dpng tii tren 30% din dudi 42% tong dien tich XMTN cua cac LVS c ^ 2
KET LUAN
Sy phan hda da dang, phiic tgp ciia cau tnic dja chat - dia hinh va Idp phii tho trong dilu kien lupng mua Idn. phin miia ciia LVS Gam da lim ting nguy co xdi mdn dat tren dign rpng. Phan c%i XMTN nhim tgo co sd khoa hpc chp viec xic dinh cic loai hinh su dyng dit tren toan luu vuc. Do khi nang iing dyng rpng rai edng cy GIS vdi cic uu the ciia nhiing ky thuat mdi trong qua trinh nghien ciiu nen tic gii da su dyng phuang p h ^ md hinh trong nghien ciiu dinh gia XMTN luu vyc. Theo dd, md hinh mat dat pho dung USLE ciia Wischmeier va Schmid dupc su dung de danh gia XMTN LVS Gam, sau khi di loai bd hg sd C v i P. Tri so XMTN (A) l i co sd cho viee phan ra 5 c ^ XMTN cho toin luu vyc vi eho eic LVS c ^ 2.
Tren toin lira vye, dign tich XMTN d cac c ^ tir rat thap (c%) 1) den rit cao (cip 5) chilm ty Ig tuong img so vdi dign tich lira vyc la: 37,3%; 26,4%; 6.9%; 5,7% v i 10,9%. Trong dd, ting dign tich XMTN cap 4 v i cip 5 chiem 16,6% dien tich luu vyc, vdi 152.644,4 ha, tap tmng d , nhiing vimg dja hinh cao. doc phia bac va ddng bac luu vyc.
Theo cic LVS cip 2. e ^ XMTN cao v i rat cao (cap 4 vi 5) tip tmng nhilu d LVS 2 (24.6%), LVS 4 (20,8%), LVS 3 (18,0%), LVS 1 (14,8%) va nhd nhat d LVS 5 (8,6%), Dien tich XMTN d cap 2 va 3 khdng chenh Igch nhilu giua cic luu vyc cip 2, trong dd tip tnmg nhilu ban ci d LVS I, chilm 43,8% tong dign tich XMTN ciia cac c ^ d LVS nay. Dien tich XMTN cip 1 dao dpng tir tren 30% den dudi 42% tong dien tich XMTN ciia cie LVS eip 2, trong dd ldn nhat d LVS 3, LVS 5 v i LVS 2.
TQMTAT
Tri so XMTN LVS Gam dupc tinh thep md hinh mit dat pho dyng USLE cua Wischmeier va Schmid, Kit qui phan c ^ XMTN tren toan lira vyc v i tren cic LVS cap 2 theo 5 cip XMTN cho thiy: Dign tich XMTN d cap rat thap v i t h ^ chilm 63,7%, d c ^ cao va rat cao chiem 16,6% tong dign tich ciia luu vyc. Giiia cac LVS c ^ 2 cd sy phin hoi khi ro ve miic dp XMTN, trong dd, LVS 2, LVS 4 va LVS 3 cd nguy co xdi mdn dit cao nhit.
f^yyeu Hgi nghi Khoa hgc Dia ly todn qudc ldn thd 7
! rvia ly viel JNam Dai hpc Thai Nguyen
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TAI LIEU THAM K H A O
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Kyyiu Hpi nghi Khoa hgc Dia lp todn qudc ldn tha 7