CHAPTER V. Spatial Distribution of Rainfall over Northeast India

Transcription

1 CHAPTER V Spatial Distributin f Rainfall ver Nrtheast India 5.1. Intrductin Knwledge f spatial distributin f rainfall f different tempral classes viz. annual seasnal r decadal, is extremely imprtant fr assessment f climate patterns and water resurces f a regin. Study f spatial variability and distributin f rainfall ver the NE regin is particularly imprtant because f the dminant rgraphic effect f hills and highlands n the passage f the mnsns and ther rain-bearing airmasses resulting in their hydrdynamic mdificatin and differential distributin f rainfall. The aim f this chapter is t examine the generic patterns f spatial variability f rainfall ver the NE regin. The general patterns f spatial variatins are explained with the help f ishyetal maps f annual and seasnal rainfall. It is sught t knw whether there is any discernible spatial pattern f rainfall ver the regin that is determined by gegraphical lcatins (defined by latitudes and lngitudes) and altitudes frm the mean sea level Analysis f Ishyets Distributin f rainfall can be effectively examined by drawing is-lines f annual r seasnal rainfall called ishyets. An ishyet is a line f cnstant rainfall amunt (McIntsh, 1972). Figures 5.1, 5.4, 5.5,5.6 and 5.7 represent ishyetal maps f respectively annual, pre-mnsn, summer mnsn, pst-mnsn and winter rainfall f NER. The ishyetal maps have been prepared using seasnal and annual data frm as many as 107 numbers f raingauge statins representing different physigraphic znes cvering the seven states f the NER. i

2 Figure 5.1 Ishyets f Annual Rainfall in NER. The database cmprises mre than 100 statins frm different physigraphic znes in NER. Lcatins f these statins are shwn in the Figure 4.4 in chapter IV. One statin lying in the Bhutanese territry but clse t Arunachal Pradesh, fr which data was available, was als used fr the sake f cntiguity. In the case f figures 5.1 and 5.5 die ishyets are drawn with multiple intervals wherever needed t preserve highreslutin infrmatin n rainfall that varies sharply between extreme values. The ishyets fr annual rainfall (Figure 5.1) make the pattern f spatial distributin f rainfall in NER amply clear. The heaviest rain-pcket f the regin is situated in the 149

3 suthern Khasi Hills famus fr Cherrapunjee (mean annul rainfall f abut mm) and Mawsynram (13000 mm) that have recrded sme f the highest pint rainfall amunts f the wrld (Oliver and Hidre, 2003). Anther high rainfall zne is the fthills f Arunachal Pradesh and adjining Assam plains where we have Pasighat (4500 mm), Lakhimpur (3500 mm) and Pathalipam (4500 mm). Far eastern fringes f Arunachal Pradesh in the Lhit valley cmprise anther high rainfall terrain near Tezu where rainfall ranges frm 3000 mm t 6000 mm. Yet anther high rainfall zne lies in the Dibang valley with rainfall varying between 4000 mm t 5000 mm. Eastern parts f the Assam valley and adjining Arunachal fthills als frm ne high rain-zne where rainfall fluctuates between 3000 mm t 4000 mm. There is a prminent lw rainfall zne in the suthern parts f Nagan and adjining Karbi-Anglng districts which is the result f the rainshadw effect cast by / the Meghalaya-Karbi plateau that bstructs the suthwest mnsns and induces heavy rgraphic rainfall n the windward hillslpes leaving scanty mistures t be precipitated in the rainshadw areas in Assam. The rainshadw belt extends frm suthern Nagam thrugh the fthills f Karbi hills up t the nrth Cachar hills. Anther lw rainfall area appears in and arund Imphal (1426 mm) in the Manipur hills. Figure 5.2 depicts the variatin f annual rainfall at Cherrapunjee (11137 mm) n the windward side and that at Shillng (2172 mm) n the immediate lee f the ridge; Lumding (1251 mm) and Lanka (1205 mm) at the cre f the rainshadw and Nagan (2016 mm) n the nrthern fringe f the rainshadw zne. It shws hw rainfall decreases gradually frm the windward side t the central parts f the rainshadw zne and then starts increasing twards the nrth central parts f the Brahmaputra valley where the rainshadw effect is weak. Figures 5.3a and 5.3b demnstrate the extremely lw amunts f annual and summer mnsn rainfall recrded in sme places f the rainshadw zne in Nagan and Karbi-Anglng districts in cmparisn t the very high rainfall at Cherrapunjee. 150

4 Annual Rainfall (mm) = Rainfall (mm) H h +»» - ( O t- < D t- ( D t- ( D t- ( P t- ( D t- O O «- ' «- C N C N 0 0 C 0 ' ^ - ^ F i r ) l f ) C D 0 ) 0 ) ( J ) C J ) 0 ) 0 ) 0 ) C D C D 0 ) 0 ) ( J ) ( 7 ) Years H----h (O <- CD (O 0) 0) ) 8 -C herrapunjee Lum ding Nagan «Lanka -Shillng 5.2 Variatin f annual rainfall in Cherrapunjee ( ) n the windward slpes f the Meghalaya plateau and statins n the leeward side viz. Shillng ( ). Lumding ( ). Lanka ( ) and Nagan ( ). Perids f data are different fr different statins. Statins Figures 5.3a Variatin f annual rainfall between the precipitus windward side f the Meghalaya plateau and different lcatins in the rainshadw area in suthern parts f Nagan and Karbi Anglng districts in Assam lying n the leeward side. 151

5 Amsi Nagan Kathiatli Kandli Kampur Nilbagan Kaki Lanka Lumding Kharikhana Bklia Diphu Brpani Kherni Cherrapunjee Mawsynram Summer Mnsn Rainfall (mm) Figures 5.3b Variatin f summer mnsn rainfall between the precipitus windward side f the Meghalaya plateau and different lcatins in the rainshadw area in suthern parts f Nagan and Karbi Anglng districts in Assam lying n the lee-ward side. Figures in the graph refer t rainfall amunts (mm). Distributin f pre-mnsn (PMR) rainfall is peculiar fr the NER and is markedly skewed ver the hills and plains tpgraphy (Figure 5.4) Nrtheast India is ne f thse areas that receive significant rainfall in the pre-mnsn seasn (March, April, May), chiefly frm frequent thunderstrms with the seasnal precipitatin nrmally exceeding 400 mm (Pant and Rupa Kumar, 1997). Like ther seasnal rainfall PMR als fllws a high-lw-high pattern in distributin frm west t east acrss the Assam plains varying frm abut an average value f 800 mm thrugh 300 mm t 700 mm. The rainiest pckets in this seasn in the regin are: suth Khasi Hills surrunding Cherrapunjee and Mawsynram where PMR ranges frm mm; far nrtheastern crner f Arunachal Pradesh in the Dibang basin where PMR may vary frm mm and the Barak valley where PMR fluctuates frm mm. Pre-mnsn rainfall in the Barak valley is cnsiderably higher than in the Brahmaputra valley. One pssible reasn f this higher PMR is the latitudinal clinearity f this zne with the head f the Bay f Bengal where mst f the 1 5 2

6 Figure 5.4 Ishyets f Premnsn Rainfall in NER. The database cmprises mre than 100 statins frm different physigraphic znes in NER. atmspheric disturbances are generated in this seasn w hich flw past the suthern M eghalaya plateau and the Bangladesh plains twards the Barak valley and adjining hills in the N rth Cachar Hills, Tripura, M izram and M anipur. It appears that the bulk f the rainfall that ccurs in these regins in the pre-mnsn seasn is generated b y these B a y disturbances that prpagate m stly in the frm f depressins system s. The im pact f the B ay disturbances ver the A ssam plains is prbably relatively lw and here pre-m nsn rainfall is m re due t the nrthwesterly disturbances (N r westers) 153

7 riginating in the Chtanagpur-Bihar plateau regin as well as lcal cnvective activity. The middle and fthills f Arunachal Pradesh in the nrth and eastern flanks are als prne t intense thunderstrms during the March-April-May mnths causing a gd amunt f rainfall in theses znes. It is prbable that the thawing f snw-ice masses, that starts in the summer in the High Himalayas with gradual rise in temperatures, supply enugh misture and ptential energy favuring thermdynamic frmatin f thunder cluds that release cnsiderable precipitatin in the Arunachal hills particularly in the middle ranges and the first ridges. The rainfall climatlgy f the pre-mnsn seasn was als discussed in Chapter-IV in sectin 4.5. The ishyets f the summer mnsn rainfall (SMR) and annual rainfall shw similar patterns (Figure 5.5). It is expected because it is the summer mnsn rainfall that determines the quantity and spatial distributin f annual rainfall n the strength f its large share (60% - 80%) in the annual amunt. Distributin f SMR within the NER is cnsiderably uneven ver the regin, thugh the amunt is high cmpared t ther parts f India. There is a high-lw-high gradient f SMR as ne mves frm western parts f the Assam valley twards the east alng the west-east axis f the regin up t far eastern parts in Arunachal Pradesh. Average SMR is relatively high in the westernmst parts f Assam varying between 2000 mm t 3000 mm. Frm there it decreases gradually twards the central Assam valley where SMR ranges between as lw as 800 mm in the suthern parts (rainshadwed by the plateaus) t 1400 mm in the nrth tuching the Bhutan and Arunachal hills. SMR increases gradually further east t' reach a peak f arund 3000 mm in the Upper (eastern) Assam plains and 4000 mm in the eastern fringes f Arunachal Pradesh. The highest SMR zne is bviusly the suthern Khasi hills where the peak values remain between mm in Cherrapunjee and Mawsynram respectively. The ther high mnsn rainfall pckets lie in the Lhit basin in the eastern flanks f Arunachal Pradesh centering arund Tezu; in the suth western Gar Hills in Meghalaya; in the nrthern Arunachal fthills and adjining Assam plains n the nrth bank f the Brahmaputra surrunding the Pasighat- Lakhimpur rain belt and in suthern Mizram hills. SMR is lw, varying frm 800 mm 154

8 Figure 5.5 Ishyets f Summer Mnsn Rainfall in NER. The database cmprises mre than 100 statins frm different physigraphic znes in NER. t 1200 mm, ver a large area cvering suthern plains f the Brahmaputra valley and adjining hills f Karbi-Anglng and Nrth Cachar hills. In the Barak valley SMR is mderate and ranges between 1000 t 2000 mm. As stated earlier, in much f the climate literature the nrtheast Indian rainfall is treated at a reginal scale and its characteristics are evaluated by using the single area weighted reginal time series f the NEI hmgeneus rainfall regin. Such a treatment bviusly leads t lss f infrmatin n the markedly high intra-reginal variabilities 155

9 in the distributin f rainfall in the regin. The spatial cntrasts in SMR described abve in case f the NER is typical f the glbal summer mnsn (the Indian summer mnsns being a part f the same) climatlgy, and are cmmnly bserved, all ver the mnsn lands in suth and sutheast Asia, generated either by the alignment f muntain (tpgraphic) barriers r by the nrmal track f the rain bearing systems f the summer mnsns (Das, 1986). Figure 5.6 Ishyets f Pst-Mnsn Rainfall in NER. The database cmprises mre than 100 statins frm different physigraphic znes in NER. 156

10 Tw prminent blcks f pst-mnsn rainfall (PMR) are bserved (Figure 5.6) in the regin-ne in the suthern Khasi Hills in and arund Cherrapunjee- Mawsynram zne and the ther in the nrtheastern Arunachal Pradesh that lie verlapping the Siang and the Dibang basins. In the frmer, PMR varies between 300 mm t 600 mm, which is the highest in the INTER. In the later it remains between 200 and 500 mm. Rainfall in the pst mnsn seasn is als seen t be relatively high / ( mm) in the Pasighat-Lakhimpur zne and in the Manipur plains ( mm). Winter rainfall is scanty (nearly 50 mm) in the Brahmaputra valley, Meghalaya, Tripura and Mizram. It is slightly higher in the Manipur hills ( mm). But winter rainfall is remarkably higher in a small pcket in the central parts f the nrthern Arunachal fthills arund the Gerukamukh-Zir transect in the Lwer Subansiri district f Arunachal Pradesh where it varies between mm wherefrm it decrease gradually t 100 mm in the plains arund Lakhimpur. Anther larger zne receiving large amunt f winter rainfall ( mm) in this seasn is the Siang - Dibang basin area in the vicinity f Anelih-Anini-Denning in the nrtheastern cmer in i Arunachal Pradesh (Figure 5.7). Beginning with the later part f the pst-mnsn and spanning the winter seasn (December, January, February), weather in the nrthern and nrtheastern Indian regins is cntrlled by the subtrpical jet streams and cnsequent develpment f high-pressure centers in Central Asia and nrthern Burma. In the higher reaches f the muntains diurnal temperatures are lwered belw freezing pint when these high-pressure systems extend their uter tngue beynd the hills. In this seasn the surface winds are predminantly nrtheasterly up t a height f 2-3 kms abve mean sea level beynd which westerlies prevail. Generally frmatin f cluds and rains are favured when wind shears are prduced at the interface f the nrtheasterlies and the westerlies at times when pulses f western disturbances are carried under the impact f the jet streams (NIH, 1992). Precipitatin by this mechanism is prbable mre ver the higher echelns f the Himalayas and adjining ranges n the nrthern and eastern margins f the NER than ver the plains. Prbably 157

11 this is the reasn why there is high rainfall in the pcket that lie in the Siang-Debang basin in the higher Arunachal Himalayas in the far nrtheastern cmer f the regin. Figure 5.7 Ishyets f Winter Rainfall in NER. The database cmprises mre than 100 statins frm different physigraphic znes in NER Latitudinal and Lngitudinal Variatin f Rainfall Spatial variability f annual and summer mnsn rainfall in the NER are further studied by examining the variatins f statin level rainfall alng latitudes 158

12 (suth t nrth), lngitudes (west t east) and with altitude (abve mean sea level). Data frm as many as 60 statins frm all ver the regin are used fr this purpse. The patterns f latitudinal and lngitudinal variatins f annual and summer mnsn rainfall are demnstrated in Figures 5.8 and 5.9. There is a high rainfall belt within 25.3 N-25.5 N latitudes that crrespnd t 91.6 E-91,72 E in lngitudinal range. This belt, lying in the Meghalaya hills having an average altitude f 1000 m abve mean sea level, directly bstructs the Bay f Bengal branch f the suthwest mnsns and as a result receives extrardinary amunts f precipitatin generated by rgraphic lifting n the windward suthern slpes. Cherrapunjee and Mawsynram belnging t this belt receive heavy rainfall in the pre-mnsn, mnsn and pst mnsn seasns. As a general pattern bth annual and mnsn rainfall increase frm N latitudes nwards twards nrth up t 25.0 N. The rate f increase becmes suddenly high ver the area between N N. Rainfall decreases sharply between N and N as ne appraches the plains f Assam. Variatins are relatively lw ver the O CD Rainfall (mm) h- cm cm m h- CD O) OOOOOCMOOi N r - CO CO O) T- ncm (N (O Tf. LO CD 05 O _ T... cd r^- OOCT) ( N C O ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ' D ' D C D C C D l p N N N N N ( M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C N C M C M Latitudes( N) SMR AR igure 5.8 Variatins f annual and summer mnsn rainfall alng latitudes ver the Nrtheastern Regin. AR: Annual Rainfall, SMR: Summer Mnsn Rainfall. 159

13 Mawsynram TO C g i i i i i i i i i i i i i i i i i i- i i i i i ONfifflsiq(' nipnss(s)r-«binffl)id O T C N O O C D l O C ' I O r '. O T - O O O O O ^ - O O O O C N C N 8 ) 0 ) 0 ) 0 1 t i ) 0 ) ( j > 0 ) 0 ) 0 ) 0 ) 0 ) 0 ) l j ) c i ) 0 ) ( j ) 0 ) 0 ) 0 ) 0 ) 0 ) ( l ) (! l 0 ) Lngitudes( E) SMR AR F i g u r e 5. 9 V a r i a t i n s f a n n u a l a n d s u m m e r m n s n r a i n f a l l a l n g l n g i t u d e s v e r t h e N r t h e a s t e r n R e g i n. A R : A n n u a l R a i n f a l l, S M R : S u m m e r M n s n R a i n f a l l. a r e a b e t w e e n l a t i t u d e s N t N w h i c h c r r e s p n d t t h e f l a t p l a i n s f t h e B r a h m a p u t r a v a l l e y u p t t h e f t h i l l s f A r u n a c h a l P r a d e s h. S h a r p v a r i a t i n s w i t h i n a s m a l l a r e a c a u s e d m a i n l y b y t p g r a p h i c a l l y i n d u c e d r a i n s h a d w e f f e c t a r e q u i t e c m m n a s c a n b e s e e n f r m t h e F i g u r e T w c l a s s i c e x a m p l e s f t h e s e p h e n m e n a a r e S h i l l n g a n d Z i r b t h b e i n g r e l a t i v e l y l w r a i n f a l l a r e a s i n c l s e p r x i m i t y t h i g h r a i n f a l l z n e s n a m e l y, C h e r r a p u n j e e - M a w s y n r a m a n d P a s i g h a t - D e n n i n g r e s p e c t i v e l y. R e l e v a n t l c a t i n s a r e m a r k e d n t h e f i g u r e t h i g h l i g h t t h e s p a t i a l v a r i a b i l i t y f r m s u t h t n r t h. T h e w e s t - e a s t v a r i a t i n s f r a i n f a l l a l n g l n g i t u d e s d e p i c t e d i n t h e F i g u r e 5. 9 s h w i n c r e a s e i n r a i n f a l l n e a r t h e M e g h a l a y a p l a t e a u b e t w e e n E a n d 9 l. 9 0 E l n g i t u d e s. R a i n f a l l s t a r t s d e c r e a s i n g f r m 9 l. 4 E, v a r i a t i n s b e i n g l w, t i l l E v e r a s t r e t c h s p a n n i n g t h e l w e r a n d m i d d l e B r a h m a p u t r a v a l l e y, B a r a k v a l l e y a n d M a n i p u r. R a i n f a l l t h e n i n c r e a s e s c n s t a n t l y f r m E t w a r d s t h e e a s t e r n h i l l r a n g e s f e a s t A r u n a c h a l a n d N a g a l a n d a n d f u r t h e r n r t h e a s t f I m p h a l. I t w a s d e e m e d e s s e n t i a l t l k i n t t h e n a t u r e f v a r i a t i n f r a i n f a l l a l n g t h e k m l n g s t r e t c h f t h e B r a h m a p u t r a v a l l e y ( f r m w e s t t e a s t ) a s a s p e c i a l c a s e 160

14 because f the bvius significance f rainfall fr the agricultural prductin f the thickly ppulated valley which is als chrnically affected by severe flds caused mainly by mnsnal rains (discussed in Chapter-VI) ver the valley and the surrunding highlands. Figure 5.10 clearly shws that rainfall n an average is high at the westernmst parts f the Brahmaputra valley neighburing Bangladesh and Nrth Bengal (SHWB subdivisin) west f 90 E. Rainfall then decreases, marked with cnsiderable spatial variability, twards east up t the west f 93 E, belnging t the Rainfall (mm) Lngitudes( E) PMR SMR AR Figure 5.10 Spatial variatins in premnsn, summer mnsn and annual rainfall at different statins in the Brahmaputra valley alng lngitudes frm west t east. lwer (western) and central parts f the valley. Lw rainfall in this stretch is a result f rainshadwing by the Khasi hills (Meghalaya) t the suth f the valley. Rainfall increases in the upper (eastern) part f the valley t the east f 93 E maintaining cnsiderable spatial fluctuatins. It is custmary t describe the rainfall pattern f Assam r the Brahmaputra valley n sundry ccasins as having an increasing trend frm the west t the east and suth t nrth (fr example Pal and Bagchi, 1975; Purkait, 2004) withut cnsidering the lw rainfall zne in the lwer and central parts especially n the suth bank f the Brahmaputra, which is climatically and envirnmentally an 161

15 3 imprtant factr that marks the flra and frest types f these areas different, fr example, frm the Assam Valley Trpical Wet Evergreen Frests and rain frests fund in the rain-drenched envirns in the Dibrugarh and Tinsukia districts f Upper Assam (Champin and Seth, 1968; HRS, 2002) Altitudinal Variatin f Rainfall The hilly terrains and muntainus envirnment exert a greater cntrl n the generatin and distributin f precipitatin in cmparisn t plain areas because rgraphy prvides the necessary uplift t the misture-laden currents striking against the tpgraphic barriers. The variatin in precipitatin with altitude is cntrlled by mean height f cluds and decrease f water vapur with altitudes (Singh et al., 1995). Orgraphic effect, which is an utcme f the cnjugal effect f tpgraphic elements (such as slpe, aspect, altitude, latitude, lngitude etc.) and strength f misture bearing wind, its misture cntent and rientatin f the hill/muntain range with respect t the prevailing wind directin (Singh and Kumar, 1997), is the main factr that cntrls differential distributin f rainfall arund high hills and grges causing cpius precipitatin n the windward slpes. Hwever, ther lcal meterlgical and reginal/glbal climatlgical factrs may als have impact n such variatins f rainfall at different elevatins f a regin. Presence f ther factrs is mre prbable in case f the plains and valleys. Infrmatin n altitudinal variatin f rainfall is useful in determining the net increase in precipitatin due t elevatin which, in turn, helps in estimatin f prbable maximum precipitatin (PMP) and prbable maximum fld (PMF) fr muntainus catchments as well as in realistic assessment f water resurces and fld ptential f hilly regins. Study f altitudinal variatins f rainfall is useful in understanding the influence f tpgraphy n the reginal r lcal weather and climate patterns especially in the vertly hilly landscape f Nrtheast India. An attempt is made here t examine the mde f variatin f pre-mnsn, summer mnsn and annual rainfall with altitude (frm mean sea level) in five majr 162

16 p h y s i g r a p h i c z n e s f t h e N E R v i z. A r u n a e h a l H i m a l a y a s ( G r e a t e r, L e s s e r a n d f t h i l l s ), B r a h m a p u t r a v a l l e y, M e g h a l a y a P l a t e a u, M a n i p u r H i l l s a n d P l a i n s L a n d s c a p e, a n d t h e T r i p u r a H i l l s a n d. P l a i n s L a n d s c a p e. T h e s e z n e s d n t s t r i c t l y c n f r m t t h s e s h w n i n t h e F i g u r e 3. 2 i n C h a p t e r - I l l. I n s t e a d f t h e p l a i n s i n M a n i p u r a n d T r i p u r a t h e e n t i r e l a n d s c a p e i s c n s i d e r e d t b r i n g u t t h e e l e v a t i n a l p a t t e r n s f r a i n f a l l m r e c l e a r l y. T h e z n e s a r e c h s e n b a s e d n t h e a v a i l a b i l i t y f s t a t i n - l e v e l d a t a f r a i n g a u g e s. A s m a n y a s 4 5 r a i n g a u g e s t a t i n s, r e p r e s e n t a t i v e f t h e s e z n e s, a r e c n s i d e r e d i n t h i s a n a l y s i s. I t i s b s e r v e d t h a t t h e p a t t e r n s f c h a n g e s f p r e c i p i t a t i n w i t h e l e v a t i n a r e d i f f e r e n t f r d i f f e r e n t z n e s ( F i g u r e s a - e ). I n t h e A r u n a e h a l H i m a l a y a s ( F i g u r e 5. 1 l a ), t h e r e a r e h i g h r a i n f a l l z n e s a l n g t h e f t h i l l s b r d e r i n g t h e B r a h m a p u t r a v a l l e y. A s n e a p p r a c h e s t h e h i g h e r h i l l s t w a r d s t h e n r t h a n d e a s t, r a i n f a l l f i r s t d e c r e a s e s a n d t h e n i n c r e a s e s w i t h a l t i t u d e r e a c h i n g t h e h i g h e s t r a i n f a l l z n e s a r u n d m. R a i n f a l l i s l w e r a t h i g h e r a l t i t u d e s. I n t h e B r a h m a p u t r a v a l l e y ( F i g u r e b ), r a i n f a l l i s h i g h a t t h e l w - l y i n g w e s t e r n m s t p a r t s i n t h e s t a t e f A s s a m. R a i n f a l l d e c r e a s e s g r a d u a l l y b u t v a r i e s s h a r p l y w i t h i n t h e a l t i t u d e r a n g e m l y i n g m a i n l y i n t h e l w e r p a r t f t h e v a l l e y. R a i n f a l l i s c n s i d e r a b l y h i g h i n b e t w e e n a l t i t u d e s m m s t f t h e s e p l a c e s l y i n g i n t h e U p p e r B r a h m a p u t r a v a l l e y. H w e v e r, t h e r e a r e c e r t a i n l w r a i n f a l l a r e a s e m b e d d e d i n t h i s a l t i t u d e z n e s u c h a s L u m d i n g i n t h e s u t h - c e n t r a l p a r t f t h e v a l l e y. I n t h e M e g h a l a y a p l a t e a u ( F i g u r e c ) t h e r e i s a b e l t f e x t r e m e l y h i g h r a i n f a l l i n t h e a l t i t u d e z n e t l m b e y n d w h i c h r a i n f a l l d e c r e a s e s s h a r p l y. I n t h e T r i p u r a P l a i n s ( F i g u r e 5. 1 I d ) p r e - m n s n r a i n f a l l i s p r m i n e n t l y h i g h a n d f l l w s a n n u a l p a t t e r n s f a l t i t u d i n a l v a r i a t i n. B e i n g a l w - l y i n g f l a t p l a i n w i t h a n a v e r a g e e l e v a t i n f 2 0 m, a l t i t u d i n a l v a r i a t i n s f r a i n f a l l a r e n t s h a r p h e r e v e r t h e s p a c e r e p r e s e n t e d b y t h e s t a t i n s c n s i d e r e d. M a n i p u r h i l l s ( F i g u r e 5. 1 l e ) a l s s h w a l t e r n a t i n g d e c r e a s i n g a n d i n c r e a s i n g p a t t e r n a s n e g e s u p i n a l t i t u d e. I n g e n e r a l i t i s a l w r a i n f a l l z n e e x c e p t a t t h e f t h i l l s ( m ) a n d a n a r r w s t r i p f m i d d l e a l t i t u d e s ( m ). 163

17 7000 Rainfall (mm) Altitude(m) -PMR SMR AR Figure 5.11a Altitudinal variatin f pre-mnsn, summer mnsn and annual rainfall in Arunachal Himalayas. Figure 5.11b Altitude(m) -PMR -SMR AR Altitudinal variatin f pre-mnsn, summer mnsn and annual rainfall in Brahmaputra valley. the 164

18 14000 CNI O 0 0 CD O c ' s i N) O O Rainfall (mm) I Altitudes(m) ---- PMR SMR AR M lz A l t i t u d i n a l v a r i a t i n f p r e - m n s n, s u m m e r m n s n a n d a n n u a l r a i n f a l l i n t h e M e g h a l a y a p l a t e a u. CO N) c n in c n Rainfall (mm) Altitude(m) ---- PMR SMR AR y. t ' CL A l t i t u d i n a l v a r i a t i n f p r e - m n s n, s u m m e r m n s n a n d a n n u a l r a i n f a l l i n t h e T r i p u r a L a n d s c a p e. 165

19 4500 Rainfall (mm) Altitude(m) PMR SMR AR Figure 5.11e Altitudinal variatin f pre-mnsn, summer mnsn and annual rainfall in the Manipur Landscape. It is seen frm the abve analysis that as a general rule, precipitatin increases with elevatin t a certain level f altitude and then decreases after a certain level with further ascent up the slpes. This mde f variatin is particularly evident in the case f the Arunachal Himalayas and Meghalaya hills. These patterns in the Eastern Himalayas are similar t the rgraphic effect n rainfall bserved in the Western Himalayas by Singh et al. (1995). Hwever, it is a simplified picture f the real situatin in case f the Eastern Himalayas where seasnal and annual precipitatin is much higher in mst f the znes and statins cmpared t thse in the Western Himalayas and changes in rainfall with elevatin are expected t be sharper and mre dramatic even within very small gegraphical areas, ft is difficult t simulate the actual patterns because f lack f data at desired numbers f statins at different elevatins n bth windward and leeward sides. 166

20 5.5. Rainfall Znes The mde f distributin f rainfall has als been examined here based n determinatin f cefficients f variatins f annual rainfall in different parts f the NER fllwed up by an assessment f the trends in the spatial patterns f the CV (%) values and subsequent identificatin f certain znes having specific limits f annual variability. Figure 5.12 depicts the rainfall znes cnstructed based n spatial trends f 1 CV (%) f annual rainfall ver the NER. Variability f rainfall is high in the Himalayan «z t F" 8je 2 N NORTH EAST INDIA ANNUAL RAINFALL VARIABILITY 28 *1* INDEX CV(%) <«11-15 E3 ifi-i9 I S 3 1»-23 mi 23*27 I H >31 j F jf" I F Figure 5.12 Spatial distributin f variability f annual rainfell ver Nrtheast India 167

21 m u n t a i n u s z n e f A r u n a e h a l P r a d e s h b e c a u s e f t h e h i g h r e l i e f. T h e e a s t e r n p a r t s f T r i p u r a h i l l s, a s m a l l p c k e t f M a n i p u r b r d e r i n g A s s a m a n d A c r e r a i n s h a d w a r e a i n s u t h e r n B r a h m a p u t r a v a l l e y a r e a l s s e e n t h a v e r e l a t i v e l y h i g h e r v a r i a b i l i t y. F i g u r e p r v i d e s a s i m p l i s t i c p r e s e n t a t i n f r a i n f a l l z n e s d e t e r m i n e d b y a n n u a l r a i n f a l l a m u n t s i n d i f f e r e n t p a r t s f t h e r e g i n. T h e f i v e p r i n c i p a l r a i n - z n e s a r e d e r i v e d f r m t h e c r r e s p n d i n g i s h y e t s f r a n n u a l r a i n f a l l d i s c u s s e d e a r l i e r i n t h i s c h a p t e r. H w e v e r, b t h t h e s e c l a s s i f i c a t i n s p r t r a y a g e n e r a l i z e d p i c t u r e f r a i n f a l l 168

22 variability because f the effect f spatial averaging wherein sme f the detailed infrmatin seen in respective ishyets are suppressed r lst Findings and Cnclusins Intra-reginal variability f seasnal and annual rainfall is fund t be extremely high in the NER as seen frm the patterns f the ishyets, and variability alng latitudes, lngitudes and altitudes. Annual rainfall in the NER varies frm as high as mm mm in the Cherrapunjee-Mawsynram zne in the suthern Khasi hills t as lw as mm in the rain shadw area in suth central part f the Brahmaputra valley. Other high rainfall pckets f the regin are - nrthern fthills f Arunachal and adjining Assam plains where rainfall varies frm 4500 mm at Pasighat t 3500 mm at Lakhimpur; far eastern cmer f Arunachal in the Lhit basin in and arund Tezu with rainfall varying frm mm; the nrtheastern parts f Arunachal in the Dibang basin with rainfall ranging frm mm. The suthern part f Nagan district in central Assam valley and adjining parts f Karbi Anglng frm a rainshadw zne where rainfall is very lw due t bstructins by the Meghalaya-Karbi plateau n the way f the surging suthwest mnsn winds that prduce cpius rainfall by rgraphic mechanism n the suthern hillslpes f Meghalaya leaving the leeward side relatively dry. The rainshadw belt extends frm suth central Brahmaputra valley thrugh the nrth Cachar hills up t the Manipur plains. The Barak valley and adjining regins receive mre rainfall in the premnsn seasn than the Brahmaputra valley indicating greater impact f lcal cnvectinal activity, lcal lws and depressins frm the Bay f Bengal - a reasn why early flds in the rainy seasn are cmmn in the Barak and its tributaries. Distributin f mnsn rainfall in the regin is erratic and varies ver a wide range. Seasnal mnsn rainfall is remarkably high in suthern Khasi hills with a mean mnsnal rainfall f mm in Mawsynram and 8000 mm in Cherrapunjee. SMR 169

23 is lw in, the rainshadw belt in the suth central parts f the Brahmaputra valley where it varies between mm. Average annual and summer mnsn rainfall in the rainshadw area is l/8th and 179th f that in Cherrapunjee and 1710th and 1/11th f that in Mawsynram. Pst mnsn rainfall is remarkably high ( mm) in parts f the Siang-Dibang valley in the nrtheastern cmer f Arunachal Pradesh. This regin als receives high winter rainfall ( mm) alng with the nrthern fthills f Arunachal ( mm), prbably due t incursin f western disturbances thrugh jetstreams and windshearing at the interface f the surface nrtheasterlies and upper westerlies. There is a high-lw-high gradient in spatial distributin f seasnal and annual rainfall alng the Brahmaputra valley frm west t east. A similar spatial trend, althugh feeble and marked with mre variability, can als bee seen in annual and summer mnsn rainfall alng the suth-nrth directin alng latitudes. A zne between latitudes 25.3 N-25.5 N, lngitudes 91.6 E E having an average altitude f 1000 m emerges as the highest rainfall zne in the NEI. This is where Cherrapunjee and Mawsynram, tw rainiest spts f the wrld, are lcated. This zne has an annual rainfall f mm, a summer mnsn rainfall f mm, a premnsn rainfall f mm and a pst mnsn rainfall f mm. These are all highest amunts in respective categries in the NE regin. 170