INVESTIGATING THE APPLICABILITY OF WEB-DHM TO THE HIMALAYAN RIVER BASIN OF NEPAL

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1 INVESTIGATING THE APPLICABILITY OF WEB-DHM TO THE HIMALAYAN RIVER BASIN OF NEPAL Maheswor SHRESTHA, Le WANG 2 ad Tosho KOIKE 3 Studet Member of JSCE, Ph. D. Studet, Dept. of Cvl Eg., Uversty of Tokyo (Bukyo-ku, Tokyo , Japa) 2 Member of JSCE, Dr. Eg., Researcher, Dept. of Cvl Eg., Uversty of Tokyo (Bukyo-ku, Tokyo , Japa) 3 Member of JSCE, Dr. Eg., Professor, Dept. of Cvl Eg., Uversty of Tokyo (Bukyo-ku, Tokyo , Japa) A dstrbuted bosphere hydrologcal model, WEB-DHM has bee evaluated from pot scale to bas scale at a sub-bas of Hmalaya rver (The Dudhkosh rver bas of Nepal). At pot scale (Syagboche CEOP referece ste), the model performace was carefully checked by the logterm hourly upward shortwave radato (USR) ad sol surface temperature (ST). I geeral, the model performed well the pot scale smulato but t was foud that the USR was uderestmated ad the ST was overestmated durg hghly sow-covered days. The results were successfully mproved by modfcatos of albedo ad bulk sol heat capacty. For bas scale smulato, the model ca reproduce the seasoal varato of the dscharges at the bas outlet, but eeds further mprovemets sow accumulato for better represetato of sowmelt ruoff the bas. Key Words : WEB-DHM, sol surface temperature, upward shortwave radato, CEOP ste, dscharge, Dudhkosh. INTRODUCTION The surface eergy ad water balace hgh elevato ad cold regos are cotrolled by fuctos of vegetato, seasoal sow cover, ar temperature, surface mosture cotet, comg shortwave ad logwave radato. Hece, physcally based models should be used for the uderstadg of lad surface hydrologc processes of such regos. Few studes have bee carred out to estmate the surface eergy ad water balace of hgh elevato ad cold regos usg Dstrbuted Hydrologcal Model (DHM) ad Lad Surface Models (LSM) )2). Idepedet applcato of these DHM ad LSM caot represet the tegrated eergy ad water budget ad hece dstrbuted bosphere hydrologcal model shall be used by couplg DHM ad LSM for mproved represetato of hydrologc systems ad estmates of water ad eergy fluxes. Water ad Eergy Budget Based Dstrbuted Hydrologcal Model (WEB-DHM) 3) s a dstrbuted bosphere hydrologcal model, developed by couplg Smple Bosphere Model 2 (SB2) 4) to geomorphology based hydrologcal model (GBHM) 5). WEB-DHM physcally descrbes evapotrasprato wth coupled water ad eergy budgets the sol-vegetato-atmosphere trasfer (SVAT) system. Although WEB-DHM has bee successfully evaluated agast several feld expermets from pot scale to bas scale 6)7)8), t has ot bee appled to hgh Hmalaya rver bas yet. I Nepalese Hmalaya rver bass, very few studes have bee coducted usg DHMs 2)9). The water balace of the Kosh rver bas of Nepal has bee studed lumped ad dstrbuted water balace approach mothly tme steps ). Research hgh head watersheds s ot well advaced due to extreme terra, mosoo clmate ad lack of feld observatos lmtg the trasferablty of hydrologc prcples, techques ad models developed elsewhere. However sce the assessmet of hydrologc respose of the Hmalaya bas s affected by the hgh degree of ucertates meteorologcal data, t s essetal to valdate the

2 feld measuremets pror to applcato of the model. Ths study focuses o the applcablty of WEB-DHM to the Hmalaya rver bas of Nepal, the Dudhkosh bas ad valdato of the model performace at Coordated Ehaced Observg Perod (CEOP) ) referece ste Syagboche sde the bas agast upward shortwave radato ad sol surface temperature. 2. STUDY AREA The study area for the research s the Dudhkosh rver bas whch s located ortheast of Nepal. It s oe of the sub bass of the Kosh bas. The bas elevato rages from 452 m to 86 m mea above sea level. The catchmet area lyg upstream of Rabuwabazar dscharge gauge s about 37 km 2 whch about 9 km 2 le Hmalaya rego. The detal of meteorologcal gauges are show Fg.. ad Table. Fg. The Dudhkosh Rver Bas Table Detal of Meteorologcal Gauges. No. Name Elev. Data Source Resoluto (m) Dktel 623 DHMNepal Daly 2 Aselukhark 243 DHMNepal Daly 3 Pakaras 982 DHMNepal Daly 4 Mae 576 DHMNepal Daly Bhajyag 5 Saller 2378 DHMNepal Daly 6 Chaurkhark 269 DHMNepal Daly 7 Syagboche 3833 DHMNepal Hourly /NCAR-EOL 8 Pherche 426 NCAR-EOL Hourly 9 Lobuche 535 NCAR-EOL Hourly The clmate of the bas s represeted by four seasos; wter (December February), pre-mosoo (March May), mosoo (Jue September) ad post mosoo seaso (October November). The source of ruoff s mosoo rafall ad sow ad cemelt. About 8 % of the precptato falls mosoo perod. The summer mosoo precptato occurs sold form hgher alttudes. 3. METHODOLODY WEB-DHM was appled to vestgate ts applcablty to a hgh Hmalaya rver bas from pot scale to bas scale. The detals of the data, model structure ad ts evaluato crtera are descrbed ths chapter. () Data The daly dscharge data at Rabuwabazar gauge ad daly precptato data for 6 meteorologcal statos (-6) were obtaed from Departmet of Hydrology ad Meteorology, Nepal (DHMNepal). Hourly surface meteorologcal datasets (dowward ad upward shortwave radato, U ad V compoets of wd speed, specfc humdty, ar temperature, atmospherc pressure ad precptato) for the three CEOP referece stes (statos 7, 8 ad 9) were obtaed from Natoal Ceter for Atmospherc Research The Earth Observg Laboratory (NCAR EOL) for Earth Observato Perod 3 (EOP-3). Syagboche stato was selected for pot scale smulato amog 3 CEOP referece stes. Hourly sol temperature data at.5 cm depth (hereafter regarded as sol surface temperature) for Syagboche stato was obtaed from DHMNepal. The topographc data was obtaed from SRTM 9m DEM. Global Leaf Area Idex (LAI) ad the Fracto of Photosythetcally Actve Radato (FPAR) MOD5_BU_C5.25 degree datasets were used ths study, whch were 8-daly compostes of MOD5BU products. LAI ad FPAR data were provded by EOS Data Gateway of NASA. FAO Sol ad USGS lad use data were used as vegetato parameters (Fg. 2). Fg. 2 DEM (left), lad use (mddle) ad sol type (rght) the bas

3 Cambsols, lthosols ad gleysols were the sol types sde the bas. The average sol texture of the study bas was classfed as 28 % clay, 46% sad ad 26% slt. Lad use type was reclassfed accordg to SB2 type. Meteorologcal data sets (dowward shortwave radato, dowward logwave radato, ar temperature, cloud fracto, specfc humdty, U ad V compoets of wd speed, surface atmospherc pressure) for the bas were processed from the Japaese 25 years Reaalyss (JRA-25) dataset. All the surface meteorologcal data cludg precptato, LAI/FPAR, sol type, lad use data were terpolated at a km grd for model smulato. (2) Model Structure The model structure of the WEB-DHM 3)6)7) ca be summarzed as follows: () The bas s dvded to sub-bass usg Pfafstetter 2) scheme. Each sub-bas s dvded to a umber of flow tervals cosderg flow dstace to ts outlet. Each flow terval comprses several model grds. () Turbulet fluxes betwee the atmosphere ad the lad surface are calculated by hydrologcally mproved SB2 o each model grd usg oe combato of sol type ad lad use. () Each model grd s treated as geometrcally smlar hllslope ut whch s the basc computg ut of WEB-DHM. (v) Hllslope module s used to smulate surface ad sub surface ruoff geerated from hllslope uts ad kematc wave approach s used to route the water flow the rver etwork. (3) Model Performace Idcator The model performace s evaluated by Mea Bas Error (MBE), Root Mea Square Error (RMSE) ad Nash-Sutclffe Effcecy (NSE) 3). MBE, RMSE ad NSE are defed as followgs; MBE = RMSE = NSE ( S O ) / () = = = = = ( S O ) 2 / (2) ( Qo Qs ) (3) 2 ( Q Q ) o o 2 where s the total umber of tme seres, S s the smulato result, O s the -stu data, Q o s the observed dscharge, Q s s the smulated dscharge, Qo s the mea observed dscharge over the smulato perod. 4. RESULTS AND DISCUSSIONS () Pot Evaluato of the WEB-DHM at the CEOP Referece Ste (Syagboche stato). Pot scale model smulato at CEOP referece ste, Syagboche stato was drve by tal codto of observed sol surface temperature. Hourly feld measuremets of dowward shortwave radato (DSR), precptato, pressure, specfc humdty, ar temperature, U ad V compoets of wd speed ad 6 hourly JRA-25 data of cloud ad log-wave radato were used as atmospherc forcg data for the model ru. The model was ru from 2 October 22 to 24 Jue 23. The model output was compared wth measured hourly upward shortwave radato (USR) ad sol surface temperature (ST). The sow cover days are cosdered whe ST s at a costat value below freezg temperature (273.5 K) ad the albedo s above 5% 4). a) Upward shortwave radato (USR) Upward shortwave radato (USR) s the crtcal parameter the radato budget of surface eergy balace. It reflects the reflectvty power of the lad surface,.e. albedo of the surface ca be calculated from the formato of USR. Hgher the USR wth respect to DSR meas the lad surface absorbs less eergy. Sce radato budget govers the whole eergy budget the hgher elevatos, USR should be correctly valdated for the eergy balace studes. The comparso betwee observed ad smulated hourly USR s show Fg. 3. The dural cycle of USR s well smulated by WEB-DHM wth hgh accuracy for the moths October December ad Aprl Jue. The smulato results showed that the USR s uderestmated durg sow cover days of the moths Jauary March. Ths dscrepacy s edorsed due to the hgh reflectvty of sow Obs. Sm. -Oct-2 8-Oct-2 5-Oct-2 22-Oct-2 29-Oct-2 -Nov-2 8-Nov-2 5-Nov-2 22-Nov-2 29-Nov-2 2 -Dec-2 8-Dec-2 5-Dec-2 22-Dec-2 29-Dec-2

4 Ja-3 8-Ja-3 5-Ja-3 22-Ja-3 29-Ja Feb-3 9-Feb-3 7-Feb-3 25-Feb Mar-3 8-Mar-3 5-Mar-3 22-Mar-3 29-Mar-3 2 -Apr-3 8-Apr-3 5-Apr-3 22-Apr-3 29-Apr May-3 8-May-3 5-May-3 22-May-3 29-May-3 2 -Ju-3 8-Ju-3 5-Ju-3 22-Ju-3 Fg. 3 Observed ad smulated hourly upward shortwave radato (USR) from 2 October 22 to 24 Jue 23 at Syagboche stato. b) Sol surface temperature (ST) Sol surface temperature s aother mportat parameter of the lad surface eergy balace. It drectly affects the sesble ad latet heat fluxes. The accurate estmato of temperature gradet betwee atmosphere ad lad surface s very mportat hgh elevato area for the estmato of evapotrasprato. It s the key varable for the estmato of upward logwave radato whe the -stu data are ot avalable. The comparso betwee observed ad smulated hourly ST s show Fg. 4. The dural cycle of ST s well smulated for the moths October December ad Aprl Jue. The smulato results show that the ST s overestmated durg sow cover days of the moths Jauary March. Statstcal aalyss of the smulato results are show Table 2. Table 2 Statstc aalyss of model smulato for whole perod. MBE RMSE Sol surface temperature (K) Upward shortwave radato (W/m 2 ) Obs. Sm. -Oct-2 8-Oct-2 5-Oct-2 22-Oct-2 29-Oct-2 -Nov-2 8-Nov-2 5-Nov-2 22-Nov-2 29-Nov Dec-2 8-Dec-2 5-Dec-2 22-Dec-2 29-Dec Ja-3 8-Ja-3 5-Ja-3 22-Ja-3 29-Ja Feb-3 9-Feb-3 7-Feb-3 25-Feb Mar-3 8-Mar-3 5-Mar-3 22-Mar-3 29-Mar Apr-3 8-Apr-3 5-Apr-3 22-Apr-3 29-Apr May-3 8-May-3 5-May-3 22-May-3 29-May Ju-3 8-Ju-3 5-Ju-3 22-Ju-3 Fg. 4 Observed ad smulated hourly sol surface temperature (ST) from 2 October 22 to 24 Jue 23 at Syagboche stato. c) Modfcato of surface albedo ad bulk heat capacty of sol We assume that the ste s covered by cotuous sow cover from 28 Jauary to 3 March of year 23 from the aalyss of observed data of ST, DSR ad USR. Durg ths perod, the model results seem accurate ad USR seems accurate from to 8 Jauary too but the observed ST s foud below the freezg pot from to 8 Jauary oly. Although the 8 days wth ST below the freezg temperature ca also be assumed as sow cover days, we gored ths short perod for modfcato durg smulato. I ths study, we

5 focused o the logterm perod wth ST below the freezg temperature for modfcato ad hece the albedo ad bulk sol heat capacty are modfed for the perod from 28 Jauary to 3 March, 23. The albedo s modfed as a fucto of ar temperature followg Meteoorm 5) as show equato 4. The orgal equatos for albedo ad bulk sol heat capacty ca be foud Sellers et. al. 4). The lear regresso equato for albedo ad modfed bulk sol heat capacty (C sol ) are as followgs; Albedo ( T 273.5) (4) sol = a C = ( η ) C + w C (5) s where T a s the ar temperature (K), η s the porosty of sol (from FAO sol data 6) ), C s s the specfc heat capacty of sol (.8 6 J m -3 K - ), w s the water cotet of sol (smulated by the model tself), C w s the specfc heat capacty of water(4.8 6 J m -3 K - ). The smulato results wth ew scheme of albedo ad bulk sol heat capacty show the good agreemets wth the observed oes as show Fg. 5. The scatterplots of USR ad ST as show Fg.6 (a,b) shows that the performace of the model s remarkably mproved after the modfcatos. The statstc dcators for mprovemet of model performace are show Table 3. Although these statstcs presets the creased performaces of the model, the eergy budget of the model should be physcally mproved further the albedo dstrbuto, physcs of sow accumulato ad sowmelt for the hgh performace the Hmalaya rver bass. Table 3 Statstc aalyss of model smulato for 28 Jauary 23 to 3 March 23. MBE RMSE a) Before modfcato Sol surface temperature (K) Upward shortwave radato (W/m 2 ) b) After modfcato Sol surface temperature (K) Upward short-wave radato (W/m 2 ) Ja-3 5-Feb-3 3-Feb-3 2-Feb-3 -Mar Ja-3 5-Feb-3 3-Feb-3 2-Feb-3 -Mar-3 Fg. 5 Observed ad smulated hourly USR ad ST from 28 Jauary to 3 March 23 at Syagboche stato. w Smulated Observed USR (W /m 2 ) Smulated Observed Fg. 6(a) Scatterplots of hourly observed ad smulated USR ad ST from 28 Jauary to 3 March 23 before modfcato. Smulated Observed USR (W /m 2 ) Smulated Observed Fg. 6(b) Scatterplots of hourly observed ad smulated USR ad ST from 28 Jauary to 3 March 23 after the modfcato of albedo ad bulk sol heat capacty. Dscharge (m 3 /s) NSE =.65 MBE = - 7 m 3 /s O-2 N-2 D-2 J-3 F-3 M-3 A-3 M-3 J-3 J-3 A-3 S-3 O-3 N-3 D-3 Fg. 7 Observed ad smulated dscharge at Rabuwabazar gauge from 2 October 22 to 3 December 23. (2) Bas scale Evaluato of the WEB-DHM Daly dscharge at Rabuwabazar gauge of the Dudhkosh bas s smulated from 2 October 22 to 3 December 23. The smulated dscharge represets the seasoal varato as compared to the observed dscharge but the volume error seems too hgh for the mosoo seaso as show Fg. 7. NSE ad MBE for the smulato s.65 ad -7 m 3 /s respectvely. The smulated dscharge s ot able to show the cosderable amout of sowmelt the mosoo ad post mosoo seaso. We smulated the dscharge wth ew scheme of albedo ad bulk sol heat capacty. The results are ot mproved because the sowmelt the pre mosoo seaso depeds upo the sow accumulato patter the wter seaso ad curret WEB-DHM s uable to smulate the sow accumulato patters. Aother reaso for the hgh dscrepacy of dscharge durg mosoo ad post mosoo seaso may be due to the characterstcs of the rver bas tself. Hgh elevato regos of the bas are covered by glacers ad glacer melt s sm obs

6 gored curret smulato. Geerally sow accumulato patter hgh elevato of sow fed rver bass are of two types: summer accumulato ad wter accumulato type. The summer accumulato type has more sow accumulato summer tha wter. Sow accumulato ad sowmelt occur smultaeously summer ad the creased summer ar temperature reduces the proporto of sow to ra causg reducto the sow accumulato. Ths wll result a decrease the surface albedo whch ultmately creases sowmelt,.e. sowmelt s strogly affected by accumulato varatos. But for the wter accumulato type, sowmelt varatos are almost depedet of sow accumulato varatos. The sow accumulato patter Dudhkosh rver bas s of summer accumulato type for whch sowmelt s strogly affected by accumulato codto sce both occur smultaeously. The curret model ca ot smulate such smultaeous accumulato ad meltg process well. The refemets o the sow physcs (both sow accumulato ad sowmelt) WEB-DHM are expected to mprove the model performace the bass wth smlar characterstcs to Dudhkosh. 6. CONCLUSIONS I ths study, WEB-DHM was evaluated pot scale at Syagboche CEOP referece ste agast the hourly sol surface temperature ad upward shortwave radato. Although the performace of the model was mproved by ew scheme of albedo ad bulk sol heat capacty, the model eeds further mprovemet the spatotemporal dstrbuto of albedo, sow accumulato ad sowmelt algorthms for the bas scale smulato. Hece, the research mprovemet of sow physcs of ths model for accurate smulato the Hmalaya rver bas s hghly ecouraged. ACKNOWLEDGMENT: Ths study was supported by the Japaese Mstry of Educato, Scece, Support ad Culture (MEXT). The authors express deep grattude to DHMNepal, Kathmadu, Nepal for provdg hydro-meteorologcal data ad NCAR-EOL, USA, for provdg CEOP/EOP-3 CAMP dataset. Mr. Mohamed Rasmy from Dept. of Cvl Egeerg, Uversty of Tokyo, was hghly ackowledged for hs techcal support. REFERENCES ) Yamazak, T.: A oe dmesoal lad surface model adaptable to tesely cold regos ad ts applcato easter Sbera, J. Meteor. Soc. Japa, Vol 79 (6), pp.7-8, 29. 2) Koz, M., Uhlebrook, S., Brau, L., Shrestha, A. ad Demuth, S. : Implemetato of a process based catchmet model a poorly gauged, hghly glacerzed Hmalaya headwater, Hydrol. Earth. Syst. Sc., Vol., pp , 27. 3) Wag, L.: Developmet of a dstrbuted ruoff model coupled wth a Lad Surface Scheme, Ph. D. thess, Uversty of Tokyo, Tokyo, 27. 4) Sellers, P.J., Radall, D.A., Collatz, G.J., Berry, J.A., Feld, C., Dazlch, D. A., Zhag, C., Colledo, G.D. ad Bououa, L. : A revsed lad surface parameterzato (SB2) for atmospherc GCMs.. Model formulato, J. Clmate, Vol. 9, pp , ) Yag, D. : Dstrbuted hydrologcal model usg hllslope descretzato based o catchmet area fucto: developmet ad applcatos, Ph.D. thess, Uversty of Tokyo, Tokyo, ) Wag, L., Koke, T., Yag, K., Jackso, T., Bdlsh, R. ad Yag, D.: Developmet of a dstrbuted bosphere hydrologcal model ad ts evaluato wth the Souther Great Plas Expermets (SGP97 ad SGP99), J. Geophys. Res.-Atmos., Vol. 4, D87, 29. 7) Wag, L., Koke, T., Yag, K. ad Yeh, P. : Assessmet of a dstrbuted bosphere hydrologcal model agast streamflows ad MODIS lad surface temperature the upper Toe rver bas, J. Hydrol., Vol. 377, pp.2-34, 29. 8) Wag, L. ad Koke, T.: Comparso of a dstrbuted bosphere hydrologcal model wth GBHM, A. J. Hydraul. Eg.-JSCE, Vol 53, pp.3-8, 29. 9) Pradha, N. R. ad Jha, R.N. : Performace assessmet of the BTOPMC model a Nepalese draage bas, IAHS Publ., 282, pp , 23. ) Sharma, K.P., Charles, J.V. ad Berre, M.: Sestvty of Hmalaya hydrology to lad use ad clmatc chages, J. Clmate Chage, Vol. 47, pp.7-39, 2. ) Koke, T. : The Coordated Ehaced Observg Perod a tal step for tegrated global water cycle observato, WMO Bull., Vol. 53(2), pp.5-2, 24. 2) Yag, D., Musake, K., Kaae, S. ad Ok, T.: Use of the Pfafstetter bas umberg system Hydrologcal modelg. I Proceedgs: Aual Coferece of Japa Socety of Hydrology ad Water Resources; pp.2-2, 2. 3) Nash, J.E. ad Sutclffe, J.V.: Rver flow forecastg though coceptual models part I A dscusso of prcples. J. Hydrol., Vol., pp , 97. 4) Ueo, K., Kayastha, R.B., Chtrakar, M.R., Bajracharya, O. M., Pokharel, A.P., Fujam, H., Kadota, T., Ida, H., Maadhar, D.P., Hattor, M., Yasuar, T. ad Nakawo, M.: Meteorologcal observatos durg at the Automatc Weather Stato (GEN-AWS) Khumbu Rego, Nepal Hmalayas, Bull. Glac. Res., Vol. 8, pp.23-29, 2. 5) Meteoorm verso 5., The global meteorologcal database for egeers, plaers ad educato, Uted Kgdom, 23. 6) FAO, Dgtal Sol Map of the World ad Derved Sol Propertes, Lad ad Water Dgtal Meda Seres Rev., Uted Natos Food ad Agrculture Orgazato, CD-ROM, 23. (Receved September 3, 29)