Distributed Model of Hydrological and Sediment Transport Processes in Large River Basins in Southeast Asia S

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 were 0.60 and 0.78, ). The simulation over 2 r ciency (NSE) and average correlation , and O. C. Saavedra ffi 1 e e 6756 6755 ff ) in the Chao Phraya River Basin and to soil detachability , C. Yoshimura k 1 ). In this study, we propose a process-based distributed model for 2 ) and the Mekong River Basin (795 000 km 2 , K. Tanuma ) between the simulated and observed suspended sediment loads were ) in the Mekong River Basin. Overall, the results suggest that the present 1,2 f r K 100 000 km > cient ( ffi ective management of sediment in rivers is becoming increasingly important from ff This discussion paper is/has beenSciences under (HESS). review Please for refer the to journal the Hydrology corresponding and final Earth paper System in HESS if available. Department of Civil Engineering, Tokyo Institute of Technology, 2-12-1-M1-4 Ookayama, Department of Civil Engineering, Faculty of Engineering, National Defense University of an economic, environmental, and ecologicalrivers perspective. with A longer-term recent records study50 % of of 145 annual of major sediment themtrend loads experienced showed (Walling a that and approximately loads statistically Fang, because of significant 2003). dams upward and The other or river majority control downward structures of trapping seasonal them sediment. Moreover, showed declining sediment E model can be used to understandriver and basins. simulate erosion and sediment transport in large 1 Introduction over land ( respectively. Sensitivity analysis indicatedto that detachability suspended by raindrop sediment ( load is sensitive Abstract Soil erosion and sediment transport havescales, been modeled yet at several spatial few andareas temporal models have been reported for large river basins (e.g., drainage assessment of sediment transportmodel at was adescribing coupled large soil basin erosion with and scale. sedimentarymodel A processes a at was distributed hillslope process-based tested units hydrological and on channels.area: distributed The two 160 000 large sediment km river10 transport basins: the years model Chao showed Phrayaboth good River basins. agreement Basin The (drainage with average the Nash–Sutcli observed suspended sediment load in coe 0.62 and 0.61, respectively, in theIn Chao Phraya River the Basin except Mekong the lowland River section. Basin, the overall average NSE and Distributed model of hydrological and sediment transport processes in large river basins in Southeast Asia S. Zuliziana Hydrol. Earth Syst. Sci. Discuss.,www.hydrol-earth-syst-sci-discuss.net/12/6755/2015/ 12, 6755–6797, 2015 doi:10.5194/hessd-12-6755-2015 © Author(s) 2015. CC Attribution 3.0 License. 1 Meguro-ku, Tokyo 152-8552, Japan 2 Malaysia, Sungai Besi Camp, 57000 Kuala Lumpur, Malaysia Received: 21 April 2015 – Accepted: 29Correspondence May to: 2015 S. – Zuliziana Published: ([email protected]) 16 July 2015 Published by Copernicus Publications on behalf of the European Geosciences Union. 5 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (Santhi 2 , subsurface ff ects of the soil ff ) of Southeast Asia. The 2 ective river basin management ff 100 000 km > 6757 6758 ecting on soil erosion. Thus, e ff cult to measure with field testing. Given the complexity of the ffi ecting sediment dynamics, it is important to develop a robust process- ff ) catchments (Duna et al., 2009). The Soil and Water Assessment Tool 2 er in terms of complexity, processes considered, and the data required for ff Process-based models are based on the solutions of fundamental physical equations This study aimed to develop a process-based distributed model that can simulate A wide range of models exists for simulating erosion and sediment transport. These A number of process-based soil erosion and sediment transport models have also describing stream flow andphysical sediment processes production observed in a in river the basin. real They world, represent the such as surface runo erosion rate, sediment transport, andlarge deposition river at basins. specific locations and especially in flow, ground flow, and evapotranspiration. Process-based modelsadvantages provide several over major empirical and conceptualspatial model, and including temporal capabilities distributions forfor estimating of an net entire hillslope soilcan or loss for estimate (or each sediment gain, pointSince in simulation on these the a on models case hillslope. are a of Further, process-based,extrapolated daily, process-based to deposition) they monthly, some models can or extent also to bethat an a seasonally broad average might interpolated range annual and of be basis. conditions, including di some conditions was confirmed insediment model large continuously catchmentsand simulates (i.e., sediment the transport. sedimentaryused Hydrologic process, as data, including input soil data. erosion distributed Soil type, hydrological loss land model and use, to itsfor and create transport large a process topography catchments comprehensive were were deposition sediment coupled in and with assessment Southeast detachment an tool in Asia. existing rivers,models. which This The paper have sediment also not describes been model applications considered of separately in the sediment most simulated model existing in two large river relationships a the sediment dynamic process at a large basin scale. The feasibility of the model requires the development of process-based models to estimate the e based model ofof sediment natural dynamics systems thatespecially as can in well be large catchments. as used human-induced to environmental predict changes the and consequences impacts, et al., 2001). It is ahydrologic semi-distributed response conceptual model, units capable of as dailyabout the simulation using combinations basic of computationalHowever, unit land semi-distributed to models use like group SWAT and do inputresolution not information soil of generally land incorporated spatial withdominant management a information, factors fine (Neitsch such a et as al., land 2002). use and soil information which are (max. 2.6 km (SWAT) was designed for(Arnold application et to al., 1998) large and river has been basins implemented in and river long-term basins of simulations over 4000 km models di been developed, but those applications are limited to individual storm events and small human activities such as deforestationor and erosion water diversion in might coastal causein regions. sedimentation the Such future sediment-related because problems of could(Walling, further be 2011; dam more construction, serious Zarfl climateserious variability, et soil and erosion deforestation potential al., anddramatic excessive 2014). sedimentation. land They Currently, are surface also river experiencing changes,hydropower construction basins and such water in diversion as (Tacio, 1993), becauseand forest Southeast of economic clearing, rapid Asia growth population in reservoir have the construction, region (Walling, and 2009). model calibration and modelmodel use for (Roberto all et applications. al.,spatial The 2012). scale, and most In characteristics general, of appropriate the there modelLoss catchment is Equation being will considered. (USLE) no depend The (Wischmeier “best” Universal on and Soil (Renard Smith, intended and 1978) Freimund, use, and 1994) its are revised widelyerosion. version used Both (RUSLE) as USLE tools for and empiricalthe RUSLE assessment long account of term soil (e.g., for for sedimentof 20 eroded years). sediment In is from these not the empirical considered catchment equations, to however, in the occur deposition in the modeled area. 5 5 10 25 20 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | at each grid ff and river routing were incorporated within ff 6760 6759 erent soil properties and hydrogeology. ff , (1) zh − kEe ) g C − ciently from upper to downstream. The water routing on the river network is ffi (1 = In the river routing system, the Pfafstetter numbering system is applied to track water In the hillslope model, each grid is divided into four layers: canopy, soil surface, In the GBHM, the target watershed is divided into grids, and a digital elevation model R D determined along the riverFurther stream details using are described one-dimensional by kinematic Yang et wave al. equations. (2001). 2.2 Sediment model 2.2.1 Soil detachment Soil detachment by raindrop impacts was estimated by Eq. (1) (Torri et al., 1987). flow e where pan observation couldwas also applied be for the used. canopy InIn water the storage, order root module, zone, Priestley–Taylor’s to surface method Richards storage, and describe equation soil is surface. the usedroot unsaturated with zone. Saturated soil zone water infiltration watermass flow rate balance and equations flow, and exchange and a Darcy’s soil with Law.estimated The the moisture the vertical simulation infiltration river contents module excess one-dimensional is of and in surface saturation describedas water the excess using lateral discharging flow into flow. basic the river system unsaturated zone, and groundwater. Vegetation covered thedirect surface soil rainfall and prevented ontovegetation coverage and the leaf area land. index.water The volume evapotranspiration The that module simulated evaporated the deficit from the of surface soil canopy and transpirated interception from the is canopy, calculated by in one hillslopemain unit. stream This in means thisrealize that model. a all The fast flow flow hillslopesa interval–hillslope computation rectangular of even system inclined a in plane enabled with flow agivens the a by large interval GBHM defined the basin. length corresponding to drain and surface The into slope. unit hillslope width. the unit The inclination is angle viewed as basins in Southeast Asia: theare Chao characterized Phraya by and di Mekong River basins. The two basins 2 Model structure The important processes of sedimentdeposition) were dynamics modeled (soil and erosion, integrated with sedimentmodel a transport, (DHM) process-based and distributed (Fig. hydrological 1).rivers In were the separately sediment modeledmodel model, and was sediment systematically developed dynamics linkeddistributed using on each hydrological FORTRAN hillslopes other. model. to and The The create sediment runo a compatible link to the adopted 2.1 Hydrological model The distributed hydrologicalhydrological model model (GBHM) used developedmomentum, by in Yang and et this al. energyrouting study (2001). module. equations It is using solves a the two continuity, geomorphology-based modules: hillslope(DEM) module is used and tothe river determine river the network. flowsubbasins, direction Each and flow subbasin accumulation intervals is patternoutlet. are divided that Lateral defined creates into flow as to a a the number main function of stream of estimated flow distance by intervals. accumulating from In runo the the subbasin’s the sediment model.on Hydrological a and daily sediment-related time-step.load The processes (SSL), overall because were model suspended calculated sediment wassediment (SS) in designed many is to of dominant the target portion world’sthe suspended rivers of suspended (Ongley, the sediment 1996), load and transported makes itMeade, up is 1983). frequently about assumed 90 that % of the total load in the world (Milliman and 5 5 25 20 15 10 20 10 15 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | is (3) (7) L E is depth of is a specific DT γ H ), 3 is hydraulic shear is the total kinetic ) of raindrop to be − τ E ) estimated for each E is an overland flow 1 ), and − f 1 ), h 1 − K − 2 ) and − 2 ), mm 1 − 2 − is subjected to calibration and ) was used in the model. − s f 1 2 K is the depth of leaf drip (net rain − is depth of overland flow (m). In − h LD H ) and 3 − ), and can be described by Eq. (3) (Brandt, 1989) is the depth of the surface water layer (mm). is critical shear stress for initiation of soil 1 6761 6762 − h D c E τ is the correction factor for water ponding where mm ), zh 2 1 − − − e s ). is sediment particle size (µm). 2 ), 1 , (2) − 1 s − − LD , (5) D h c H ). 2 L − DT E ), (9) C H s τ > τ ) and ective height of the plant canopy in meters. This study assumed , (8) ) 5.87 (4) 3 s C ff C I − ( D − + − for cient (kgm ) is the specific weight of sediment particles (Nm , (6) γ ffi c s DT γ 0.5 − (TC H is the value of the dimensionless shield parameter obtained from the ) s s 1 D ) as given in Eq. (7). is an index of the detachability of the soil (gJ γ 2 E τ < τ 8.44log ( − is an overland flow detachment (kgm ) PH wv − k is soil detachment rate by raindrop impact (gm ( c s C is canopy cover in the model (i.e., in each grid) and was estimated from land + τ τ F is a specific weight of water (Nm is the slope of the ground surface. In Eq. (5), β sheilds C R C )), which was estimated by deducting the interception loss of water from the is rain intensity (mmh , D 1 N D γ S f = I C − sheilds 15.8 − 8.95 K 0 for N (1 γhS is the kinetic energy of direct throughfall drops (Jm is the proportion of soil surface in each grid. Raindrop impacts were categorized = = = is the kinetic energy due to leaf drip, also as proposed by Brandt (1990) and shown = river = The kinetic energy for direct rainfall = depends on soil texture (0.9–3.1), and = F F g D D L L C compute soil detachment basedshear on stress. comparison of critical shear stress andD hydraulic where D where E For soil detachmentRehman due to and overland Akhtar flow, (2004) we and used shown equations as derived by Eqs. Habib-ur- (5) and (6). These were used to use data on a scaleE of 0.0 to 1.0 (0direct for throughfall bare drops, land for and which 1.0 rain for intensity highly (mmh dense forest area). E in Eq. (4). PH isthat the PH e is 1 m following Kabir et al. (2011). E Shield’s curve, weight of water (Nm 2.2.2 Transport and deposition of sediment Soil detachments by flow and sedimentoccur deposition simultaneously. Flow in rivers detachment are or generallydescribed deposition considered by can to Morgan be et expressed al. by (1998). Eq. (9),DF as where where τ into direct rainfall and leaf drip, allowing the total kinetic energy ( detachability coe where time step, energy of the rainfall (Jm C described by Eq. (2). E the kinetic energy of leaf(mmh drip (Jm depth of rain intensity ( particle motion as obtainedstress from (Nm the Shield’s curve (Nm where the critical shear stress values are obtained byτ the following equation. z this study, the depth of overlanddepth. flow is assumed to be the corresponding surface water 5 5 10 20 20 15 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ). is 50 1 d s − (14) (15) (12) (13) ) for ), is the 1 1 − − w m 1 ), − 3 s ) by dividing − 3 3 m ) by using the is accumulated s 3 C qs is the cross-section ), A 1 − ), 1 ) calculated with Stokes’s − 1 s − 3 is mean flow velocity (cm V , erence approximation were applied ff ). V s 3 − ) (16) 10 s C = is stream velocity (m − ω 6764 6763 V ), 1 (TC − s ), TC is the transport capacity (m 3 − wv is the critical value of unit stream power (0.40 cms s m β cr 3 is particle settling velocity (m ω + s river (m v ) in flow-interval and iflow is the number of flow intervals in s 1 cients that depend on the median particle size of the soil ( of soil density was used for the conversion unit in both case DF , was calculated at its upstream flow interval right before the − C ffi 3 AV in + m − (iflow) ) = Q 1 0.6 − s qs − s 0.25 Q 3 ; = iflow , (11) ( are coe , (10) ) η s J is the flow detachment or deposition of sediment (m ) AV η 0.32] 300] qs V cr / / QC = ∂t 0.85 = ω ( 5) 5) − river , (17) Q ∂ and s − ; e is river discharge from the hydrological model (m t is a correction factor used to calculate cohesive soil erosion as shown in ) of water and sediment flow, + + is the unit stream power (cm ∀ is the soil cohesion (kPa). Several methods have been developed to estimate s c + 2 ∂t d ω . Using Eqs. (9) and (15), Eq. (14) was converted to Eq. (16). d ∂A s ( J Q ) ω 50 50 Q s β c d d = QC 0.79 + [( [( = s O = = QC ) was determined by the one kinematic wave approximation in the river node. The ∂x = = ( s s − ∂x Q ∂ ∂Q Q to simulate sediment transport both over land and in the river. On the land grids, in µm). c η The movement ofmovement sediment with in waterand each discharge, momentum grid based similar cellThe to on one-dimensional was the kinematic the wave flow determined and principle simulation finite by of di in associating conservation the the distributed of hydrological mass model. the slope in percentage, and In this study, 2.67 gcm studies. where TC. This study adoptedstructure Eq. and (11), available proposedestimate input by SSL, Govers parameter not (1990), including database. bed because loads. Equation of its (11)TC simple was only used to where Eq. (10) (Kabir et al., 2011). β given the movement ofa soil weighting and system waterthe that accumulated flow flow was was was based streamed( accumulated into on the at the river as eachkinematic distance lateral wave flow equation from flow. shown The interval the in waterto Eq. discharge with main (14) calculate was stream. the also Then, applied movement to of the river suspended routing sediment model concentration ( sediment yield (m river routing, the unitwith of the specific sediment weight mass of (g) sediment (2.67 was gcm changed to volume (m each subbasin. An accumulated sediment yieldflow was and considered was as added the at lateral the sediment inlet of the control volume (i.e., the river routing part). In the where 2.3 Dam model The inflow to adam dam, location on adescribed river by network change by of GBHM. reservoir storage The in balance time of using dam Eq.I inflow (17) (Ponce, and 1989). outflow is area (m where DF sediment concentration Law. width of the riverthe flow hydrological (m) model in and each subbasin as estimated from the input parameter of 5 5 15 20 10 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ). They 2 curve data. ), the Wang 2 ∀ – h ) (Kummu et al., 2010) 1 − s was obtained per Valeriano 3 are discharge flow into the 2 ∀ 2 in Q and 1 in ), and the Nan River (34 557 km 2 Q ected by the Southeast Asian monsoon and 6766 6765 ff is change in storage volume within a time interval , (18) ∀ . The basin is traditionally the center of Thailand’s t 2 ∆ # 2 out from head to mouth. In this study, the target basin covered Q 2 − is discharge from the dam and is assumed to be constant ! 2 out curve, the water level can be calculated. In normal conditions, the 2 in is an outflow, d Q ∀ Q ), the Yom River (24 720 km – O 2 + 2 h 1 in Q needs to be set as the initial volume condition to read the " 1 is inflow, ∀ + I 1 ∀ ). Then, the reservoir storage at the current time step = In the following case studies, the dam operation rule was applied, and release The climate in Thailand is strongly a t 2 dam and are provided byvolume the simulation using GBHM. The value for the last time step Then, using the was assumed to bein equal the to Chao observedannual Phraya discharge River release from Basin. from the In the Lancang contrast, Bhumibol subbasin in and (2332.29 m the Sirikit Mekong dams River Basin, the mean release can be calculatedthe using flow a can dam be operational routed downstream rule. by Once GBHM. the release is defined, was used asstabilizes its downstream dam river discharge. In releasesedimentation the Chao at Phraya was River Basin, Manwan the estimatedreservoir reservoir sedimentation in Dam, by the Mekong assuming Brune’s Riverof Basin dam was curve that observation made, due data. to (Brune, hydropower limited availability station 1953). No estimation of characterized by distinct rainythe and middle dry or end seasons. ofthe Basically, Chao May the and Phraya rainy River lasts season Basin untilDepartment, varies starts the 2012). between middle at 1000 of and October. 1500 mm Annual (Thai precipitation Meteorological in 3.1.1 Model set-up and calibration Geographical information for the Chaoand Phraya land River use) Basin was (e.g., collected topography,obtained soil from for type, the the Shuttle development Radar of Topography Mission (URL: ahttp://dds.cr.usgs.gov/srtm/ hydrological model. A DEM was River (11 708 km converge at Nakhoncovered Sawan. by In forest and the bareof northern soil. the These mountainous headwaters valleys of stretch region, the south Chao to there north, Phraya are River which Basin. is valleys the area rice production, because the monsoonto weather October. typically brings Land moreincluding cover rainfall from in evergreen, May the deciduousfields Chao and (7.1 %), Phraya mangrove bodies River of forests),(UNEP,1997). Basin water The (0.6 croplands consists soil %), in and of (56.4 the areas %), Chaoand forest for Phraya contains River which paddy Basin 38.2 (30.2 no % %, is sand, data predominantly sand 25.2 isChao % clay available loam Phraya silt, (5.7 and River %) 36.7 has % clay four on major average (Kyuma, tributaries: 1976). the The Ping River (36 018 km from sources toa the catchment Chao area Phraya of Dam 117 375 (C13) km (the gray area in Fig. 2), which has 3 Model application 3.1 Chao Phraya River Basin The Chaoapproximately Phraya 160 000 km River Basin covers about one third of Thailand, which is where the subscriptcurrent 1 refers time to step. thebetween last time time steps step, 1 and the and subscript 2. 2 Both refers inflows to the where (d et al. (2010), ∀ 5 5 20 25 15 10 15 10 20 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ect ff ) were J , k ) for 2001– r ciency criteria: ffi cients ( ) in the basin were f ffi K ed Complex Evolution (SCE) algorithm ffl ) and from sheet flow ( 6768 6767 k in Table 2 (refer to Fig. A1 for hydrographs). The r cient (NSE) and correlation coe ffi ciency coe at P73 were closer to 1 than in other drainage basins and were ffi r ecting the hydrological process and soil erosion, such as land use ff e e ff ) was assumed to be 50 µm based on the suspended sediment distribution 50 River discharge and SSL were calibrated and validated in combination with dam Model evaluation revealed that the river discharge simulation performed Regarding SSL, NSE for all stream gauges was larger than 0.5 except at C2 ) was determined in each subbasin. In the sediment model, the sediment particle J ( size (d in a river nearproposed Chiang model, Mai (unpublished the data). model To was demonstrate the calibrated applicability and of validated the with two e the lowest at C2downstream among gauge the is gauges. related The toNormally, the reason the flooding for river situation the in discharge lowestregion the NSE because overflows Chao the occurring Phraya every discharge River at year capacity Basin. discharge the around during is C2 likely rainy is to low. season Therefore, overflow1.3 the to % in overestimated land in the in the real lower lowersurface situations. basin, water Moreover, the can whereas average inflow it sloperetardation smoothly is is time to 3.1 and % the river in river dischargewithdrawal the channel. for gets Thus, upper irrigation stuck the canals, mountainous in which condition region the was lengthens where lower not modeled regions. in In this addition, study, water also has an e values of NSE and on the lowest hydrological simulation in the lower region. (Table 2). The simulation results captured the high peak of SSL during rainy season, 2010. Lastly, the eleven soil types intypes: the clay, sand, Chao Phraya and Basin silt. were reclassified into three 3.1.2 Results and discussion of modelThe performance monthly calibration forwith the SCE hydrological in and Chao sediment Phraya in process 2001 was (Table 1). implemented The parameters for sediment ( operation for the period(P73-Ping from 2001 River, to W3A-Wang 2010 River,stream at Y37-Yom gauge four River, stream in gauges and theavailability in N13A-Nan outlet of the (C2-Chao data, River) upper the Phraya region andthe River) monthly one (Fig. calibration river discharge 2). model and Taking2002–2010 at into SSL were all account data used five for the for 2001 streamcalibration validation. method were For gauges, was used the whereas used. for parameter the(Duan It calibration, et was observation a the al., semi-automatic data Shu 1992).dominant from It factors a was implemented in 2001 to identifyAs suitable for parameters. parameters The relateddataset to was sediment used transport to and considersediment soil spatial detachability erosion, distribution from of the rain soilcalibrated FAO drop global properties. respectively soil ( The based parameters on of the observed SSL at Khong Chiam. Soil cohesion the Nash–Sutcli and soil characteristics, were considered for parameter calibration, as listed in Table 1. satisfactorily, as shown by NSE and ).version2_1/SRTM3/ The model hasresolution was 90 m aggregated to spatialDigital 1 resolution. km Soil for In Map simulation. of the Soil theof study type World the (version classification United area, 3.6) Nations relied the (FAO/UN). from The onand the dominant the sandy Food soil and silt is clay Agriculture in and Organization theDevelopment sand Department lower in region. the of upper Land Thailand region ). use (http://www.ldd.go.th/ are The data paddy land (2001) field, use were categories farm obtainedDaily from land, precipitation the forest, data Land grassland, were collected bareThe first from land, is two urban open-source kinds rain area, ofManagement gauge and rain network Center gauge water data for provided network body. the by systems. (RID). upper the northern Hydrology The and region Water other ofdata the is sources Royal have managed Irrigation daily temporal by Department level, resolution. storage, the Daily inflow, dam and Thai operational outflow data (release) Meteorologicalgathered such for from as the Department RID Sirikit water and Dam (TMD). the and Both Electricity Bhumibol Dam Generating were Authority of Thailand (E-GAT). calibrated to be largerwithin than the the reasonable reported range2005). for values the (Morgan et Chao al., Phraya River 1998), Basin but (Bhattarai they and are Dutta, 5 5 20 15 25 20 15 25 10 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 − tyr in the 6 1 , where − 1 10 − , Bhattarai × tyr 1 6 − ) from 7.0 (for 10 k ) was larger than × 1 showing a ten-year ), and soil cohesion − f 1 K − tyr in the upper basins (P73, 6 3 75 % from those calibrated − − 10 erent locations of the control ff × , primarily reflecting the trapping ff 50, and cult to detach by raindrops (Sharma − ffi . This estimate was slightly lower than the 50, 1 + − 6769 6770 in the outlet, C2. Basically, we confirmed two tyr 3 for dominant clay soil (7.0 gJ ) by erent estimates of SSL. 6 − 1 ff k − 10 × larger than 0.5 except at the watershed outlet, C2 r , Morgan et al., 1998). Basically, the soil detachability is 1 ), soil detachability from sheet flow ( − k in the 1960s and early 1970s to around 6 1 to 8.10 − 6 tyr 10 6 cient accuracy for long-term simulation (Table 2). The results from × ffi 10 × ), as reported by FAO/AGL (2005). The di 1 − tyr 6 10 × It is inferred that the process of soil loss was strongly influenced by rainfall intensity. The simulated SSC also shows good correlations with observed data at all upper First, results were obtained by changing the detachability of soil ( ). The target period for this analysis was one year 2005 at P73. J of sediment by athe larger large Bhumibol number and ofthis Sirikit study, small dams the dams (Walling, observed 2009). SSL andchange, But at population irrigation C2 for growth, structures shows the no land 10 andand years decreasing clearance, other also targeted trend. land infrastructure in Nevertheless, by development use climate canSSL change, over be reservoir the expected longer-time construction, to scale causeBasin of (Walling, some 2011). 50 changes years in in the large river basins like the Mekong River over 10 years baseda on decreasing simulation trend in with the a 2000s. decline In in fact, annual the runo observed SSL shows lowest at the Wang(1181.3 mm). River due Walling (2009) tofrom reported that around area that 28 having the the1990s. annual lowest SSL In annual in this average Chao study, rainfall Phraya the declined average annual SSL was estimated to be 1.28 This was clearly shownis by the higher simulated (1341.8 SSL mm) at than the Nan at River other (Fig. tributaries. 3), where In rainfall contrast, the simulated SSL was points could be a reason for these di W3A, Y37, and N13A) and 0.09 kgm stream gauges, indicated by (Table 2). Possible errorsdischarge simulation. in The overall simulated average relativesimulated mean SSC and square observed error at SSC (RMSE) ranged from between the 0.07 to outlet 0.09 kgm couldpeaks be every related year to inW3A, river both Y37, observed and N13A andof (Fig. simulated 4). the SSC The rainy in firstincreased four season. peak from stream occurred Beginning the gauges: in in upper May, P73, occurred which stream, June, in is due the the the to main concentration beginning monsoon theheavy periods fell starting rainfall (August, that while rainy September, increases and season. river the October), The volume which discharge second of have river peak discharge. 3.1.3 Sensitivity of SSL to sediment-relatedThe parameters sensitivity ofthe modeled SSL input was parameters.detachability also The from rain investigated drop target for ( parameters the reasonable for ranges this of sensitivity analysis were soil C2, located at thethe lowest results reach, were indicate not underestimation,at as even the good though as lower the inSSL reach total other in was simulated stream the river overestimated gauges. lower At discharge error and C2, reach may supposedly than have resulted been inThe in larger the average total higher at upper annual the simulated SSL reach. lowerincrease was However, estimated from reach it to 5.72 because be appears 1.28 of thereported accumulating simulation estimate uncertainty. of(11 the average total annual SSL in the Chao Phraya River Basin ( clay and silt) and 9.1values. (for The sand) theoretical (gJ range of this soil detachability index is 0.01–10 gJ as shown in Fig.Basin 3. indicated Overall, su the performance of the model in the Chao Phraya River the minimum is foret clay, al., and 1998; the Morgan, maximum 2001).(Fig. is The 5a). peak for The of sand calibrated SSL (Gumiere increased parameter et as the al., detachability 2009; increased Morgan associated with soil texture,Thus, showing a soils higher having detachability a with high a lower clay clay content content. are di one for the dominantand sandy Dutta, soil 2005) in andagriculture the one in for Chao Europe a Phraya (2.0 wide gJ River range Basin of (3.5 gJ soil texture that are commonly used for 5 5 25 20 15 10 15 10 20 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 k × − ) contributes erent type of s , not including ff 2 < C (sand)) were shifted ) was relatively high, 1 k − s 50 % from the calibrated 2 − − , which is possibly because the k 25 and − than 25, f K + (west of Vietnam), respectively (Kite, 2001). 1 6772 6771 − (range from 2 to 7 kPa) (Nearing et al., 2005). J is related to erodibility and limits detachment within J 2min) resolution was used to delineate the Mekong needs to be adjusted considerably to properly predict × J , which indicates soil detachability from sheet flow (Eq. 5). under saturated SSC condition (i.e., TC f K J ) was shifted by increased, although the changes were small and invisible J f (0.6 (clay), 1.0 (silt), and 1.1 mgm K f K increased (Fig. 5c). In this case, the lateral inflow of sediment was the J (grid area: 2min . The Mekong is the largest trans-boundary river in Asia (Fig. 6). It 2 2 ) that describe soil erodibility showed the significant influence on SSL in the Chao 20 km J ∼ Second, we focused on In this basin, acrisols were found to be the dominant soil type. These are tropical Sensitivity analysis was conducted for three parameters to evaluate the reliability Third, soil cohesion ( The initial values of value (3.0 kPa). Each model outputdetachment was in confirmed streams influenced to by understand transport theincreased capacity. degree The as peak of of net SSL soil in September of the model forand simulating sediment dynamics. Overall,Phraya River the Basin. The two range input ofreference input parameters for parameters ( sediment-related used in research this in model the could Chao be Phraya a River useful Basin. same as the initialincreases results SSC with as 3.0 kPa. higher Equations (10) and (11) infer that soil erosion precipitation is the main agent for sediment yield. in Fig. 5b. Thus, SSL is less sensitive to by the factors 100, 10,The 0.1, results and show 0.01 thatincreased (Fig. the 5b) slightly simulated in suspended as each sediment type peaks of (August soil; to clay, silt October) and sand. to less deposition. Generally, the measured net soil loss,river since sediment. The simulated SSLmeasured from SSL and the with LISEM increasing model consistently increased with 3.6km resolution, and land cover and soil data were aggregated by reclassifying the 3.2 Mekong River Basin The second study area was the795 Mekong 000 River km Basin, covering an areaoriginates of approximately in Tibet4600 km. and The flows minimum down and maximum to annual Southern rainfalls Vietnam, in the a basin distance are 1000 of mmyr more than River Basin. TheLand land Cover cover 2000 and (http://edc2.usgs.gov/glcc/eadoc2_0.php)the soil and world type the (FAO, 2003), for FAO respectively. The soil the elevation map data basin of was were first converted obtained to from 3.6km Global The input data for the modelland include cover. weather In data, this topography study, data, theof soil GTOPO30 properties global and DEM data with a horizontal grid spacing (northeast of Thailand) and 4000 mmyr the Mekong River Basinof is 30.5 the %. areas The are agricultural2000). This land shrubland study coverage (17.2 examined %), is the model urban 40.7hydrologic outputs %. stations; (2.1 (river The %), discharge, 1-Chiang SSL, rest Sean, and and 2-Khong SSC) water Chiam, at bodies three and 3-Phnom (8.7 %) Penh3.2.1 (Fig. (MRC, 6). Model set-up and calibration soils that have aand leached. high Their clay characteristics accumulationused include in for low a arable fertility cultivation. horizon andare The ease and 27.1 % average of are sand, textures erosion extremely 30.4 of if % weathered they soils silt, are in and 42.5 the % Mekong clay River (Kyuma, Basin 1976). The forest coverage in indicating high soil detachmentpresented and resulting results in reveal high thesoils, SSL especially importance transport for of clay into soil raindrop the in river. detachment the The Chao for Phraya di River Basin. The wet season lastsreaches from around 80–90 May % to oflasts October. the until April. annual During In total. this the study, The thethe wet area dry delta of season, in the season modeled southern average starts basin Vietnam. rainfall is in 786 335 November km and et al., 1994). In the Chao Phraya River Basin, detachability ( 5 5 25 20 15 10 25 15 10 20 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) is wider , whereas 1 f − K is not important and k k , and k ) at monthly intervals from 1991 to r erent physical characteristic such as ff for the Mekong (7.0–100.0 kgJ ) (Table 1). This is probably because of the 1 k − cients ( 6773 6774 ffi between observation and simulation discharge r 62 µm in diameter (i.e., silt and clay), and sediments < ), 50 µm were adopted in this study area because all 50 0.51). The model simulation underestimated at the upper = 0.8). These indicators imply the GBHM satisfactorily describes r > narrowed slightly due to di J e, 1970) and correlation coe ff For the sediment model, we calibrated the same parameters as in the Chao Phraya The river discharge and SSL were simulated by considering an existing dam in Annual records of river discharge and SSC in the study were extracted from the Figure 7 shows the simulated results of monthly SSL compared with measurements was equally high ( River basin. In the Mekong River, wider ranges were determined for the seasonal cycle andRiver spatial Basin, distribution although of the simulated the discharge hydrology showed processes slightly in higher peaks. the Mekong the range of topography (e.g., rill, interill,of and soil gully) content). For and example, structure the of range of soil (e.g., erodibility and type than that for the Chao Phrayasoil (7.0–9.1 kgJ structure and content. Thesoil, Chao which Phraya can River be Basin easilycovered is by detached. mostly clay covered In soil. with contrast, This sandy the may Mekong be River why Basin SSL is is not dominantly sensitive to for sediment yield in the Mekong River Basin. at three gaugingagreement stations with observations, fromfor as 1991 the summarized upper, to middle, in and 2000.(1996–2000) Table lower periods, 3. stations The except in NSE in simulated the(Chiang the was calibration results case (1991–1995) larger Sean) of and are than the validation (NSE validation 0.6 in period good for the upper station 2000. 3.2.2 Results and discussion of modelThe performance river discharge of(refer to the Fig. Mekong B1 was forKhong hydrographs). well Chiam The simulated and NSE Phnom at values Penh for all1991 were river to larger three discharge 2000. than The at stations 0.7 average Chiang for correlation (Table Sean, calibration 3) and validation from observation at Phnom Penhand was observed used results for (river theand discharge lower Sutcli basin. and The SSL) fit was between evaluated using simulated the NSE (Nash calibration, whereas thesediment data particle from 1996 sizethe to (d sediment 2000 is were commonly used for validation. For the and then calibrated accordinggauges. All to the the parameters were observed calibratedChiang river by Sean SCE discharge was (Duan used and etwhereas al., for SSL observation 1992). calibration at at Observation of Khong at the parameters Chiam that was three used reflect for only calibration the for the upper middle basin, basin and the Chinese section ofdischarge, SSL, the and main SSC streamalong for (Manwan the 10 Dam). main years stream The fromriver were model 1991 discharge adopted to simulated and for 2000, river calibration sediment and and three data validation stream for (Fig. gauges the 6). period The daily from 1991 to 1995less were than used for 2 µm1996). make The up five 45 parameters % shown of in the Table section 1 near were Vientiane initialized (Ahlgren with and empirical values Hessel, completeness of riverdischarge, which discharge was measured and daily, SSC was sedimentcollected monitored near monthly. records SSC the samples were at surface(MRC, of the 2000). the station. river In (0.3 this Unlikemeasured m SSC study, depth) river and monthly in the observed the corresponding middle measured SSL daily of was river the computed discharge. main from stream the monthly historical record publishedhistorical record by tabulated the measurementsother of physical MRC characteristics river at gauging (Mekong discharge, stationsand SSC, located River along water those the quality, Mekong Commission, River of and Basin from 2005). the the river’s The three tributaries.SSL targeted In simulation. stations The this were stations study, were identified river selected and discharge based used and on their to SSC relative calibrate records locations and and the validate land use data for nine classesair and temperature the soil data data from for 65 eightRiver classes. station Commission Daily weather (MRC). precipitation stations and were obtained from the Mekong 5 5 10 20 25 15 10 15 20 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 1 − ect ff erent tyr ff erence 7 ff 10 ) between r × decreases by k 3 kPa. The same cient ( = ffi and 5.6 J 7 10 × , and 1 − s 2 − was larger than 0.5 between monthly 1 mgm ) (FAO/AGL, 2005). Lu and Siew (2006) r 1 = − f K tyr cient , 6776 6775 7 ) at the middle region, whereas the upper and 1 ffi 1 − at Phnom Penh. The results reveal a decreasing erent methods for SSL estimation could explain − 10 3 ff × − based on rating curve estimation. The di tyr 7 gJ ). This study shows a 57 % lower SSL than the 7 1 1 = − − 10 k tyr tyr × 7 7 10 10 × × 6.86 = ). The average SSC was lowest at Phnom Penh (estimated to be 3 − ). The low value at Phnom Penh was due to the sediment deposition in 3 − The simulated total annual SSL at Phnom Penh fluctuated over the period 1991– The monthly SSC simulation at the stream gauging stations for the period 1991–2000 The results revealed that SSL increases slightly in September when 2000 (average respectively. Walling (2009) reported thatbefore the increasing annual further SSL downstream. isin Our high the simulated middle in results region the also (i.e., middledue showed Khong higher region to Chiam SSL station) the thanthe in large Chinese other tributary regions. boundary, This including drainageincreases. is the area. probably middle The region) annual tends SSL to increase at as Lower basin Mekong area (after reported average annual SSL (16.0 average RMSE between the observed0.25 at and Khong simulated Chiam SSC and 0.14 was kgm 0.31 at Chiang Sean, locations of the control stations.Mekong In River FAO/AGL (2005), Basin SSL was area,area estimated to including for Phnom the the whole Penh. Mekong In delta. addition, This di study covered only the show that the SSC wasthe higher dry in the season rainy atmainly season all (July, caused August, by three and heavy September) stationsJuly precipitation than (Fig. in at 8). the the This rainy upperrecorded is season. the The station due highest high to showed concentrations simulated the a occurringmid-August, SSC intensive early the good in in soil observed agreement rainy erosion SSC with season,September. began in This the to mid-July. trend observed After decline, matches SSC, continuingAugust the and which to simulated September. decline SSC through trend, early which showed a3.2.3 decline in Sensitivity of SSL to sediment-relatedA parameters sensitivity analysismanner was as applied for incalibrated SSL the values, which in Chao were the Phraya Mekong River River Basin. Basin First, in all the the same parameters were set to 50 % from the initialsmaller changes (Fig. with 9a). further In decreases (at addition, a the factor of peak 25 %) SSL from kept the initial increasing value. and Thus, showed 0.13 kgm the lower region. This trendwhich was due promotes to sediment the deposition decreasein in and mean the decreases monthly main SSC. SSC stream In was waterwater observed fact, velocity, quality along a measurement the began decreasing entire in trend length 1985 of (Lu the and Siew, Mekong 2006). River The since model results also trend in the SSCThe along average the three monthlybe regions SSC from 0.33 was kgm upstream to the the highest downstream at (Fig. 8). Chiang Sean station (estimated to reported the averageBasin annual was SSL about forin 14.5 SSL the between period those reports 1962–2003 and in our the estimate here Mekong is River probably due to thethe di variances. is shown in Fig.observed 8. The and correlation simulated coe SSC at the three gauging stations (Table 3). The overall factor was used incompare the the Mekong River sensitivity Basin ofsimulated and at the Chao Khong parameters Chiam Phraya for station River for SSL Basin all in in the order the parameters to in both 1999. basins. The SSL was (Chiang Sean) station (Fig. 7), and this error may have been caused by the e lower regions showed average annual SSL values of 3.4 of the ManwanBasin Dam (Lu and on Siew, the 2006).simulated Nevertheless, main the and linear stream observed correlation in coe Generally, the SSL SSL the was was fairly upper well inresults simulated basin at the describe the of three the range stations the seasonal (Fig.SSL of 7). pattern Mekong The is 0.80–0.86 of simulated River expected for SSL duringmodel in all results the the reveal three rainy Mekong that, season, stations. for Rivervalues the as Basin, were entire the described and period highest higher also of (8.9 1991–2000, by the Walling average (2008). annual The SSL 5 5 15 10 15 20 10 20 25 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) k ) in f K erent group soils , as soil cohesion ff f K cient modeling of SS ffi and ) in both basins. . f f (with factors 0.1 and 0.01). k K K decreases as soil strength f , K in New Zealand (Russell and k k than in the Chao Phraya River 1 − k s implies that soil strength and clay 2 − ) is the most sensitive parameter in k J ). This result implys that soil detached 1 − s erent 2 ff − ) in the Chao Phraya and Mekong rivers, and 6778 6777 f K processes and sediment transport on hillslope (Fig. 9c). In contrast, the SSL peak decreases , ff k J ) shows that the SSL peak in September increases J values (0.4 to 0.6 mgm ected by both processes. f ff ) and detachability ( K , the peaks in SSL in 1999 decreased by 40 % with the multiplying f J K (Lukey et al., 2000; Bathurst et al., 2002). For example, sediment modeling using with a small range from 0.0019 to 0.0045 mgm Regarding Nevertheless, the outputs from this model at the basin scale may provide useful However, the present model assumed a single SS size instead of a wide range of SS The analysis of soil cohesion ( The three input parameters that describe soil erodibility have a significant influence f f size distribution might have limitedstudies. the Therefore, applicability the of model themulti-size performance sediment sediment may model be particles in further into theparameters improved and case the structure by may model. incorporating also have Uncertainties influencedthe the in simulation estimation results. terms For of example, ofthe net model equations. inputs, sediment Currently, detachment thisdetached (limitation equation (Eq. to (Eq. 9) deposition) 9)this and could limited assumed equation be by that should factors improved theerosion such be by as soil and improved soil revising particles deposition, cohesion. bybasins were Thus, especially considering is highly for the a river reasonable basins. balance Sediment between information management to in developers, river and decision implementing makers, and appropriate other basin-wide stakeholders sediment when management planning strategies, which sizes, due to limited information in both case studies. Thus, insu a factor of 100 % (Fig.simulated SSL 9b) also due drastically to decreased the with increase decreasing of soil detachability from sheet flow. The in the Chao Phraya. 4 Conclusion In this study, ascale physically-based was model developed of and sedimentenables coupled transport us with targeting to a a simulate large distributed rainfall–runo basin hydrological model. The model increases. Such resultsraindrop from contributes a little to sensitivity SSL analysis generation in suggest the that Mekong River soil Basin. detachment by the Mekong, whereas SSL change was more sensitive to detachability by raindrop ( content are not important, althoughof studies sands, of loams, soil strength and for clays the di (Sharma, 1999) show that and within a river network. Inbasins, its application the to sediment the Chao dynamics Phrayahillslope River (i.e., areas. and yield Mekong As River it and is erosion)dynamics a were grid-based by model, reasonably it a simulated can in soil identify fine locations cohesion grid of ( serious scale. sediment Moreover, the present model applications estimated Basin. The subtle response of SSL for di The simulated SSL revealsDutta the (2005), who opposite found trendswith the from increasing simulated that SSL reported peaks from by August Bhattarai to and October increased revealed the high sensitivity of SSL to soil detachability ( from sheet flow isBasin. important The input literature for doesK SSL not transport contain in the conclusiveK river results in on the the Mekong sensitivity River range of in the Mekong River Basin, SSL is less sensitive to by 70 and 80 %The by change in decreasing SSL the is soil more cohesion sensitive factor to by soil 0.75 cohesion and than 0.5, respectively. Sandy, 2006) showed therange, inverse where trend the simulated of SSL the increased result with increasing shown inby Mekong 150 % for the with same a factor of 1.25 for indicates soil detachabilityrivers within (Eqs. rivers. 9 In and 10),is the soil recognized equations cohesion to for limits the bea net detachment single related soil size of of to sediment. detachment soil Soil erodibility, in (Govers cohesion and et al., no 1990). uniqueon relationship the output exists of evenboth SSL. for river Generally, basins. soil The cohesion SSL ( change was sensitive to soil detachability over land ( 5 5 15 20 25 10 10 15 20 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent ff and sediment yield at ff cient global optimization for ffi ciency of emerging reservoirs long ffi ective and e ff 6779 6780 models, Water Resour. Res., 28, 1015–1031, 1992. ff eld, J., Vicente, C., White, S. M., and Romano, N.: Modelling large ciency of reservoirs, Trans. AGU, 34, 407–418, 1953. S. Zuliziana and K. Tanuma made substantial contributions to model This research for Chao Phraya River was supported by Asian Core ffi ffi eld, J., Bathurst, J. C., Hiley, R. A., and Mathys, N.: Test of the SHETRAN ffi constituents adsorbed to suspended306, matter Swedish of the University Mekong ofUppsala, River, Agricultural 1996. Lao Sciences, PDR, International Working Paper Rural Development Centre, and assessment, Part I: Model development, J. Am. Water Resour.basin As., 34, hydrology 73–89, 1998. andin: sediment Mediterranean yieldGeeson, Desertification: N. A., with a Brandt, C. sparse Mosaic J., and data: Thornes, of J. theprocess B., Processes Wiley, Chichester, based Agri 397–415, and 2002. models, basin,Institute Responses, in: of Technology, southern Thailand, Proceedings edited 6–7 June, of Italy, by: 215–226, the 2005. MTERM507–524, International 1989. Conference, Asian Earth Surf. Proc. Land., 15, 687–689, 1990. Draix, France, J. Hydrol., 235, 44–62, 2000. conceptual rainfall–runo Brownb, R. E., and Xud, A.for C.: forest Adapting applications, the J. Water Hydrol., Erosion 366, Prediction 46–54, Project 2009. (WEPP) model Rome, Italy, 2003. development in mainstream of the Lancang River, Chinese Sci. Bull.,transport 51, and 119–126, deposition 2006. processes, IAHS Publ., 189, 45–63, 1990. parameterization and sensitivity, Catena, 77, 274–284, 2009. and sediment transportProceedings of model; Pakistan Engineering its Congress, Lahore, calibration Pakistan, April, and 231–245,analysis 2004. validations, of in: sedimentdoi:10.5194/hess-15-1307-2011, 69th dynamics 2011. in Annual a Session river basin, Hydrol.Hydrol., Earth 253, Syst. 1–13, 2001. Sci., 15, 1307–1321, the Mekong, Geomorphology, 119, 181–197, 2010. Southeast Asian Studies, Kyoto University, Kyoto, 77 pp., 1976. in the Lower Mekong River:10, possible 181–195, impacts doi:10.5194/hess-10-181-2006, of 2006. the Chinese dams, Hydrol. Earthtechnology Syst. for Sci., modeling the impact of reforestation on badlands runo Platforms) and Collaborative Research Program (CRA) of AUN/SEED-Net. References Ahlgren, J. and Hessel, K.: The suspended matter ofArnold, Mekong. J. G., A Srinivasan, R., study Muttiah, R. of S., and Williams, the J.Bathurst, R.: chemical Large J. area hydrologic C., modeling She Bhattarai, R. and Dutta, D.: Analysis of soil erosion andBrandt, sediment C. J.: yield The using size distribution empirical of and throughfall dropsBrandt, under vegetation C. canopies, J.: Catena, Simulation 16, of size distribution and erosivity of raindrops and throughfall drops, Brune, G. M.: Trap e Duan, Q., Soorooshian, S., and Gupta, V.: E Program of Japan Society forwork the for Promotion Mekong River of was Science supported (JSPS). by Also, JSPS the Core-to-Core part Program (B. of Asia–Africa the Science modeling scenarios in large river basins. Author contributions. develop, simulations, data collectionC. Yoshimura and and O. analysis, C.edit and Saavedra the developed drafting manuscript. the research and framework editing and the modelsAcknowledgements. and manuscript. helped Duna, S., Wua, J. Q., Elliot, W. J., Robichaudb, P. R., Flanaganc, D. C., Frankenbergerc,FAO: The J. Digital R., Soil Map of the World and Derived Soil Properties,FAO/AGL: World version 3.6, River FAO/UNESCO, Sediment Yields Database,Fu, FAO, Rome, K. 2005. D., He, D. M.,Govers, and G.: Li, Empirical S. relationships J.: for Response theGumiere, of S. transport downstream J., capacity sediment Le to of Bissonnais, water overland Y., resource Habib-ur-Rehman, flow, and in M. Raclot, D.: erosion, and Soil Naeem resistance Akhtar, to M.: interill Development erosion: of model Kabir, regional M. scale A., Dutta, soil D., erosion and Hironaka, S.: Process-based distributedKite, G.: modeling Modelling approach the for Mekong: hydrological simulationKummu, M., for Lu, environmental X. X., impact and Varis, studies, O.: Basin-wide J. trapping e Kyuma, K.: Paddy Soils in theLu, Mekong X. X. Delta and of Siew, R. Vietnam, Y.: Water discussion discharge paper and sediment 85, flux CenterLukey, changes B. over for T., the She past decades can also be integratedused also with to water project resource the anthropogenic management. impacts The on model sediment dynamics could under also di be 5 5 10 15 20 25 10 15 20 25 30 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | s, ff depth and soil cohesion, ff to changes in precipitation and cover, Catena, ff 6782 6781 aut, C., Cerdan, O., Couturier, A., Hernandez, M., ff ects of reservoirs and land use changes, J. Hydrol., 422–423, ff e, J. V.: River flow forecasting through, Part I: A conceptual models ff Catena, 14, 149–155, 1987. Assessment and Monitoring,Bangkok, Thailand, Environment 1997. Assessment Technical Reports,season Volume using 8-A, a distributed hydrological580–586, model 2010. and a heuristic algorithm, J. Hydrol. Eng.,to 15, the Mekong River Commission, Vientiane, 61 pp., 2005. of an International2009. River Basin, edited by: Campbell, I.sediment C., management Elsevier, in Asian New rivers York, basin, 113–142, Walling ford, IAHSrivers, Publ., Global 349, Planet. 37–51, Change, 2011. 39, 111–126, 2003. Mekong River, River Res. Appl., 27, 33–46, 2011. conservation planning, in:Agriculture Research Agriculture Service, Washington, Handbook, D.C., 244–252, No. 1978. 282, No. 537, US Department of Validation of the SWATAm. model Water Resour. on As., 37, a 1169–1188, large 2001. river basintransport from with interill point areas, Soil and Sci. nonpoint Soc. Am. sources, J.,for J. 59, uplands, 727–734, Agroforest. 1995. Syst., 22, 145–152, 1993. October 2012), Thai Meteorological Department, Bangkok, Thailand, 2012. 61, 131–154, 2005. 202, Texas Water Resources Institute, College Stations, Texas, 02–06, 2002. of Fresh WaterOrganization, Geneva, Quality 301–315, 1996. Studies and MonitoringPrentice Hall, Programmes, 1989. chap. 13, Worldagricultural soils, Health J. Soil Sci., 39, 111–124, 1988. the revised USLE, J. Hydrol., 157, 287–306, 1994. in the Lo river17–29, (Vietnam): 2012. e under a large rainfall simulator, Hydrol. Process., 20, 2253–2270, 2006. discussion of principles, J. Hydrol., 10, 282–290, 1970. Bissonnais, L. Y., Nichols, M. H.,Modeling Nunes, J. response P.,Renschler, of C. S., soil Souchere, erosion V., and and Oost, runo K. V.: 91, 1–21, 1983. model, Catena, 44, 305–322, 2001. Chisci, G., Torri, D.,a and dynamic Styczen, M. approach E.:Earth for The Surf. predicting Proc. European Land., sediment Soil 23, transport Erosion 527–544, 1998. Model from (EU-ROSEM): fields and small catchments, UNEP Environment Asessment Programe for Asia andValeriano, the O. Pacific C. Bangkok: S., Land Koike, T., Cover Yang, K., and Yang,Walling, D.: D. E.: Optimal Evaluation and dam Analysis of operation Sediment Data during from flood theWalling, Lower D. Mekong E.: River, The Report changingWalling, sediment D. E.: load The of sediment the load of Mekong the river, Mekong Ambio, River, 37, in: The 150–157, Mekong 2008. Biophysical Environment Walling, D. E.: Human Impact on theWalling, sediment D. load E. of and Asian Fang, rivers, D.: sediment RecentWang, problems trends J. and in J., Lu, the X. suspended X., sediment and loads Kummu,Wischmeier, M.: of Sediment W. the load world’s H. estimates and and variations in Smith, the D. Lower D.: Predicting rainfall erosion soil losses: a guide to Santhi, C., Arnold, J. G., Williams, J. R., Dugas,Sharma, P.P.,Gupta, W. S. A., C., and Srinivasan, Foster, G. R., R.: Raindrop-induced and soilTacio, detachment Hauck, H. and D.: sediment L. Sloping Agricultural M.: Land Technology (SALT): a sustainable agroforestary scheme Thai Meteorological Department: Weather outlook forTorri, Thailand D., Sfalanga, during M., rainy and season Delsette, M.: (June– Splash detachment–runo Neitsch, S. L.: Soil and Water AssessmentOngley, Tool E.: User’s Sediment Manual measurement, in: Version Water 2000, Quality SWRL Monitoring, Report Design and Implementation Ponce, V. M.: Engineering Hydrology: PrinciplesRauws, and G. Practices, vol. and 640, Govers, Englewood G.: Cli Renard, K. Hydraulic G. and and Freimund, J. soil R.: Using mechanical monthlyRoberto, precipitation aspects R., data Thanh, to of H., estimate and the rill Maria, R-factor in C.: generation A RUSLE on approach to model suspended sedimentRussell, load A. and Sandy, E.: Physically based modeling of sediment generation and transport Nearing, M. A., Jetten, V., Ba MRC: Lower Mekong Hydrologic Yearbook,MRC: Mekong River Annual Comm., report Phnom 2005 Penh, [CD-ROM],Milliman, 2000. Mekong J. River D. and Comm., Meade, Vientiane, R. Mekong, H.: 2005. World-wide delivery ofMorgan, river R. sediment P. to C.: the A oceans, simple J. approach Geol., to soilMorgan, loss prediction: R. a revised P. Morgan–Morgan–Finney C., Quinton, J. N., Smith, R. E., Govers, G.,Nash, Poesen, J. W. E. A., and Auerswald, Sutcli K., 5 5 15 20 30 10 25 15 20 25 30 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Chao Phraya River Mekong River ) 0.16–0.17 0.16–0.19 1 6784 6783 − ) 0.6–1.1 1.0–10.0 1 − ) s 1 2 − − ) 7.0–9.1 7.0–100.0 (mgm 1 (kPa) 3.0–7.5 3.0–5.0 f − K J (gJ k Residual soil moisture, wrsd (mmh Sediment model Raindrop, Overland flow, Soil cohesion, Hydrological model Saturated hydraulic conductivity ofsurface soil, ksat1 (mmh 89.0–255.6 4.6–30.4 Model parameters calibrated for Chao Phraya River and Mekong River basins. the Asian monsoon tropic regionMeteorol. with Soc. a Jpn., perspective 79, of 373–385, coupling 2001. with atmospheric models,reference J. to land use and climate changes, Hydrol. Process.,hydropower 17, 2913–2928, dam 2003. construction, Aquat.2014. Sci., 77, 161–170,, doi:10.1007/s00027-014-0377-0 Table 1. Yang, D., Herath, S., Oki, T., and Musiake, K.: Application of distributed hydrologicalYang, model D., in Kanae, S., Oki, T., Koike,Zarfl, T., C., and Lumsdon, Musiake, A. K.: E., Global Berlekamp, potential J., soil Tydecks, erosion L., with and Tockner, K.: A global boom in 5 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | cient ciency ffi ffi e e ff r ciency coe ffi r e e ff 0.15 0.31 1.210.27 0.52 0.61 1.94 0.62 3.24 0.10 NSE − − − − − 1.02 0.78 0.25 0.73 4.33 0.31 NSE − − − r r stands for the Nash–Sutcli r (2001) (2002–2010) 0.14 0.29 1.960.11 0.15 0.57 0.05 0.68 NSE − − − − 0.07 0.66 0.07 0.43 NSE (1991–1995) (1996–2000) − − 6786 6785 stands for the Nash–Sutcli r cient, respectively. ffi Stations Performance indicators Stations Performance indicators cient, respectively. ffi Phnom Penh 3 Suspended sediment load (SSL) Chiang SeanKhong ChiamPhnom 1 Penh 2Suspended sediment concentration (SSC) 3Chiang Sean 0.62Khong 0.62 Chiam 0.85 1 0.64 0.86 2 0.80 0.51 0.62 0.65 0.08 0.83 0.64 0.58 0.87 Location CodeRiver discharge CalibrationChiang SeanKhong ChiamPhnom 1 Penh Validation 2 3 0.71 0.77 0.81 0.84 0.87 0.87 0.71 0.74 0.86 0.86 0.85 0.86 Suspended sediment concentration (SSC) Ping RiverWang RiverYom RiverNan RiverChao Phraya River P73 W3A C2 Y37 N13A 0.52 No observation 0.45 0.28 0.57 LocationRiver discharge CodePing RiverWang RiverYom River CalibrationNan RiverChao Phraya River P73 W3ASuspended Validation sediment C2 load (SSL) Y37Ping River 0.88 0.90 N13AWang River 0.68 0.94Yom 0.89 River 0.81 0.82Nan River 0.93 0.95Chao Phraya River 0.93 P73 0.89 W3A 0.91 0.95 C2 0.90 0.69 Y37 0.82 0.54 0.94 0.76 0.83 N13A 0.96 0.94 0.70 0.43 0.49 0.70 0.50 0.80 0.55 0.78 0.71 0.45 0.51 0.72 0.51 0.80 Model performance indicators for monthly river discharge, SSL, and SSC in Chao Model performance indicators for monthly river discharge, SSL, and SSC in Mekong cient and correlation coe ffi Table 3. River Basin from 1991 to 2000. NSE and and correlation coe coe Table 2. Phraya River Basin from 2001 to 2010. NSE and Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

river:

Raindrop erosion Raindrop Flow detachment detachment Flow Erosion & deposition & Erosion Sediment model Sediment (SSC)

  Sediment transport in in transport Sediment  Soil erosion from hillslope: from erosion Soil

6788 6787 process model Hillslope Hillslope River Routing River Hydrological Hydrological Suspended sediment load (SSL) (SSL) load sediment Suspended River discharge River

Suspended sediment concentration sediment Suspended

Climate (rainfall, air temperature), DEM, Land use, Soil type, Vegetation type, Soil use, Land DEM, temperature), air (rainfall, Climate Dam Dam model Dam operation Dam

Structure of the distributed sediment model integrated with a process-based

The target area of Chao Phraya River Basin.

Precipitation (mm) 0 400 800 1200 1600 0 400 800 1200 1600 0 400 800 1200 1600 400 800 1200 1600 0 0 400 800 1200 1600 Jan-10 Simulation Observation Jan-09 Jan-10 Jan-08 Jan-09 Precipitation Simulation Observation Jan-08 Jan-07

Jan-07 Year - Jan-06 Year - Month Jan-06 6790 6789

Jan-05 Month

Jan-05

Jan-04

Jan-04 Jan-03 Nan River Nan River Wang River Wang - - - Yom River Yom Jan-03 Wang River Wang Ping Ping River - Chao Phraya ChaoRiver Phraya Yom River Yom - - - - Ping Ping River Chao Phraya River Chao Phraya - - C2 Jan-02 N13A W3A Y37 N13A P73 Y37 C2 P73 W3A Jan-02 0 0 0 Jan-01 1 0 0

Jan-01 0.2 0.1 0.4 0.3 0.4 0.3 0.2 0.1 0.4 0.3 0.2 0.1 5 0 0.5 1.5 5 0 0.5 0.4 0.3 0.2 0.1 8 6 4 2 0 8 6 4 2 0 8 6 4 2 0

20 15 10 25 20 15 10

SSC (kg m (kg SSC ) ) ) month t (10 SSL

-3 1 - 5

Monthly suspended sediment load (SSL) at stream gauge stations in Chao Phraya Average monthly suspended sediment concentration (SSC) at stream gauge stations Figure 4. in Chao Phraya River Basin for 2001–2010. River Basin for 2001–2010. Figure 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . f K soil cohesion

(c)

Vietnam

China ) and ) Cambodia Dec f

Dec Dec

K Laos

Nov Nov Nov respectively

, Thailand Mekong Basin River Oct Oct Oct sand Myanmar

Sep Sep Sep for sand)

clay, silt, and 1 Aug - Aug Aug for boundary

1 - s

Jul 2 Jul Jul - mg mg m Jun Chinese Jun Jun Month in 2005 inMonth 6792 6791

for clay andfor 9.1clay silt, J g May May May

100 10 0.1 0.01 1.25 0.75 0.50 0.6, 1.0, and 1.1

1.5 0.5 0.25 ( detachability from sheet ﬂow ( Apr

    Apr Apr

(3.0 kPa)    f f f f f

   (7.0 J g

J J J J K K K K K k k k k (b) Mar Mar Mar Initial Initial Initial Initial Initial Observation Initial Initial Initial Initial Initial Initial Observed Initial J (kPa),Initial 3 J x 1.25Initial J x 0.75Initial J x 0.50 Observation Initial Initial Initial Initial Observation Initial Initial Observed Initial Kf (mg/m2/s)Initial Kf (0.6 x 100 forInitial clay, Kf 1.0x 10 forInitial silt, Kf 1.1 x 0.1forInitial sand) Kf x 0.01 Observed Initial k (g/J) Initial(7.0 for k xclay 1.5 Initialand silt, k x 9.10.5 Initialfor sand) k x 0.25 is invisible due to the minor response of SSL to ), k Feb Feb Feb (b) Jan Jan (a) Jan (c) (b)

8 6 4 2 0 0 8 6 4 2 8 6 4 2 0

10 10 10

) month t (10 SSL

1 - 5

Sensitivity of suspended sediment load (SSL) at P73 in Chao Phraya River Basin to The target area and the modelled river network of Mekong River Basin.

Precipitation (mm) Jan-00 0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000 Simulation Observation Jan-99 Jan-00 Jan-98 Precipitation Simulation Observation Jan-99 Jan-97 Jan-98

Year Jan-97 Jan-96 - Year - 6794 6793 Jan-96 Jan-95 Month Month Jan-95

Jan-94

Jan-94 Jan-93 Jan-93 Phnom Phnom Penh Khong ChiamKhong Chiang Sean - - - 3 2 1 Jan-92 Jan-92 Phnom Phnom Penh Chiang Sean Khong ChiamKhong - - - 3 1 2

Jan-91 4 2 0 8 6 4 2 0 8 6 2 0 6 4

Jan-91

10 10

) month t

SSL (10 SSL 1.0 0.5 0.0 2.0 1.5 2.0 1.5 1.0 0.5 0.0 1.0 0.5 0.0 2.0 1.5 1 - 7

) m kg ( C SS Monthly suspended sediment concentration (SSC) at stream gauge stations in

Monthly suspended sediment load (SSL) at stream gauge stations in the Mekong -3

Precipitation (mm) Dec Dec Dec 400 800 1200 0 0 400 800 1200 0 400 800 1200 800 1200 0 400 0 400 800 1200 Nov Nov Nov Oct Oct

Oct Jan-10 Sept Sept Sept for sand) Jan-09

1 - Precipitation Simulation Observation

Aug Aug Aug Jan-08 detachability from sheet ﬂow ( July July July

(b) Jan-07

) 1 - ), Year s - June June

June 2 - k Month in 1999 inMonth Jan-06

for clay andfor clay 100silt, J g

) 1 6796 6795 -

May May

is invisible due to the minor response of SSL to Month

May

Jan-05 100 10 0.1 0.01

1.00 mg m 1.50 0.50 0.25 ( 1.25 0.75 0.50

   

3.00 kPa   f f f f  f (a) (    (7.00 J g April April

April K K K K K J J J J k k k k Jan-04 Observed Initial(mg/m2/s), Kf 1.00 Initialx Kf100 Initialx Kf10 Initialx Kf0.1 Initialx Kf0.01 Initial Initial Initial Initial Initial Initial Initial Observation Observed Initial (kpa), J 3.00 Initial x 1.25 J Initial x 0.50 J Initial x 0.75 J Observed Initial k(g/J) (7 for clay and silt, 100 sand) for k x 1.5 k x 0.5 k x 0.25 Observation Initial Initial Initial Initial Initial Initial Initial Initial Initial Observation Initial Initial March March March Nan River Wang River Wang - - Yom River Yom Ping Ping River - Jan-03 - Chao Phraya River Chao Phraya - Feb Feb Feb Y37 N13A P73 W3A C2

Jan-02 Jan Jan Jan (b) (c) (a) 8 6 4 2 0 8 6 4 2 0 2 0 8 6 4 Jan-01

12 10 12 10 12 10 0 0 0

0 0 50 80 60 40 20 50 40 30 20 10

30 20 10 50 40 30 20 10

250 200 150 100

) s m (10 discharge River ) month t (10 SSL

1 - 3 3 -1 7

detachability from rain drop ( erence among lines in ﬀ (a) Monthly average river discharge at stream gauge stations from 2001 to 2010 at ). Di Sensitivity of suspended sediment load (SSL) at Khong Chiam station in Mekong J cohesion ( Figure A1. Chao Phraya River Basin. Figure 9. River Basin to Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Precipitation (mm) 0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000 Jan-00 Precipitation Simulation Observation Jan-99 Jan-98

Jan-97 Year - Jan-96 6797 Month Jan-95

Jan-94 Jan-93 Phnom Penh Phnom Khong ChiamKhong Chiang Sean - - - 3 2 1 Jan-92 Jan-91

0 4 3 2 1 0 0

30 20 10 20 10

) s m (10

discharge River

-1 3 5

Monthly average river discharge at stream gauge stations from 1991 to 2000 at Mekong River Basin. Figure B1.