GWD-LR ver1.2

August 2014

1The Global Width Database for Large Rivers2(GWD-LR) version 1.2

345Dai Yamazaki

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JAMSTEC – Japan Agency for Marine Earth Science and Technology
d-yamazaki@jamstec.go.jp

1. Introduction

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The Global Width Database for Large Rivers (GWD-LR) is a satellite-based global-scale8

database of river width [Yamazaki et al., 2014a]. GWD-LR is calculated from satellite water

9mask and flow direction map using fully-automated width calculation algorithm. The

10GWD-LR is freely distributed for research purposes. Please visit the GWD-LR webpage11( http://hydro.iis.u-tokyo.ac.jp/~yamadai/GWD-LR/index.html ). In order to acquire the12password to download the GWD-LR, please e-mail to the developer (Dai Yamazaki).

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Note that both the input dataset and the calculation algorithm has been updated from theprevious versions. A brief description for river width calculation is available online

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2728292. Product Description
The GWD-LR is prepared as a raster data. The data format is “plain binary” (i.e. Fortrandirect access, GrADS binary, ArcGIS BIL/BSQ/FLT format).

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Effective river width (channel width excluding islands) is represented in the GWD-LR. A

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land area <500 km2 surrounded by water mask is treated as an island. River width is32

calculated for each river centerline pixel, and the upstream-downstream relationships of33

centerline pixels are given by flow direction maps. Therefore, river width data is available

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sequentially along river networks.

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The effective width data, flow directions, drainage area (and additional data such aselevation) are prepared at three different resolutions.

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2.1 Original 3-sec resolution database

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River width is calculated at 3-sec resolution (about 90 m at the equator), and the40

GWD-LR is available at the original 3-sec resolution. The data is divided into 5 degree by 541

degree tile (6000 pixels by 6000 pixels). Filename indicates the west and south corner of the42domain, e.g. n30w140_wth.flt is the width file for the domain of N30-N35, W140-W135.

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Effective river width, flow direction map, drainage data is available in the original 3-sec

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resolution database.

45[Effective River Width] ${LAT}${LON}_wth.flt

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Effective river width (in [m], >0) is calculated for each centerline pixel. Land pixel,

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non-centerline water pixel, and island pixel are represented by 0, -1, -2, respectively. Ocean

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pixels are filled with undefined value (-9999). Data is prepared in 4 byte real format (in little49

endian).

50[Flow Directions] ${LAT}${LON}_dir.bil

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Flow direction of each pixel is represented by (1:N, 2:NE, 3:E, 4:SE, 5:S, 6:SW, 7:W,

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8:NW). River mouth is represented by 0, while inland river terminating point is represented

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by -1. Ocean pixels are filled by undefined value (-9). Flow direction is also a raster data,54

thus all land pixels have a downstream direction. Note that the flow directions in GWD-LR is55

modified from the original HydroSHEDS and GDBD flow directions, in order to represent the56

centerline of each river segment. Flow direction map is prepared in 1 byte integer format.

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57[Drainage Area] ${LAT}${LON}_upa.flt

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Drainage area of each pixel is saved in [m2]. Ocean pixels are filled by undefined value

59(-9999). Drainage area is saved in 4 byte real format in little endian.

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Figure 1: (a) Effective river width. Non-centerline water bodies are represented by gray,while islands are represented by black. (b) Drainage Area of modified flow direction map.

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2.2 Regional 0.005 degree resolution database64

The original 3-sec resolution database was upscaled to 0.005 degree resolution by the65

FLOW algorithm [Yamazaki et al., 2009] in order to reduce the file size for easy data

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handling. The global domain is divided into 14 areas tagged by 3 characters.

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Table 1: List of areas in 0.005 degree resolution database

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Area Tag
ID
West
East
North
South
nx
ny
cell size

sa1
1-85-30
15-60
11000
15000
0.005

ca1
2-120-60
40
5
12000
7000
0.005

na1
3-130-50
60
25
16000
7000
0.005

af1
4
5
60
35-35
11000
14000
0.005

eu1
5-20
20
60
08000
12000
0.005

eu2
6
5
70
60
20
13000
8000
0.005

as1
755
100
60
59000
110000.005

Area Tag
IDWestEast
North
Southnxny
cell size

as2
890
150
60
20
12000
8000
0.005

as3990
155
35-15
13000
10000
0.005

oc1
10110
180-10-50
14000
8000
0.005

na2
11-170-55
75
50
23000
5000
0.005

eu3
12070
80
45
14000
7000
0.005

si1
1355
115
80
45
12000
7000
0.005

si2
14100
195
75
50
19000
5000
0.005

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ID, domain boundary, number of cells, cell size are listed.

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Figure 2: Areas for 0.005 degree resolution database.

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72[Effective River Width] ${AREA}.rivwth.flt

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Effective river width (in [m], >0) is calculated for each centerline cell. Land cell,74

non-centerline water cell, and island cell are represented by 0, -1, -2, respectively. Ocean75

cells are filled with undefined value (-9999). Data is prepared in 4 byte real format (in little76

endian).

77[Downstream XY] ${AREA}.nextxy.bsq

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Downstream cell (jx,jy) of each cell (ix,iy) is saved. Record=1 represents downstream jx,

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while record=2 represents downstream jy. River mouth is represented by -9. Ocean cells are80

filled with undefined value (-9999). Data is prepared in 2 byte integer format.

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The “downstream XY format” is used because upscaling of river network is82

mathematically impossible when “D8 flow direction format” is used. For detail, see

83[Yamazaki et al., 2009].

84[Drainage Area] ${AREA}.uparea.flt

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Drainage area of each cell is saved in [km2]. Ocean cells are filled by undefined value

86(-9999). Drainage area is saved in 4 byte real format in little endian.

87[Adjusted Elevation] ${AREA}.elevtn.flt88

Elevation of each cell is saved. Ocean cells are filled by undefined value (-9999).

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Elevation is saved in 4 byte real format in little endian.
Elevation is based on SRTM3 DEM (GTOPO30 DEM above 60N), and is adjusted(smoothed) along river networks to remove depressions [Yamazaki et al., 2012].

92[Longitude / Latitude of outlet pixel] ${AREA}.lonlat.flt

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Longitude and latitude of the outlet of each pixel are saved. Record=1 is for longitude,

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record=2 is for latitude. Longitude and latitude is saved in 4 byte real format in little endian.

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Each 0.005 degree cell has 36 3-sec pixels. The outlet pixels of each cell represent the96

position of river channel within the 0.005 degree cell.

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2.3 Global database (1/40, 1/10, 1/4 degree resolution)

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The 0.005 degree resolution database is again upscaled by the FLOW algorithm to101

construct global coverage database. Global database is prepared at various resolutions.

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This global database can be used in the global hydrodynamic model CaMa-Flood103[Yamazaki et al., 2011, 2013, 2014b].

104[Effective River Width] width.flt

105Effective river width (in [m], >0) is calculated for each centerline grid. Land grid,106

non-centerline water grid, and island grid are represented by 0, -1, -2, respectively. Ocean

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grids are filled with undefined value (-9999). Data is prepared in 4 byte real format (in little108

endian).

109[Downstream XY] nextxy.bsq

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Downstream grid (jx,jy) of each grid (ix,iy) is saved. Record=1 represents downstream jx,

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while record=2 represents downstream jy. River mouth is represented by -9. Ocean grids

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are filled with undefined value (-9999). Data is prepared in 4 byte integer format.

113[Associated Maps]

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Global database contain files listed in the below table.

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Table 2: List of files in global domain database

FileDescription

nextxy.bsq

Downstream X(rec=1)
Downstream Y(rec=2)GWD-LR Effective Width
Base Elevation
Upstream Drainage Area
Channel Length
Bain ID
Basin Color Patern for Visualization
Outlet Longitude(rec=1)
Outlet Latitude(rec=2)width.flt
elevtn.flt
uparea.flt
rivlen.flt
basin.bsq
bsncol.bsq
lonlat.flt
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Unit

Formatinteger

[m]
[m]
[m2]
[m]
deg
deg

real
real
real
real
integer
integerreal
real

Note: data format is 4 byte real or 4 byte integer, both in little endian.

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3. Input Data & Method

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3.1 Flow Direction Map120

HydroSHEDS 3 arc-sec flow direction map [Lehner and Grill, 2013] is used as baseline

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data between 60N and 60S. GDBD 1km flow direction [Masutomi et al., 2009] is used for122

region above 60N. GDBD 1km flow direction map was downscaled to 3 arc-sec resolution,

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and fused to HydroSHEDS flow direction map [Downscaling and fusing method: not124

published yet]125

3.2 Water Mask

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SRTM Water Body Data (SWBD) [NASA/NGA, 2003] is used as baseline data between

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60N and 60S. GLCF’s MODIS Water Mask [Carrol et al., 2009] is used for region above 60N.

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MODIS Water Mask is downscaled to 3 arc-sec resolution for calculating river width.

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3.3 Elevation Data

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For adding elevation to river network map, SRTM3 DEM (CSI version 4.1) [Reuter et al.,131

2007; Farr et al., 2007] is used between 60N and 60S. The area above 60N is covered by

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GTOPO30 [USGS]. Elevations are adjusted to remove depressions along river networks

133[Yamazaki et al., 2012]. Then, integer-value elevations are converted to real-value in order134

to smooth stepwise elevation increase due to integer representation [Smoothing method not135

published yet].

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3.4 River Width Calculation

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Brief description on the width calculation algorithm is explained in Yamazaki et al., 2014a.

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Detailed description document and sample source code are available online.

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3.5 Upscaling Method

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Flow direction and river width at 3 arc-sec resolution are upscaled to 0.005 degree

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database by the FLOW algorithm [Yamazai et al., 2009]. Then, the 0.005 degree database142

is again upscaled by the FLOW algorithm to create global database.

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4. Update in version 1.2

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4.1 Database above 60N

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GWD-LR is extended to regions above 60N, using GDBD flow direction map and MODIS

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water mask. Because low-resolution baseline data is used, the accuracy of river width is149

lower in regions above 60N, compared to the accuracy between 60N and 60S.

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4.2 Channel/Floodplain Separation

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The algorithm is updated in version 1.2, and is now able to separate channels and152

floodplains more strictly then the previous version. The accuracy of river width calculation is153

improved.

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Figure 3: River centerlines calculated by (a) ver 1.2 algorithm and (b) ver 1.1 algorithm.

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4.3 Seamless 3-sec resolution data

Channels and floodplains was not perfectly separated in version 1.1.

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In the previous version (ver. 1.1), the original 3-sec database is divided into 10 areas.

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River width calculation algorithm is applied to global domain in version 1.2, thus 3-sec

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resolution database (prepared as 5deg x 5deg tiles) has no area boundary.

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4.4 Parallelized Fortran90 code

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The width calculation algorithm was parallelized by OpenMP. Now, global calculation at163

3-sec resolution can be done within one day. Previously, it took 3-4 days to finish global

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calculation.

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5. Future Tasks

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5.1 Lake Mask

167In GWD-LR, width is calculated for all water bodies, without separating river channels,

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floodplains/wetlands, and lakes. This may not be ideal when GWD-LR is applied to169

hydrodynamic model simulations or any other researches.

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In order to distinguish river channels and lakes, a lake mask is currently under171

construction.

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5.2 High-resolution water mask

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The accuracy of river width is low in the area above 60N because of low-resolution

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MODIS water mask is used as baseline data. A new high-resolution water mask is under175

development using LANDSAT GLS database.

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References

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Carroll, M. L., J. R. Townshend, C. M. Di Miceli, P. Noojipady, and R. A. Sohlberg (2009), A
new global raster water mask at 250 m resolution, Int. J. Digital Earth, 2(4), 291–308,
doi:10.1080/17538940902951401.

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Farr, T. G., et al. (2007), The Shuttle Radar Topography Mission, Rev. Geophys., 45,RG2004, doi:10.1029/2005RG000183.

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Lehner, B, and G. Grill (2013), Global river hydrography and network routing: baseline dataand new approaches to study the world's large river systems, Hydrol. Proc., 27,2171-2186, doi:10.1002/hyp.9740.

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Masutomi, Y., Y. Inui, K. Takahashi, and U. Matsuoka (2009), Development of highly
accurate global polygonal drainage basin data, Hydrol. Processes, 23, 572 – 584,
doi:10.1002/hyp.7186.

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NASA/NGA (2003), SRTM Water Body Data Product Specific Guidance, Version 2.0,
available online: http://dds.cr.usgs.gov/srtm/version2_1/SWBD/SWBD_Documentation/

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Reuter H.I, A. Nelson, A. Jarvis (2007), An evaluation of void filling interpolation methods for
SRTM data, International Journal of Geographic Information Science, 21:9, 983-1008

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Yamazaki, D., T. Oki, and S. Kanae (2009), Deriving a global river network map and its
sub-grid topographic characteristics from a fine resolution flow direction map, Hydrol.
Earth Syst. Sci., 13, 2241 – 2251, doi:10.5194/hess-13-2241-2009.

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Yamazaki, D., S. Kanae, H. Kim, and T. Oki (2011), A physically-based description offloodplain inundation dynamics in a global river routing model, Water Resour. Res., 47,
W04501, doi:10.1029/2010WR009726.

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Yamazaki, D., C. Baugh, P. D. Bates, S. Kanae, D. E. Alsdorf, and T. Oki (2012), Adjustmentof a spaceborne DEM for use in floodplain hydrodynamic modeling, J. Hydrol., 436-437,
81 – 91, doi:10.1016/j.jhydrol.2012.02.045.

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Yamazaki, D., G. A. M. de Almeida, and P. D. Bates (2013), Improving computational
efficiency in global river models by implementing the local inertial flow equation and avector-based river network map, Water Resour. Res., 49, 7221 – 7235,doi:10.1002/wrcr.20552.

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Yamazaki, D., F. O’Loughlin, M. A. Trigg, Z. F. Miller, T. M. Pavelsky, and P. D. Bates (2014a),
Development of the global width database for large rivers, Water Resour. Res., 50,
doi:10.1002/2013WR014664.

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Yamazaki, D., T. Sato, S. Kanae, Y. Hirabayashi, and P. D. Bates (2014b), Regional flood
dynamics in a bifurcating mega delta simulated in a global river model, Geophys. Res.
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