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Global rivers that will be observed by SWOT and their characteristics
K. Andreadis1, E. Clark1, M. Durand2, S. Biancamaria3, E. Rodriguez4, D. Lettenmaier1, D.
Alsdorf2
SWOT Hydrology Workshop9/15/2008, Columbus, OH
1University of Washington 2Ohio State University3LEGOS 4NASA JPL
Motivation
● Obvious question on what rivers will SWOT be able to “see”
– What are their characteristics?– How many times will they be observed per
orbit cycle?
● What are the expected errors in discharge estimation?
– How far upstream the drainage network can we estimate discharge with reasonable accuracy?
● Availability of global measurements of river discharge, width, depth etc?
General approach● Power law relationships for river width and
depth (functions of discharge)● Regional regression of mean annual
discharge with drainage area
Moody and Troutman, 2002
● In-situ discharge measurements and global river dataset are required
● Estimate potential discharge errors from simple stage-discharge relationship
Datasets● HYDRO1K
– Topographically derived dataset based on GTOPO30
– Aspect, fow direction/accumulation, stream lines, drainage basins
● GRDC (Global Runof Data Center)– 3,035 stations worldwide– Mean monthly discharges
● HYDROSHEDS– Based on SRTM at 3-arcsec resolution– Hydrologically conditioned dataset
Caveats
● Major limitations with existing datasets and power law relationships
● Peak versus mean annual fow● Not looking at temporal variability here● However, we need a consistent approach
as a “frst-cut” for evaluating the potential for SWOT observations
● We need a realistic dataset of river characteristics
Algorithm
● Q: discharge● A: drainage
area● W: width● D: depth
● σz: height error
● σQ: discharge error
SWOT orbital coverage
● 22-day repeat cycle
● 78-degree inclination
● Australia from HYDROSHEDS, rest from HYDRO1K
Global river discharge
Mean annual river discharge (m3/s)
Global river width
Derived river width (m)
Global river depth
Derived river depth (m)
Sanity check for derived characteristics
● Maximum river widths and mean annual discharge
● North America: Q=13600 m3/s, W=845 m● South America: Q=145000 m3/s, W=2740
m● Africa: Q=32000 m3/s, W=1290 m● Asia: Q=17400 m3/s, W=950 m● Europe: Q=19900 m3/s, W=1020 m● Oceania: Q=12450 m3/s, W=800 m
What kind of rivers would SWOT see?
1280 km2 2430 km2 5700 km2 21000 km2 5880700 km2
Drainage Area
1
2
3-4
5-9
>10
SWOT Observations
per 22-day orbit cycle
Per
cent
age
of r
iver
s be
long
ing
in b
in
26 m 35 m 53 m 88 m 2740 m
River Width
What kind of rivers would SWOT see?
1
Strahler Order
1
2
3-4
5-9
>10
SWOT Observations
per 22-day orbit cycle
Per
cent
age
of r
iver
s be
long
ing
in b
in
2 3 4 5 6
0.00001 0.0004 0.0011 0.0026 0.485
Gradient (Δh/ΔL)
Calculating the expected error in river discharge
● We're using a simple stage-discharge relationship
● Need to take into account: i) measurement error and ii) model error (discharge)
=2 b z
z 2
– Assuming model and measurement errors are independent
Q=azb Q=QQ=∂Q∂ z
zQ
Discharge fractional error
Amazon watershed (Branco River)● Small domain in the
Amazon basin
● Example validation of methods
● River width in-situ measurements
Amazon watershed maps
● We can evaluate what order streams SWOT will observe (discharge) depending on required accuracy
Dis
char
ge (
m3/s
)
Riv
er w
idth
(m
)
Riv
er d
epth
(m
)M
onth
ly σ
Q/Q
σQ/Q
Global river discharge errors
Discharge fractional error σQ/Q
Discharge errors with river characteristics
Discharge (m3/s) Width (m)
Strahler OrderDepth (m)
σQ/Q
σQ/Q
N. America
S. America
Africa
Asia
Europe
Oceania
Australia
Discharge errors with river characteristics
Discharge (m3/s) Width (m)
Depth (m)
σQ/Q
σQ/Q
N. America
S. America
Africa
Asia
Europe
Oceania
Australia
Global river discharge monthly errors● Based on mean Q relationship ft from
gauge measurements
Future work
● Swapping HYDRO1K with more accurate HYDROSHEDS dataset
● Improve discharge estimates by using additional in-situ datasets
● Perform analysis regionally on basins where in-situ width, depth etc measurements are available
● Incorporate other models of stage-discharge and other techniques of estimating discharge
Questions?