<s>
GSSHA	B-Application
(	O
Gridded	O
Surface/Subsurface	O
Hydrologic	O
Analysis	O
)	O
is	O
a	O
two-dimensional	O
,	O
physically	O
based	O
watershed	O
model	O
developed	O
by	O
the	O
Engineer	O
Research	O
and	O
Development	O
Center	O
of	O
the	O
United	O
States	O
Army	O
Corps	O
of	O
Engineers	O
.	O
</s>
<s>
It	O
simulates	O
surface	O
water	O
and	O
groundwater	O
hydrology	O
,	O
erosion	B-Algorithm
and	O
sediment	O
transport	O
.	O
</s>
<s>
The	O
GSSHA	B-Application
model	O
is	O
used	O
for	O
hydraulic	O
engineering	O
and	O
research	O
,	O
and	O
is	O
on	O
the	O
Federal	O
Emergency	O
Management	O
Agency	O
(	O
FEMA	O
)	O
list	O
of	O
hydrologic	O
models	O
accepted	O
for	O
use	O
in	O
the	O
national	O
flood	O
insurance	O
program	O
for	O
flood	O
hydrograph	O
estimation	O
.	O
</s>
<s>
Input	O
is	O
best	O
prepared	O
by	O
the	O
Watershed	B-Application
Modeling	I-Application
System	I-Application
interface	O
,	O
which	O
effectively	O
links	O
the	O
model	O
with	O
geographic	B-Application
information	I-Application
systems	I-Application
(	O
GIS	B-Application
)	O
.	O
</s>
<s>
GSSHA	B-Application
uses	O
a	O
square-grid	O
,	O
constant	O
grid-size	O
representation	O
of	O
watershed	O
topography	O
and	O
characteristics	O
,	O
similar	O
to	O
a	O
digital	B-Application
elevation	I-Application
model	I-Application
representation	O
.	O
</s>
<s>
The	O
GSSHA	B-Application
model	O
was	O
derived	O
from	O
the	O
CASC2D	O
hydrologic	O
model	O
.	O
</s>
<s>
GSSHA	B-Application
represents	O
a	O
significant	O
improvement	O
on	O
CASC2D	O
in	O
terms	O
of	O
capabilities	O
,	O
options	O
,	O
and	O
numerical	O
procedures	O
.	O
</s>
<s>
GSSHA	B-Application
includes	O
dynamic	O
time-stepping	O
depending	O
on	O
stability	O
criteria	O
,	O
different	O
time	O
steps	O
for	O
different	O
numerical	O
processes	O
,	O
and	O
the	O
ability	O
to	O
run	O
on	O
multi-processor	O
computers	O
.	O
</s>
<s>
Processes	O
included	O
in	O
GSSHA	B-Application
include	O
surface	O
and	O
ground	O
water	O
flow	O
,	O
channel	O
hydraulics	O
,	O
evapotranspiration	O
,	O
erosion	B-Algorithm
and	O
sedimentation	O
,	O
storm	O
drainage	O
networks	O
,	O
tile	O
drains	O
,	O
a	O
variety	O
of	O
hydraulic	O
structures	O
,	O
and	O
contaminant/nutrient	O
fate	O
and	O
transport	O
.	O
</s>
<s>
GSSHA	B-Application
uses	O
a	O
regular	O
square	O
grid	O
computational	O
discretization	O
of	O
the	O
watershed	O
.	O
</s>
<s>
Elevation	O
data	O
are	O
taken	O
from	O
a	O
digital	B-Application
elevation	I-Application
model	I-Application
.	O
</s>
<s>
GSSHA	B-Application
uses	O
a	O
vector	O
channel	O
representation	O
.	O
</s>
<s>
The	O
GSSHA	B-Application
model	O
was	O
developed	O
from	O
the	O
outset	O
to	O
be	O
capable	O
of	O
'	O
long	O
term	O
 '	O
simulations	O
consisting	O
of	O
multiple	O
events	O
.	O
</s>
<s>
Overland	O
and	O
channel	O
flow	O
hydraulics	O
are	O
based	O
on	O
explicit	O
,	O
finite-volume	B-Algorithm
,	O
diffusive	O
wave	O
schemes	O
.	O
</s>
<s>
A	O
two-dimensional	O
finite-difference	B-Algorithm
groundwater	O
solver	O
is	O
coupled	O
to	O
streams	O
through	O
a	O
stream	O
bed	O
conductance	O
layer	O
.	O
</s>
<s>
There	O
are	O
a	O
number	O
of	O
optional	O
methods	O
to	O
calculate	O
erosion	B-Algorithm
and	O
sediment	O
transport	O
.	O
</s>
<s>
Current	O
additions	O
to	O
the	O
GSSHA	B-Application
model	O
include	O
source/sink/transport	O
of	O
nutrients	O
and	O
contaminants	O
.	O
</s>
<s>
GSSHA	B-Application
is	O
programmed	O
in	O
C++	O
,	O
and	O
runs	O
on	O
Windows	O
or	O
Linux	B-Application
computers	O
.	O
</s>
<s>
Parallel	B-Operating_System
computing	I-Operating_System
is	O
enabled	O
at	O
present	O
using	O
the	O
MPI	O
or	O
the	O
OpenMP	O
approach	O
.	O
</s>
<s>
Work	O
is	O
underway	O
to	O
port	O
the	O
code	O
to	O
run	O
on	O
massively	B-Operating_System
parallel	I-Operating_System
distributed	B-Operating_System
memory	I-Operating_System
architecture	O
machines	O
.	O
</s>
