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Water Balance of the 1993 Midwest Flood
CE 394K.2 Surface Water Hydr=
ology
University of Texas at Austin
Prepared by David R. Maidment=
and
Pawel Mizgalewicz
Table of Contents
In 1993, the Midwestern States of the United States su=
ffered
their worst flood in more than 40 years. Heavy rains sustained throughout t=
he
summer flooded Iowa and portions of surrounding States. President Clinton
appointed a team called the Scientific Assessment and Strategy Team (SAST) =
to
examine the flood events, collect and analyze data, and report on the policy
implications of the flood. How could the effects of future floods of this t=
ype
be mitigated through policy changes? Should levees that breached in the flo=
od
be rebuilt? Has the draining of wetlands in the Midwest created flood hazar=
ds
downstream? If so, what effect would recreating some of these wetlands have=
on
flood discharges? Here =
are
some articles about the 1993 Midwest flood:
http://mo.water.usgs.gov/Reports/1993-Flood/
http://www.=
nwrfc.noaa.gov/floods/papers/oh_2/great.htm
http://en.wikipedia.org/wiki/Great_Flood_of_1993
The SAST team met and worked for several months at the US Geological
Survey's EROS Data Center where they compiled a considerable volume of data,
some of it displayed at http://egsc.=
usgs.gov/isb/pubs/factsheets/fs10399.html
In February, 1994, the SAST t=
eam
convened a meeting of hydrologists and hydraulic engineers from around the
United States to review the team's work and to suggest new lines of activit=
y.
It was fairly clear at this meeting that hydrologic practice at the time co=
uld
not address very well the huge regional scope of the flood: the affected re=
gion
has an area of approximately 700,000 km2, and most of the studies presented=
at
the meeting dealt with areas of the order of 10 to 100 km2.
This exercise is based on some of the data generated in that study. The =
data
presented here are for the SAST region taken as a single unit, comprising a=
ll
the drainage area of the Upper Mississippi River and a portion of the Misso=
uri
River drainage, primarily capturing westward flowing water from Western Iow=
a.
There are five inflow points to the study region where flow coming into the
Missouri River from Eastward flowing tributaries enters the region. There is
one outflow point, the Mississippi River at Thebes, Illinois, just above the
confluence of the Mississippi and Ohio Rivers. All inflows and outflow poin=
ts
are monitored by USGS gages reporting daily discharge.
Daily precipitation over the region is estimated by interpolation from
records of more than 1000 gages. Daily evaporation over the region is estim=
ated
by taking National Weather Service Maps of potential evaporation, and findi=
ng
the factor (0.775) which when used to multiply the potential evaporation
ensures that the annual water balance from January 1 to December 31, 1993, =
is
approximately closed.
- To provide experience in
analyzing the water balance of a large region
- To practice using spreads=
heet
programs for hydrology
- To learn something about =
the
1993 Midwest flood.
This exercise is intended to be performed on a spreadsheet program on a
personal computer.
Download the water balance data.
Excel file: http://www.ce.utexas.edu/prof/maidment/GradHydro2010/Hmwk2/Midwest.xls=
The data will display on the screen.
The data consist of:
Column 1: Date expressed as PYYMMDD, where YY is the year (93) MM=
is
the month, and DD is the day of the month
Column 2: Outflow [m3/s] from the region (discharge measured in the
Mississippi R. at Thebes, IL.)
Column 3-7: Inflow [m3/s] into the region measured at five sites whe=
re
rivers drain into the study region from the West:
- Osage River near St Thoma=
s,
Missouri
- Gasconade River near Rich
Fountain, Missouri
- Missouri River at Yankton,
South Dakota (the discharge from the upper Missouri basin which enters=
the
study region)
- Platte River at Louisvill=
e,
Nebraska
- Kansas River at DeSoto,
Kansas
Column 8: Average Precipitation over the Region[mm/d]
Column 9: Average Evapotranspiration over the Region[mm/d]
Drainage area within the study boundary =3D 698525 km2
Conversion factors:
1 d =3D 86400 s
1 m =3D 1000 mm
1 km =3D 1000 m =3D 1,000,000 mm
1 cfs =3D 0.028317 m3/s
1 in =3D 2.54 cm
1 m =3D 3.2808 ft
The daily water balance is to be constructed for 365 d=
ays, 1
January to 31 December 1993.
- Convert the outflow serie=
s of
the Mississippi River at Thebes, Ill, into a series of equivalent dept=
hs
in mm/day by converting the daily discharge into an equivalent volume =
in
m3/day and then dividing by the drainage area.
- Aggregate the five inflow
series into a single total inflow series and similarly convert the tot=
al
to an equivalent depth of water in mm/day by dividing by the drainage
area.
- Construct a spreadsheet w=
ith
date in the first column and the succeeding four columns being
precipitation, evaporation, inflow and outflow, all in mm/day. To make=
a
date field in Excel, type 1/1/93 in the first column, use a formula to=
add
1 to get the date in the next row in this column, then copy the result=
to
succeeding rows.
- Add two additional column=
s,
the change in storage each day computed as:
Change in Storage =3D Precip - Evap + Inflow -
Outflow
and the total accumulated storage in the regi=
on, assuming
that the storage was initially 30mm on 1 January 1993.
- Determine the annual total
precipitation, evaporation, inflow, outflow and change in storage for =
the
region. Is the annual water balance reasonably closed?
- Plot a graph showing the
outflow in mm/day and the storage in mm against the date. Prepare the
graph in the usual way in Excel and then click on the outflow line once
the graph is plotted, bring up a dialog box, click on "axis"=
and
select "secondary" to get both the outflow and the storage
plotted with different y axes.
- It appears that the bulk =
of
the flood occurred over a two month period from 1 July to 31 August 19=
93.
Prepare a new spreadsheet for this period. Determine the series
(precipitation - evaporation) and (outflow - inflow), all in mm/day and
plot them.
RESULTS TO BE TURNED IN=
:
- The total annual precipit=
ation,
evaporation, inflow, and outflow in mm for 1993. What is the annual net
outflow (outflow - inflow) in mm? What percentage of the annual
precipitation within the region evaporated and what percentage drained=
out
of the region in the Mississippi River?
- A graph showing the stora=
ge
(mm) and the discharge (mm/day) for the period Jan 1 to December 31, 1=
993.
What was the peak depth of storage on the land surface? When did it oc=
cur?
What was the peak discharge (mm/day and m3/s) at the outflow point? Wh=
en
did it occur? What was the lag time between the peak storage and the p=
eak
discharge?
- Present a spreadsheet sho=
wing
the daily water balance analysis for July and August 1993. This should
include date, precipitation, evaporation, inflow, outflow, change in
storage and storage, in mm. Plot a graph showing (precipitation -
evaporation) and (outflow - inflow). What effect does flow through the
region have on smoothing the time variations in precipitation?
- For this two month period,
what was the total precipitation, evaporation, inflow, outflow and cha=
nge
in storage in mm? What was the net outflow (outflow - inflow) in mm? W=
hat
percentage of the precipitation left the region as evaporation? What
percentage of the precipitation flowed out of the region as net outflo=
w?
- Read some articles about =
the
1993 Midwest flood and write a 1-page summary describing the character=
of
the flood and its impact on communities in the Midwest.
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