GG/EE 275 Lab3
Water Balance of the 1993 Midwest Flood
In 1993, the Midwestern States of the United States suffered their worst flood in more than 40 years. Heavy rains sustained throughout the 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 type be mitigated through policy changes? Should levees that breached in the flood be rebuilt? Has the draining of wetlands in the Midwest created flood hazards downstream? If so, what effect would recreating some of these wetlands have on flood discharges?
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. In February 1994, the SAST team 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 activity. It was fairly clear at this meeting that hydrologic practice at the time could not address very well the huge regional scope of the flood: the affected region 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.
At this meeting, Dr Maidment proposed that a daily water balance study should be carried out to look in a spatially distributed way at the amount of precipitation, evaporation, change in storage, inflow and outflow of water in the region. This study was eventually funded by the US Geological Survey and jointly undertaken by the Center for Research in Water Resources of the University of Texas at Austin, and the Department of Geography of the University of Utah.
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 all the drainage area of the Upper Mississippi River and a portion of the Missouri River drainage, primarily capturing westward flowing water from Western Iowa. 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 points 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 estimated by taking National Weather Service Maps of potential evaporation, and finding 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 spreadsheet programs for hydrology
- To learn something about the 1993 Midwest
flood.
The data you will use in this lab is saved in an Excel file "lab3_startup.xls" which is in our class folder.
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 where rivers drain into the study region from the West:
- Osage River near St Thomas, 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 Louisville, Nebraska
- Kansas River at DeSoto, Kansas
Column 8: Average Precipitation over the Region [mm/d]
Column 9: Average Evaporation over the Region [mm/d]
Drainage area within the study boundary = 698525 km2
Conversion factors:
1 d = 86400 s
1 m = 1000 mm
1 km = 1000 m = 1,000,000 mm
I. Copy the file lab3_startup.xls from the class folder to your own folder.
II. The daily water balance is to be constructed for 365 days, from 1 January to 31 December 1993.
1. Convert the outflow series of the Mississippi River at Thebes, Ill, into a series of equivalent depths in mm/day and save them in another column.
A. Input "Out_Flow" in J1
B. Leave J2 blank, input "mm/d" in J3
C. Define the number format of column J as "0.00"
D. In J4, input formula "=B4*86400/1000/698525 (think why you can change the unit from m3/s to mm/d in this way)
E. Select J4 to J368, then choose Edit ˆ Fill ˆ Down
2. Aggregate the five inflow series into a single total inflow series and similarly convert the total to an equivalent depth of water in mm/day, and also save to another column.
A. Input "In_Flow" in K1
B. Leave K2 blank, input "mm/d" in K3
C. Define the number format of column K as "0.00"
D. In K4, input formula "=SUM(C4:G4)*86400/1000/698525
E. Select K4 to K368, then choose Edit ˆ Fill ˆ Down
3. Add two additional columns:
(1) The change in storage each day, which is computed as:
Change in Storage = Precip - Evap + In_Flow - Out_Flow
A. Input "Change in Storage" in L1
B. Leave L2 blank and input "mm/d" in L3
C. Define the number format of column L as "0.00"
D. In L4, input formula "=H4-I4+K4-J4"
E. Select L4 to L368, then choose Edit ˆ Fill ˆ Down
(2) The total accumulated storage in the region, computed as:
Accumulated storage = Previous day's Accumulated storage + Change in Storage
Here we assume that the storage was initially 30mm on 1 January 1993.
A. Input "Accumulated Storage" in M1
B. Leave M2 blank and input "mm" in M3
C. Define the number format of column M as "0.00"
D. In M4, input formula "=30+L4"
E. In M5, input formula "=M4+L5"
F. Select M5 to M368, then choose Edit ˆ Fill ˆ Down
4. Plot a graph showing the Out_Flow in mm/day and the Accumulated Storage in mm against the date.
You need to select columns A, J and M to create the chart. Use line #1 and use different line style for Out_Flow and Accumulated Storage.
Prepare the chart in the usual way in Excel and after you finish, choose and edit the Out_Flow line, then in the "format data series" window, click on "axis" and select "secondary axis" to get the Out_Flow and the Accumulated storage plotted with different Y axes.
RESULTS TO BE TURNED IN AS THE RESULTS SECTION OF YOUR LAB REPORT: (due by next lab)
1. Compute the annual total precipitation, evaporation, inflow, outflow and change in storage for the region. Is the annual water balance reasonably balanced? Discuss (Use your computation results to support your argument)
2. Hand in the plot showing the Out_Flow (mm/day) and the Accumulated Storage (mm) for the period Jan 1 to December 31, 1993. Also answer the following questions: What was the peak depth of storage? When did it occur? What was the peak discharge (mm/day) at the outflow point? When did it occur? What was the lag time between the peak storage and the peak discharge?
3. Plot a graph showing (Out_Flow Ð In_Flow) and (Precipitation Ð Evaporation) for July and August. For this two months period, what was the total precipitation, evaporation, inflow, outflow and change in storage in mm? What was the net outflow (outflow - inflow) in mm? What percentage of the total input water (precipitation + inflow) left the region as evaporation?