TABLE OF CONTENTS

PAGE

ABSTRACT
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1\. INTRODUCTION
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2\. THE PHYSICAL SYSTEM
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3\. PRECIPITATION DATA
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4\. RUNOFF DATA
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5\. LAKE EVAPORATION
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6\. NET BASIN SUPPLY
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6.1 Residual Method Errors
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6.2 Component Method Erros
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6.3 Ogoki Diversion Inclusion
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7\. CONNECTING CHANNEL FLOWS
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8\. DIVERSIONS
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9\. BEGINNING-OF-MONTH LAKE LEVELS
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10\. CHANGES IN STORAGE
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11\. DATA AVAILABILITY
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12\. REFERENCES
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TABLES

Table 1 \-- Comparison of residual with component monthly NBS over the
evaluation period

> (August 1982 - December 1988)
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Table 2 \-- Coordinated Great Lakes drainage areas (Coordinating
Committee, 1977) \...\...\...\...\...\...\...\...\...\...\...\... 10

GREAT LAKES MONTHLY HYDROLOGIC DATA[^1]

Thomas E. Croley II, Timothy S. Hunter, and S. Keith Martin

> ABSTRACT. Accurate hydrologic data (over-land precipitation, over-lake
> precipitation, runoff, lake evaporation, net basin supplies,
> connecting channel flows, diversion flows, beginning-of-month lake
> levels, and changes in storage) are required for simulation,
> forecasting, and water resource studies on the Laurentian Great Lakes
> and their basins. This report is an update of an earlier report
> presenting Great Lakes monthly hydrologic data (Quinn and Kelley,
> 1983). It has been expanded and revised to include all available data
> through 1999 and to reflect improved computational techniques. The
> data and a program for combining the data are available separately.

**1. INTRODUCTION**

Accurate hydrologic data (over-land precipitation, over-lake
precipitation, runoff, lake evaporation, net basin supplies, connecting
channel flows, diversion flows, beginning-of-month lake levels, and
changes in storage) are required for simulation, forecasting, and water
resource studies on the Laurentian Great Lakes and their basins. In
1983, the Great Lakes Environmental Research Laboratory (GLERL)
published a report (Quinn and Kelley, 1983) compiling monthly values of
these data through 1980 in a single source. This report serves as an
update to the 1983 report by providing additional data through 1999 and
revising some data to reflect improved computational techniques. We also
provide files of all of the tables in this report as well as some
derived quantities and FORTRAN software for computing additional derived
quantities. As in the previous report, numerous sources are involved so
the period of record for the data will vary.

As in the preceding report, all hydrometeorological data are uncorrected
for gage errors or systematic measurement biases (such as precipitation
gage undercatch). Data are averaged spatially and temporally without
regard to their relative quality, except that obvious errors of two
types are trapped: 1) all daily minimum and maximum air temperatures
that improperly match are regarded as missing data (i.e., minimum
exceeds maximum), and 2) all air temperature or precipitation data that
exceed range limits are regarded as missing. These range limits are, for
the U.S. side, -60°F to 115°F for air temperature and 0 to 10 in for
precipitation; for Canada, they are -53°C to 43°C for air temperature
and 0 to 25.4 cm for precipitation.

**2. THE PHYSICAL SYSTEM**

The Laurentian Great Lakes contain 23,000 km^3^ of water (about 20% of
the world\'s fresh surface water) and, with their surrounding basins,
cover 770,000 km^2^ in the United States and Canada. Their surface areas
comprise about one-third of the total basin area. The basin extends over
3,200 km from the western edge of Lake Superior to the Moses-Saunders
Power Dam on the St. Lawrence River. The water surface cascades over
this distance more than 182 meters to sea level. The most upstream,
largest, and deepest lake is Lake Superior. The lake has two interbasin
diversions of water into the system from the Hudson Bay basin: the Ogoki
and Long Lake diversions. Lake Superior water flows through the lock and
compensating works at Sault Ste. Marie, Michigan and down the St. Marys
River into Lake Huron where it is joined by water flowing from Lake
Michigan through the Straits of Mackinac.

Another interbasin diversion takes place from Lake Michigan at Chicago.
Here, water is diverted from the Great Lakes to the Mississippi River
basin. The water from Lake Huron flows through the St. Clair River, Lake
St. Clair, and Detroit River system into Lake Erie. From Lake Erie the
flow continues through the Niagara River and Welland Canal diversion
into Lake On­tario. The Welland Canal diversion is an intrabasin
diversion bypassing Niagara Falls and is used for navigation and
hydropower production. There is also a small diversion into the New York
State Barge Canal system which is ultimately discharged into Lake
Ontario. From Lake Ontario, the water flows through the St. Lawrence
River to the Gulf of St. Lawrence and the Atlantic Ocean.

Lake Superior and Lake Ontario are the only regulated lakes in the
system. Their regulation is conducted under the auspices of the
International Joint Commission (IJC) and its Boards of Control. The
criteria, or guidelines, for the regulation of these two lakes are set
forth in Orders and Supplementary Orders of Approval issued by the IJC.
Regulation plans which strive to satisfy these criteria have been
developed and are incorporated in hydrologic routing models maintained
by the U.S. Army Corps of Engineers and Environment Canada.

The hydrologic cycle of the Great Lakes basin and meteorology determine
water supplies to the lakes. Runoff comprises a significant part of the
Great Lakes water supply, particularly during the snowmelt season, late
March through early June. Because the lakes are so large, lake
precipitation and evaporation are of the same order of magnitude as
runoff. On a monthly scale, precipitation is fairly uniformly
distributed throughout the year. Lake evaporation typically has the
greatest effect on water supplies during the winter months as dry air
and warm water result in massive evaporation. Condensation on the cool
lake surface from the wet overlying air occurs in the summer. Net
groundwater flows to each of the Great Lakes are generally ignored. Net
supplies (runoff and precipitation less evaporation) typically reach a
maximum in late spring and a minimum in late fall.

**3. PRECIPITATION DATA**

We present monthly estimates for over-land and over-lake precipitation
for each of the Great Lakes in Appendices A and B, respectively. Due to
the quality of the available station data, estimates were derived in
three ways, corresponding to the available periods of record for
different segments of the historical time series. Data prior to 1930 (
1918 for Lake Superior) were computed by the Lake Survey District of the
U.S. Army Corps of Engineers and by the National Ocean Survey. Data for
1930-1947 (1918-1947 for Lake Superior) were computed at the Great Lakes
Environmental Research Laboratory (GLERL) from monthly station values.
Data for 1948-1999 were computed at GLERL using daily station data. The
methodology for each period is briefly described below.

The Lake Survey District of the U.S. Army Corps of Engineers, and later
the National Ocean Survey, computed monthly precipitation estimates for
data earlier than 1930 with an areally weighted \"district\" approach
(Quinn and Norton, 1982). Monthly over-land precipitation data begin in
1882 (Superior), 1883 (Michigan), 1883 (Huron), 1900 (St. Clair), 1882
(Erie), and 1883 (Ontario). Districts (large areas) were designated and
divided into sub-districts (smaller areas). Arithmetic means for each
sub-district were computed from all stations chosen for that
subdistrict. The sub-district values were areally weighted to compute
district precipitation. Over-land values were computed by areally
averaging district values. Over-lake values came from the use of
nearshore stations. Monthly over-lake precipitation data begins in 1900
for all lakes.

Quinn and Norton (1982) computed 1930-1947 monthly precipitation by
using a modified Thiessen weighting approach. They areally weighted
monthly station data on a 5-km grid to calculate each monthly value.
Each grid square, belonging to the basin or lake, was evaluated to find
its nearest station; the relative counts for each station within the
basin or lake were used as Thiessen weights to areally combine station
values for the basin or lake. All available stations within 25
kilometers of the basin were used, and the weights were recomputed for
each month as necessary.

We computed 1948-1999 monthly precipitation from all available daily
data from stations in the basin or within approximately 0 - 30 km of the
basin, depending upon station density near the edge of the basin. The
distance was chosen to assure that we obtain the same non-zero Thiessen
weights as if no stations were eliminated. Station data for the U. S.
were obtained from the National Climatic Data Center (2000), and station
data for Canada were obtained from the Meteorological Service of Canada
(1999). We then used Thiessen weighting (Croley and Hartmann, 1985)
similar to the above, but defined for a 1-km grid and recomputed every
day for all watersheds within the basin and the lake. The basin
watersheds were defined by the U. S. Geological Survey and the Water
Survey of Canada. We constructed daily over-land basin precipitation by
areally weighting the areally-averaged daily precipitation for each
watershed and summing over the days in each month. We also constructed
total monthly over-lake precipitation by summing the areally-averaged
daily values for the lake surface.

**4. RUNOFF DATA**

We computed watershed runoff estimates, in the spreadsheet entitled
runoff.xls, by using streamflow records from major rivers, available
from the U.S. Geological Survey (Showen, 1980) for U.S. streams and the
Inland Waters Directorate of Environment Canada for Canadian streams
(Inland Waters Direc­torate, 1980). Updated values of U.S. stream data
were obtained from the U.S. Geologic Survey web page. The Canadian
stream data was updated using the HYDAT software obtained from Greenland
International Consulting Inc. Complete years of historical daily runoff
data begin in 1908 (Superior), 1910 (Michigan), 1915 (Huron), 1935 (St.
Clair), 1914 (Erie), and 1916 (Ontario). Daily runoff values provided by
these agencies were summed for each watershed within a lake basin. The
runoff was extrapolated over ungaged areas; between 22% and 43% of the
Great Lakes basin remains ungaged (Lee, 1992) and runoff error potential
exists. Weights were assigned to each non-overlapping stream­flow gage by
dividing its drainage area by the watershed area. Daily watershed runoff
estimates were computed by summing all daily station values in the
watershed and then dividing by the sum of their weights, to extrapolate
for ungaged areas.

Daily lake basin runoff estimates were computed in a similar manner by
summing the watershed estimates, for which there were data available,
and then dividing by the ratio of the area of those watersheds to the
total basin area. Monthly basin runoff was computed by simply summing
the daily basin runoff estimates for all days in each month.

The earlier methodology for computing the runoff values (Quinn and
Kelley, 1983) diff­ers from our methodology in the handling of missing
data. Quinn and Kelley (1983) used data from upstream gages to fill in
missing data. We did not fill in missing data, but rather removed any
stations with missing data from the computations for those days that
data were missing. This effectively makes a station\'s drainage area
part of the ungaged area of the subbasin, and the normal area
extrapolation is used with no further adjustments required.

We wish to note that, as in the previous report, the Ogoki diversion
flow is indirectly included in our runoff estimate for Lake Superior. It
cannot be separated from measured runoff at Lake Superior since it is
added upstream, in Lake Nipigon, and its timing obscured by routing
through Nipigon and connecting channels to Lake Superior. The Ogoki
diversion is about 0.8% of Lake Superior runoff.

**5. LAKE EVAPORATION**

Monthly evaporation estimates (spreadsheet entitled evaporation.xls)
were derived from daily evaporation esti­mates generated by the Great
Lakes Evaporation Model (Croley, 1989a,b, 1992; Croley and Assel, 1994).
This is a lumped-parameter surface flux and heat-storage model. It uses
areal-average daily air temperature, windspeed, humidity, precipitation,
and cloudcover. These data are sufficiently available since 1948 (1953
for Georgian Bay), and 2 years are used for model ini­tialization.
Over-land data are adjusted for over-water or over-ice conditions.
Surface flux processes are represented for short-wave radiation and
reflection, net long-wave radiation ex­change, and advection. Atmospheric
stability effects on the bulk transfer coefficients are formu­lated and
used with the aerodynamic equation for sensible and latent heat surface
fluxes.

Energy conservation and superposition mixing are used to account for
heat storage in the lake. The effects of past heat additions or losses
are superimposed to determine temperatures at all depths. Each past
addition or loss is parameterized by its age and allowed to mix
throughout the volume accordingly. Mass and energy conservation are used
to account for ice formation and icepack decay in the lake.

These relations are solved simultaneously through iterative
determination and use of water surface temperature and ice cover. The
model was calibrated to give the smallest sum-of-squared-errors between
model and actual daily water surface temperatures observed by satellite
during the calibration period of generally 1979-88 and smallest
sum-of-squared errors between model and actual daily ice cover during
the calibration period of 1958-88. Statistics compiled over independent
verification periods agree well with the calibrations. The models were
also assessed partially by comparing model evaporation with water
balance derivations for 1951-88. Low annual residuals resulted.

Turnovers (convective mixing of deep lower-density waters with surface
waters as sur­face temperature passes through that at maximum density)
occur as a fundamental behavior of GLERL\'s thermodynamic and heat
storage model. Hysteresis between heat in storage and sur­face
temperature, observed during the heating and cooling cycles on the
lakes, is preserved. The model also correctly depicts lake-wide seasonal
heating and cooling cycles, vertical temperature distributions, and
other mixed-layer developments.

Comparisons between model results and actual data include 23 years of
daily aerial and satellite observations of water surface temperature on
all lakes, 8 years of bathythermograph observations of depth-temperature
profiles on Lake Superior, and 1 year of independently-de­rived weekly or
monthly surface flux estimates on Lakes Superior, Erie, and Ontario (2
esti­mates). The daily lake evaporation estimates were summed over all
days in each month to cal­culate monthly lake evaporation.

**6. NET BASIN SUPPLY**

The water supplies to a lake, referred to as the net basin supplies
(NBS), are defined in terms of their components as:

NBS = P + R - E (1)

where P is over-lake precipitation, R is basin runoff to the lake, and E
is lake evaporation. (Note: since Lake Superior runoff indirectly
includes the Ogoki Diversion, NBS values reported here include the Ogoki
diversion on Lake Superior.) Over-lake precipitation values were used to
compute the values in the spreadsheet entitled PrecipLake.xls, but GLERL
often uses the over-land precipitation as the best estimate of over-lake
precipitation (see section 6.2). NBS have also been computed indirectly
by both the U.S. Army Corps of Engineers and Environment Canada as a
residual in a water balance:

∆S = NBS + I - O + D (2)

where ∆S is change in lake storage computed from lake-level changes, I
and O are inter-basin inflow and outflow through a natural channel,
respectively, and D is inter-basin diversions into the lake. Equation
(2) ignores thermal expansion, consumptive use, and groundwater.

Lake evaporation is estimable from 1950 (1955 on Lake Huron), since wind
speed and humidity data exist only since 1948 (1953 on Georgian Bay) and
2 years are used for model initialization. Thus, NBS from equation (1)
from 1950 (1955 for Lake Huron) are reported in the spreadsheet nbs.xls.
Residual NBS, from equation (2), depend on connecting channel flows and
water level data, which exist preceding January 1900. Residual NBS can
be found in the spreadsheet resid_nbs.xls

Water balance errors can result in significant differences between NBS
estimates com­puted from equations (1) and (2). Table 1 summarizes a
comparison of the two monthly NBS series over an evaluation period of
August 1982 through December 1988. While correlation is good, there are
large differences between the component and residual methods for all
lakes, as reflected by the bias and root mean square error (RMSE)
statistics. On all lakes, the RMSE is about one third of the standard
deviation of component NBS. On Lakes Superior, Michigan, Huron, and
Erie, the bias is about one tenth of the mean component NBS, and on Lake
Ontario it is about 4%. Comparison plots of average seasonal
characteristics of NBS from both methods (Lee, 1992) reveal little
difference in the timing of the seasonal maximum and minimum, but reveal
significant differences in the monthly means during the late summer and
fall months. Annual comparison plots (Lee, 1992) show the residual NBS
are consistently lower than the component NBS throughout the evaluation
period used here for Lakes Superior, Michigan, Huron, and Ontario. This
agrees well with the bias in Table 1. The differences in Table 1 are
also due to inconsistent application of equations (1) and (2) on Lake
Superior; see section 6.3.

6.1 Residual Method Errors

The differences between residual and component NBS largely result from
residual errors in the water balance of equation (2). These include
errors in estimating change of storage, inter-basin inflows, outflows,
and diversions, and errors in ignoring thermal volumetric changes,
consumptive use, and groundwater. The change in storage on a lake is
computed from the dif­ference between the beginning-of-period and the
end-of-period water levels (Lee, 1992). Both are estimated with 2-day
averages of spatial means. Average estimates of instantaneous values and
remnant short-term fluctuations cause errors in estimates of storage
change and, ultimately, NBS. Croley (1987) evaluated errors in storage
change for Lakes Superior and Erie and found the largest error in fall
and winter months, when storm activity is most frequent.

Lake outflows are determined by direct measurement (Lakes Superior and
Ontario), stage-discharge relationships (Lakes Michigan, Huron, and St.
Clair), or a combination (Lake Erie) and are generally considered
accurate within 5%. However, small outflow errors can result in large
errors in residual NBS. Quinn and Guerra (1986) have shown that a 5%
error in the Detroit or Niagara River flows can result in a 34% error in
the residual NBS of Lake Erie. Errors in outflows estimated for Lakes
Michigan, Huron, and St. Clair may exceed 5% during ice jams, since ice
retardation must often be estimated. Other sources of large errors in
flow estimation include flow reversals on the Detroit River during
seiche activity on Lake Erie, past dredging activities in the St. Clair
River, and negotiated changes in flow rating equations.

Omissions in equation (2) further contribute to error in residual NBS
estimation. Ignor­ing thermal expansion or contraction of the lake water
body can result in a 100% error in month­ly residual NBS (Meredith, 1975)
on each lake. This difference is largest for June through November, when
the lakes heat storage increases and then declines. Quinn and Guerra
(1986) verified this in a Lake Erie water balance. Ignoring the total
consumptive use \[about 2,850 cfs in 1989, (Great Lakes Commission,
1991)\] will affect residual NBS (computed for the entire Great Lakes
basin) as much as omitting the Lake Michigan diversion; they are about
the same magni­tude. Groundwater contributions to the Great Lakes are
generally taken as small (Great Lakes Basin Commission, 1975) and
conveniently ignored since they are unknown. However, any groundwater
errors are directly reflected in the residual NBS estimation. While no
residual net basin supplies are presented here, they may be computed
from equation (2) from changes in storage (computed from lake levels),
interconnecting channel flow rates, and basin diversions; these data are
presented here in following sections. They may be used with available
software to compute residual net basin supplies in a variety of manners.

6.2 Component Method Errors

There are, of course, errors associated with the component method of
estimating NBS. These include errors in estimating precipitation,
runoff, and evaporation. Over-lake precipitation estimates can be
different, depending on which land-based meteorologic stations are used
and how lake/land precipitation ratios are determined from short-term
studies. For the Great Lakes, where lake effects on nearshore
meteorology are significant and the drainage basins have rela­tively low
relief, the use of all available meteorologic stations throughout the
basin may be less biased than the use of only nearshore stations. GLERL
often estimates over-lake precipitation from over-land precipitation
which is, in turn, measured throughout the basin and Thiessen weighted
(Croley and Lee, 1993). Direct measurement of over-lake precipitation
may be possi­ble in the future with the implementation of the Next
Generation Radar (NEXRAD) as part of the National Weather Service
modernization program.

Likewise, there are errors in evaporation modeling. While the model
agrees well with monitored water surface temperatures, ice cover,
temperatures measured at depth, and estimated thermodynamic surface
fluxes, there are no evaporation measurements to compare. However, even
though there are significant differences in monthly component and
residual estimates of NBS, they agree well over the long term; this
indicates the long-term evaporation estimates, at least, are reasonable.
Based on our review of the sources and magnitudes of errors in the
compu­tation of NBS, we believe that actual NBS are determined more
accurately now by estimating all components directly, instead of as a
water balance residual.

6.3 Ogoki Diversion Inclusion

Some diversion into Lake Superior is measured before it enters the basin
even though it passes through regulated Lake Nipigon before it enters
Lake Superior. This is because the timing and amount of diverted water
from Lake Nipigon cannot be separated from its natural flow. Likewise,
diversions from Lake Michigan, in equation (2), include actual
withdrawal from the lake and redirected lake drainage (International
Joint Commission, 1985), even though only lake withdrawal should be
used. Both cause errors in the NBS estimates.

Users of this data are cautioned to note that the Lake Superior runoff
to the lake, and hence Lake Superior net basin supply, include the Ogoki
diversion, since it cannot be separated from the measurement at the
lake. Users of this data should omit separate Ogoki diversion data in a
water balance with this data. Table 1 compares Lake Superior component
net basin supplies, with the Ogoki diversion included, to residual net
basin supplies, with the Ogoki diversion removed (albeit crudely by
subtracting values reported at the basin divide rather than values at
the lake edge). By adding the Ogoki diversion back to the residual net
basin supplies, the removal is reversed. The comparison between
component and residual net basin supplies, both with the Ogoki diversion
included, is more meaningful. Then, the *Mean Residual NBS* in Table 1
for Lake Superior becomes 71 mm; the *Standard Deviation Residual NBS*
becomes 64 mm; the *RMSE* drops by 2 mm; the *Correlation* is unchanged;
and the *Bias* drops by 3 mm.

**7. CONNECTING CHANNEL FLOWS**

The connecting channels of the Great Lakes system consist of the St.
Marys, St. Clair, Detroit, Niagara, and St. Lawrence Rivers. Historical
flows in these channels are presented in the spreadsheet channels.xls.
The basic reference for the St. Marys, Detroit, and St. Clair River
flows was the Coordinating Committee on Great Lakes Basic Hydraulic and
Hydrologic Data (1988). The reference for the Niagara River flows was
the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic
Data (1976). The flow data for the St. Lawrence River were obtained from
the Levels and Flows Work Group (1969). Updated values for the St.
Marys, Niagara, and St. Lawrence Rivers were obtained from the Detroit
District of the Army Corps of Engineers. **[Note:]{.underline}** Flows
for the Detroit and St. Clair Rivers for the time period 1991-1999 are
provisional and were generated by the Great Lakes Environmental Research
Laboratory.

**8. DIVERSIONS**

Data for the Long Lac-Ogoki, Chicago, and Welland diversions were
obtained from the Levels and Flows Work Group (1969) and updated by the
various responsible U. S. and Cana­dian agencies and international boards
and committees. They are presented in the spreadsheet diversions.xls.
Note that the Chicago diversion includes water that would have gone into
Lake Michigan as streamflow runoff, but is now discharged into the
Mississippi River basin. The diverted area of the basin has already been
removed from consideration in our runoff estimates for Lake Michi­gan.
Using this diversion data in a water balance with the runoff data
presented here would double count (negatively) Chicago runoff. That is,
runoff to Lake Michigan does not include Chicago runoff to the lake, but
Chicago runoff is included as a diversion from the lake. The user of
this data will need to account for this if performing water balance
calculations. The diversion values do not accurately reflect the amount
of water removed from the lake, but are significantly higher. Through
discussions with the U.S. Army Corps of Engineers in Chicago (Kiel,
personal communication, 1993), we determined that it is not possible for
us to separate the components for most of the period of record. The
numbers are, therefore, presented as supplied to us.

Additionally, in late 1992, the U. S. Geological Survey performed a
verification of the rating curve used to compute flows through the
Chicago diversion. The USGS found that the flows were actually much
higher than indicated by the equations which were used during the period
October, 1985 - December, 1990. Accordingly, data for this period is
presented from the revised values supplied by the U.S. Army Corps of
Engineers, derived from the USGS acoustic current meter at Romeoville,
Illinois.

**9. BEGINNING-OF-MONTH LAKE LEVELS**

We computed the beginning-of-month lake levels, in the spreadsheet
Levels_BOM.xls, by using the same meth­odology used to compute the values
published in the 1983 report (Quinn and Kelley, 1983). See Quinn
(1975a,b), Quinn and Derecki (1976a,b), Kelley (1976), and Quinn et al.
(1979). A minimal set of Thiessen polygons were constructed and used
without further modification. Missing station data were filled in by
various techniques by using nearby gages so that Thiessen weights never
required recomputation due to missing data. Networks changed only when
new stations became available or old stations were discontinued.

The values in this publication are reported in IGLD 1985 datum. The 1983
report (Quinn and Kelley, 1983) used the IGLD 1955 datum. We do not have
access to the station data prior to 1973 in a computer-readable form, so
we did not recompute the values prior to 1973. We simply computed
lake-wide conversion factors (from IGLD 1955 to IGLD 1985) based on the
networks employed and the conversion factors for the individual gages in
each of the networks. The conversion factors were then applied to the
IGLD 1955 values. The IGLD 1955 to IGLD 1985 conversion was unavailable
for Pte. Petre, Ontario, so it was estimated by the average of those at
Cobourg, Ontario and Kingston, Ontario. We recomputed the values from
1973 to 1990 by using the actual IGLD 1985 gage data. Our network was
the largest network used by Quinn et al. (1979). In those instances
where a station from that network had been discontinued, we recom­puted
the network weights.

The beginning-of-month level for a gage is defined as the level at
12:00:00 a.m. (mid­night) on the first day of that month. Since this
value is generally unknown for practical reasons, it is standard
practice to compute the value by averaging the daily mean level for the
last day of the previous month with the daily mean level for the first
day of the current month, when both are available. When one or both of
the daily mean values are unavailable, several alternative methods for
filling in the missing data are available. We used the following
methods, listed in order of preference, to estimate the
beginning-of-month lake level (BOM) from daily lake level i days after
the beginning of the month, Li:

BOM = 0.50 L~+0~ + 0.50 L~-1~

= 0.67 L~+0~ + 0.33 L~-2~

= 0.33 L~+1~ + 0.67 L~-1~

= 0.75 L~+0~ + 0.25 L~-3~

= 0.25 L~+2~ + 0.75 L~-1~

= 0.50 L~+1~ + 0.50 L~-2~

= 0.60 L~+1~ + 0.40 L~-3~

= 0.40 L~+2~ + 0.60 L~-2~

BOM = 0.50 L~+2~ + 0.50 L~-3~ (3)

= L~+0~

= L~-1~

= L~+1~

= L~-2~

= L~+2~

= L~-~

These alternatives were sufficient to compute the beginning-of-month
level at each gage for each month.

**10. CHANGES IN STORAGE**

Monthly changes in storage for each lake, presented in the spreadsheet
ChangeInStorage.xls, were computed by multiplying the difference between
two consecutive beginning-of-month levels (as determined by Thiessen
weighting) by the area of the lake in Table 2. Changes in storage were
then convert­ed from volumes into average rates by dividing by the number
of seconds in the month. Leap years were taken into account.

**11. DATA AVAILABILITY**

All spreadsheets referred to in this report can be found at
[ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-083/new/](ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-083/)
. Inquiries should refer to this report and be addressed to:

Publications

NOAA, Great Lakes Environmental Research Laboratory

2205 Commonwealth Blvd.

Ann Arbor, Michigan 48105-1593

U.S.A.

**12. REFERENCES**

Meteorogogical Service of Canada. Climatological Station Data Catalogue,
Ontario. Environ­ment Canada, Downsview, Ontario (1999).

Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic
Data. Lake Erie Outflow 1860-1964, With Addendum 1965-1975.
International Joint Commission, Chicago, Illinois and Cornwall, Ontario
(1976).

Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic
Data. Coordinated Great Lakes Physical Data. International Joint
Commission, Chicago, Illinois and Cornwall, Ontario (1977).

Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic
Data. Lakes Michi­gan-Huron Outflows, St. Clair and Detroit Rivers,
1900-1986. International Joint Commission, Chicago, Illinois and
Cornwall, Ontario (1988).

CROLEY, T.E., II. Minimizing Long-Term Wind Set-up Errors in Estimated
Mean Erie and Superior Lake Levels. NOAA ERL Technical Memorandum ERL
GLERL-64, 40 pp. (1987).

CROLEY, T.E., II. Verifiable Evaporation Modeling on the Laurentian
Great Lakes. *Water Re­sources Research*, **25**(5):781-792 (1989a).

CROLEY, T.E., II. Lumped Modeling of Laurentian Great Lakes Evaporation,
Heat Storage, and Energy Fluxes for Forecasting and Simulation. *NOAA
Technical Memorandum ERL GLERL-70*, 48 pp. (1989b).

CROLEY, T.E., II. Long-Term Heat Storage in the Great Lakes. *Water
Resources Research*, **28**(1):69-81 (1992).

CROLEY, T.E., II, and R.A. ASSEL. A One-Dimensional Ice Thermodynamics
Model for the Laurentian Great Lakes. *Water Resources Research*,
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CROLEY, T.E., II, and H.C. HARTMANN. Resolving Thiessen Polygons.
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CROLEY, T.E., II, and D.H. LEE. Evaluation of Great Lakes Net Basin
Supply Forecasts. *Water Resources Bulletin*, **29**(2):267-282 (1993).

Great Lakes Basin Commission. Great Lakes Basin Framework Study -
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Michigan (1975).

Great Lakes Commission. 1989 Annual Report of the Great Lakes Regional
Water Use Data Base Repository, Appendix A (Draft) (1991).

HYDAT Software, version 99-2.00, Greenland International Consulting
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Inland Waters Directorate. Supplying Hydrometric and Sediment Data to
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Ottawa, Ontario (1980).

International Joint Commission. Great Lakes Diversions and Consumptive
Uses Report. Inter­national Joint Commission, Windsor, Ontario (1985).

KELLEY, R.N. Lake St. Clair beginning-of-month water levels and monthly
rates of change of storage. NOAA Technical Report ERL 372-GLERL 13, 15
pp. (1976).

LEE, D.H. Computation of net basin supplies: a comparison of two
methods. Final Report Subtask 19.1.2a (Scenarios Based Upon 1900-1989
Supplies), Task 19.1.2, Task Group 2, Working Committee 3, Phase II -
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Environmental Research Laboratory, Ann Arbor, Michigan (1992).

Levels and Flows Work Group. General Great Lakes Information.
International Joint Commis­sion, Chicago, Illinois (1969).

Meredith, D.M. Temperature effects on Great Lakes water balance studies.
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National Climatic Data Center. Surface Land Daily Cooperative Summary of
the Day TD-3200. National Climatic Data Center, Asheville, North
Carolina (2000).

QUINN, F.H. Lake Huron beginning-of-month water levels and monthly rates
of change of storage. NOAA Technical Report ERL 348-GLERL 4, 30 pp.
(1975a).

QUINN, F.H. Lake Michigan beginning-of-month water levels and monthly
rates of change of storage. NOAA Technical Report ERL 326-GLERL 2, 35
pp. (1975b).

QUINN, F.H., and J.A. DERECKI. Lake Erie beginning-of-month water levels
and monthly rates of change of storage. NOAA Technical Report ERL
364-GLERL 9, 37 pp. (1976a).

QUINN, F.H., and J.A. DERECKI. Lake Ontario beginning-of-month water
levels and monthly rates of change of storage. NOAA Technical Report ERL
365-GLERL 10, 30 pp. (1976b).

QUINN, F.H., J.A. DERECKI, and R.N. KELLEY. Great Lakes
beginning-of-month water levels and monthly rates of change of storage.
*Journal of Great Lakes Research* **5**:11-17 (1979).

QUINN, F. H., and B. GUERRA. Current perspectives on the Lake Erie water
balance. *Journal of Great Lakes Research* **12**(2):109-116 (1986).

QUINN, F.H. and R.N. KELLEY. Great Lakes Monthly Hydrologic Data. NOAA
Data Report ERL GLERL-26, 79 pp. (1983).

QUINN, F.H. and D.C. NORTON. Great Lakes Precipitation by Months,
1900-80. NOAA Data Report ERL GLERL-20, 29 pp. (1982).

Showen, C.R.. Data formats for U.S. Geological Survey computer files
containing daily values for water parameters. U.S. Geological Survey,
Reston, Virginia (1980).

USGS. Surface-Water Data for the Nation. Web Page
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[^1]: ^1^ GLERL Contribution No. 902