City of Belle Isle

"On beautiful Lake Conway"

Lake Conway Present Information

High resolution chart in PDF format

Lake Conway Water Surface Level Chart

We are glad you are interested enough in your lake to take a look at this chart. It will be updated monthly just before the Orange County Lake Conway Navigation Advisory Board meeting which is the second Tuesday of each month. Currently these meetings are being held at the Edgewood City Hall at 6:30 PM.

The primary purpose of this chart is to show you the relationship between the current elevation of the lake and the normal level. To accomplish this we have included markers indicating the average elevation of the lake for each month and the daily elevation for the last year. Based on the last 40 years of data we also can provide a 2 month projection of where the lake level may go. It also contains other information that may be of interest such as the amount of rainfall and water surface temperature. The following description of the data should make it much easier to understand.

Each data description is preceded by a symbol which represents how the data will look on the chart.

2005 ELEVATION - This is the daily lake elevation in feet according to Orange County Datum. Its scale is on the left side of the chart. It has been found that normally without rain the lake level will fall 1/4" per day in the summer and 1/8" per day in the winter. It is presumed that this is simply due to evaporation. 1/8" is approximately 0.01 foot.

2004 ELEVATION - This shows the daily lake level for last year to help show the expected variation throughout the year. Its scale is also on the left side of the chart.

AVERAGE MONTHLY ELEVATION - This is the average of the lake levels for each particular month over the last 53 years. Its scale in feet is on the left side of the chart.

PROJECTED AVERAGE - This is the lake level we would expect if all conditions in the next month or two match the averages recorded over the last 40 years for these months. Its scale in feet is on the left side of the chart. This average was obtained by averaging all the level rises and falls for each month for the last 40 years.

PROJECTED HIGH - This is the lake level we would expect if all conditions in the next month or two match the average increase recorded over the last 40 years for these months. Its scale in feet is on the left side of the chart. This average was obtained by averaging all the level rises for each month for the last 40 years. We have occasionally seen this exceeded in the past 4 years (since projections have been made).

PROJECTED LOW - This is the lake level we would expect if all conditions in the next month or two match the average decrease recorded over the last 40 years for these months. Its scale in feet is on the left side of the chart. This average was obtained by averaging all the level decreases for each month for the last 40 years. We have infrequently seen this exceeded in the past 4 years (since projections have been made).

RAINFALL - This is the daily total rainfall. Its scale in inches is on the right side of the chart. It is the total rainfall which occurred each day at a gauge on the north shore of the middle lake. It has been found that normally the lake level will rise about 80% of the rainfall amount with events that are less than 2". The lake is a large enough area that it is unusual for the rainfall to cover the entire lake causing a net rise less than the rainfall measured at one point. On the flip side of this, there have been times that no rainfall was recorded and the lake rose anyway because of rain over another part of the lake. Surprisingly, there is little change in the lake level due to the surrounding watershed until the rainfall exceeds 2". At this point it can take two to three days after the event before the level starts dropping at the usual rate for the season.

TEMPERATURE - This is the average daily water temperature taken at depth of about one foot. This is often several degrees warmer than the deeper water. Usually the thickness of this warm surface band is between 1 to 1.5 feet in winter, 3 feet in spring, and over 6' in mid summer. Its scale in tens of degrees Celsius is on the right side of the chart. (This is the only way to put it on the chart in a location that does not interfere with other data.)

So here is how to read it. Use the numbers on the right side and multiply them by 10. 2 would be 20 degrees and 3 would be 30 degrees Celsius. Since most of us are used to Fahrenheit here is a conversion table.

Chart Deg. C Deg. F
Reading

1.5               15               59.0
1.6               16               60.8
1.7               17               62.6
1.8               18               64.4
1.9               19               66.2
2.0               20               68.0
2.1               21               69.8
2.2               22               71.6
2.3               23               73.4
2.4               24               75.2
2.5               25               77.0
2.6               26               78.8
2.7               27               80.6
2.8               28               82.4
2.9               29               84.2
3.0               30               86.0
3.1               31               87.8
3.2               32               89.6
3.3               33               91.4
3.4               34               93.2
3.5               35               95.0

2005 LAKE STAGE - Simply put this is how full the lake is. Its scale in percent is on the right side of the chart and can be applied to all the traces that use the elevation scale. At 100% the lake has not been above this level in the last 30 years. At 0% the lake has not been lower than this in the last 30 years. At 50% the lake has been above this level for 15 years and below this level for 15 years.

Lake Conway does not have a fixed water elevation. It is a natural body of water and is subject to the fluctuations of nature and man. Since 1952 when data started being collected on this lake its level has consistently varied about 5' over generally 10 year periods and the average level has been dropping slightly. From 1968 to 1973 the level was artificially maintained using large groundwater pumps. Now those pumps have been removed and current regulations prevent it from happening again. Since the total rainfall over the past 100 years has not changed but water use by mankind has substantially increased, causing a net decrease in the water table elevation, it can be surmised this is at least one reason for the average lake level decrease.

With this in mind, it is apparent that very old data may have little bearing on what we can expect from our lake today. In general, the government agencies who monitor and establish "normal" water surface conditions use between 15 and 20 years of data as their standard. To be conservative the 2005 Lake Stage is based on the previous 30 years of data (1976-2005). The Lake Stage is calculated by finding the average lake elevation for each month then tallying how many months the lake was at each 0.1' elevation range (creating a histogram). Based on the number of months in each elevation range a percentage of months below that can be calculated resulting in the Lake Stage percentage for that elevation range.

Since the Lake Stage is calculated using 30 years worth of data a 1 year shift in the data does not create a substantial difference in the results so the Lake Stage data will only be updated every 5 years. The "2010 Lake Stage" data will be recalculated using information from 1981 through 2010.

Lake Conway has a weir on the south lake at a nominal elevation of 86.4 (Since the datum elevation was adjusted on 8/31/2000 and the weir has settled some by February 2008 the actual elevation is between 86.11 and 86.36). According to Orange County regulations the Normal High Water (NHW) is established as 6" above the weir at 86.9. The NHW level corresponds to a 2005 Lake Stage of 98.35%. The "NORMAL" water elevation is a Lake Stage between 15% and 85% which equates to actual water elevations between 83.8 and 86.3 according to the 2005 Lake Stage data.

City of Belle Isle

Phone: 407.851.7730

Fax: 407.240.2222

1600 Nela Ave.
Belle Isle, Fl 32809