Irrigation or Evaporation?

Part of the Young Naturalist Awards Curriculum Collection.

by Jacob, Grade 7, Florida - 2008 YNA Winner
Figure 1.

My family lives in a nice house on a cul-de-sac off a lake called Lake Catherine in Maitland, Florida. We have been living in this house for about four years, and I have been watching and worrying about the constant drop in lake water levels. Every so often—like after a hurricane—it goes up a great deal, but then it quickly drops again! Now the lake is very low, and every dock is three or more feet out of the water, even though they were covered during the hurricanes in 2004. So now that the water level is getting dramatically low, I thought about studying this further and seeing why it is dropping so fast.

I started my project by going to my neighbor, Duane Bergstrom, who owned all the cul-de-sac property for at least 20 years before he sold the property to new neighbors. He said, "People have been watering their lawns for 40 years and have had docks in the water deep enough to dive into the water from them." He said that lake water levels had been dropping for 40 years and that people have been irrigating the whole time.
I already knew that there were district limits on how much lawns could be watered, and I knew that irrigation could be done with city water, well water, or lake water. I also basically knew what evaporation was from my science classes. I went looking around for information on lake access irrigation (LAI) and evaporation, and eventually my question went from "What is causing our lake water level to drop?" to "Is the water level dropping more because of LAI or evaporation?" My hypothesis was that lake access irrigation uses more water and causes a larger drop in the lake water level than evaporation.

Figure 2.

My dad and I poked around for information on both these subjects and gathered some interesting information. For LAI I learned that a trench is dug from the pump to the lake, and a PVC pipe (usually either one inch or two inches in diameter) is buried in the trench. Connected to the pipe is a pump and a control box, which controls how long and at what times the pump runs. It sucks water from the lake according to the pump specifications, usually between 30 and 100 gallons per minute.
Evaporation occurs when a body of water heats up, mostly because of the Sun (solar irradiance). The molecules in the water heat up and spread around and leave the water surface (turning into a gas) and go up into the air, thus evaporating. In order to estimate the amount of evaporation, I learned about U.S. class-A evaporation pans. I also found information on Penman's equation for estimating evaporation. Both of these were too complicated. However, I was able to find a paper titled "Data-Sparse Estimation of Lake Evaporation Using a Simplified Penman Equation," by Edward T. Linacre. I would be able to use this equation once I replaced the variables with climate data like temperature, wind speed, and dew point.

I now developed my procedure. I would count all the LAI pumps around the lake and determine which houses they came from. I would create a survey to get a better understanding of how people used LAI. I would ask all the homeowners the same questions (Science Survey)and I would use the information to make an estimate of how much the lake drops on any given day from LAI.

Figure 3: Science Survey 
Hi, my name is Jacob Polfer. I live down the road on Lake Catherine Ct. I am a student at Lake Highland, and for my science project I am trying to determine whether evaporation or lake access irrigation is the main cause in the drop of water level in our lake.
May I ask you a few questions about your watering habits?
Do you use lake access irrigation?
May I record the make and model of your pump?
How many times do you water a week?
How long do you water a session? (How many zones do you have, and how long per each zone?)
Do you have any comments or questions?

Figure 4.

Thank you very much.For evaporation, I would need to get mean temperatures, wind speeds, and dew points. To get this data I would need equipment (like a wind gauge) that I did not have. However, I found out that has historical weather data and graphs, and that they have a station in Maitland, Florida. I decided to use the graphs to get the climate data I needed to estimate evaporation with the simplified Penman's equation.

Data and Analysis 
I began my data collection by walking around the lake, counting pumps and taking pictures. I came to a house with a dock that had been above, then below (during the hurricane), and then again above the water within the last couple of years. I hoped that the paint could help us a bit with the lake level history. Sadly, they had painted the dock that very week! During the first trip around the lake, I counted 15 pumps (see map). I made two more trips around the lake over the next few weeks to take surveys. I was able to complete a total of 11 surveys and collect information on four pumps. The table below shows the information that I gathered on my trips (see Table 1).
If I know the flow rate for a pump and the length of time that a pump runs, I can figure out how many gallons of water are used. The following equation can be used to see how many gallons of water are used each week by the four houses in the survey:

Table 1.

G  is the number of gallons of water pumped from the lake per week
N  is the number of times watered per week
Z  is the number of zones
M  is the number of minutes each zone runs
F  is the flow rate of the pump when running
From the Web I estimated that the 2HP pumps had a flow rate of around 40 gallons per minute, but the 3HP pump had a flow rate of 60 gallons per minute.
Using the formula above for each of the four survey pumps, I was able to calculate the number of gallons of water each house used per week (see Table 2).

Table 2.

The average of the four houses' withdrawal from the lake is 15,800 gallons per week. I was only able to get pump information from the people who gave me permission, so I am using the average of the four houses for all 15 pumps around the lake. That means that I estimate that 237,000 gallons of water per week are taken from the lake.
Lake Catherine is 23 acres, and says that an acre-inch of water is 27,154 gallons. That means 624,548 gallons of water must be taken from Lake Catherine before it will drop one inch, and that LAI causes Lake Catherine to drop 0.379 inches per week. That converts to a 1.38 mm/day drop in the water level caused by LAI.
So now I turned my attention to the evaporation part of the experiment. The simplified Penman's equation is a way of "simply" figuring out an estimation of evaporation. Different climate data are used as variables in the following equation:

A mathematical equation.

  R s  is the solar irradiance of the Lake Catherine's surface, which is the energy that the Sun gives to the lake. It can be calculated with:


A  is the latitude (°)
y   is the annual mean temperature (°C)
r  is the annual temperature range (°C)
z  is the elevation (m)

A mathematical equation.

For Lake Catherine  A  and  z  were found using a Garmin nüvi 370 GPS.  A  is 28.62° and  z  is 30m.  T y  was calculated using monthly Orlando mean temperatures on the World Weather Information Service.  T y is 22.39°C.  T r  is the difference between the July and January temperatures in Maitland, and it is 12.55°C.  F  is 0.9974.
Using the numbers above, the equation turns into:



T  is the monthly mean temperature (°C)
u  is the wind speed at 2m (m/s)
d  is the dew point temperature
In order to figure out the temperature, wind speed, and dew point, I went to and looked at the historical weather graphs for high temperature, low temperature, dew point, and wind speed. I put the data into Microsoft Excel tables and averaged it to get the mean for September, October, and November of this year.
I then used the data for the climate variables in the simplified Penman's equation to get the daily evaporation estimates for each month. The results are in Table 3 below:

The table shows that depending on the month, evaporation causes Lake Catherine's water level to drop between 2.6 mm/day and 5.4 mm/day.

The table shows that depending on the month, evaporation causes Lake Catherine's water level to drop between 2.6 mm/day and 5.4 mm/day.



My research took me around the lake to meet some interesting people who share my concern for Lake Catherine. They gave many theories for the drop in the lake level, including:

  • Irrigation or evaporation (many were passionate about these theories, and even suspected their neighbors of watering more than they were allowed)
  • A natural seven-year cycle
  • Sinkholes at the bottom of the lake

Also, there were many questions about what I was including in the paper. For example, people wanted to know about storm water refilling the lake and about irrigation water running back into the lake as their lawns were being watered. This taught me that just because water is taken out of the lake, it doesn't mean that it won't go right back in. I also realized that there are many other factors that could affect the outcome of my project.
I was only able to survey four houses around the lake, but I got some good information. I figured out that, on average, LAI causes a drop in the lake level of 1.38 mm/day, while evaporation causes a drop between 2.6 mm/day and 5.4 mm/day, depending on the month and the season. Even if my estimates are wrong by as much as 0.5 mm/day, it is clear that evaporation—not lake access irrigation—is the main reason the level of Lake Catherine is dropping.


Bergstrom, Duane. Interviewed by Jacob Polfer. Lake Catherine Ct., Maitland, 14 October 2007.
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Lake Advisory Council, Maitland.  City of Maitland Minutes of the Lakes Advisory Board (LAB).  Maitland Lake Advisory Council. Retrieved from the World Wide Web on 21 November 2007.
Linacre, Edward T. "Data-Sparse Estimation of Lake Evaporation Using a Simplified Penman Equation." Agriculture and Forest Meteorology  64 (1993) 237-256. Elsevier Science Publishers B.V., Amsterdam.
Linacre, Edward T.  Estimating U.S. Class-A Pan Evaporation from Few Climate Data.  Geography Department, School of Resource and Environmental Management, Australian National University, Canberra, Australia. Retrieved from the World Wide Web on 16 October 2007.
Linacre, Edward T.  Ratio of Pan to Lake Evaporation Rates.  Geography Department, School of Resource and Environmental Management, Australian National University, Canberra, Australia. Retrieved from the World Wide Web on 4 November 2007.
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