EVAPORATION



EVR 4211L – WATER RESOURCES LAB

Laboratory 2

Title:

Objectives:

Students will familiarize themselves with fundamental components of the hydrologic cycle. Specifically, students will learn how to collect hydrologic data from archived databases and analyze the data to determine values for precipitation and evaporation. Such values are critical in determining the water balance for a water resource.

Materials:

Students will need a straightedge ruler, a calculator, and access to the internet (before handing in the lab report).

Introduction:

Throughout this lab, Lake Okeechobee will serve as the water body under investigation. Lake Okeechobee is important to study because of its important role within South Florida. Lake Okeechobee is the second-largest freshwater lake in the continental U.S. It has an area of 730 miles2 and an average depth of nine feet. It is located in South-Central Florida and serves as an important source of water, supplying the greater Kissimmee-Okeechobee-Everglades ecosystem. (SFWMD, 2005)

|Table 2.1 Precipitation - Lake Okeechobee |

|Station |Average Annual Precipitation |

| |in. |from records of |

|S72 |40.23 |1995-2005 (11) |

|S77 |47.18 |1994-2004 (11) |

|S3 |46.92 |1993-2004 (10) |

|S308 |48.28 |1993-2004 (10) |

|S133 |47.07 |1993-2005 (11) |

|LZ40 |35.07 |1994; 1996-2004 (10) |

WATER BALANCE

Begin by writing a hydrologic continuity equation for the water budget of Lake Okeechobee. Include all of the components that likely are significant and explain how each component fits into the hydrologic system of Lake Okeechobee.

PRECIPITATION

Arithmetic Average Method

Precipitation data taken at meteorological monitoring sites record the depth of precipitation at those physical locations only. Consequently, in order to get an idea of the volume of precipitation within a larger area (e.g. a watershed), it is necessary to extrapolate the point data over the entire area of interest. Different methods exist for achieving this; however, the simplest one is arithmetic averaging method. For this method, one simply multiplies the average of the point data by the area of interest.

1. Use the arithmetic averaging method to determine the average annual precipitation (m3) of Lake Okeechobee. Use the data from the six monitoring stations given in Table 2.1.

Thiessen Method

Another method of determining precipitation within a given area is the Thiessen Method. This method is more accurate than the previous one because it accounts for any non-uniform distribution of monitoring stations and weights each station’s contribution differently. For instance, this method assumes the value of precipitation of an area within a watershed is most accurately reflected by the measurement of the nearest station. Furthermore, it provides a simple way of delineating the area that each monitoring station represents. (Refer to attached handout for a description of the Thiessen method)

2. Using the Thiessen method, determine the average annual precipitation (m3) of Lake Okeechobee. Make use of the map of Lake Okeechobee in Figure 2.1. Use the grids to estimate the area.

How do the results compare with the precipitation calculated using the arithmetic averaging method?

Isohyetal Method

A third method for calculating the precipitation of a region is to use the isohyetal method. The main benefit of using this method is that it takes into account the topography for an area, along with the non-linear variations in precipitation resulting from sudden changes in elevation (orographic effects). Furthermore, contour lines of equal precipitation (isohyets) are plotted within the basin, making use of precipitation values of the monitoring stations. The area within the contours is then found and multiplied by the average precipitation value between the two bounding contour lines. (Refer to attached handout for a description of the isohyetal method)

NOTE: Variations in topography for Lake Okeechobee are negligible.

3. Use the isohyetal method to compute the average annual precipitation (m3) of Lake Okeechobee. Use of the map of Lake Okeechobee in Figure 2.2.

How do these results compare with the other two methods? Which method would be most advantageous to use in the case of Lake Okeechobee? Explain your reasoning.

Further compare the results from the three methods with the average annual precipitation values published map in Figure 2.3. Do they agree? Which method agrees most? What are the likely reasons for any differences between your results and the value in the map?

EVAPORATION

Estimation of the evaporation out of Lake Okeechobee will be carried out in two ways. First, refer to a published map. Second, withdraw relevant evaporation data from a meteorological monitoring station at Lake Okeechobee.

1. Estimate average annual lake evaporation (m) by viewing the evaporation map in Figure 2.3.

The data for the annual lake evaporation map (Fig. 2.3) were derived from measurements using a standardized evaporation pan. A ‘Class A’ pan was used, which is a 4-foot diameter, galvanized pan. Since the amount of water evaporated out of an evaporation pan is typically 140 percent greater than the evaporation out of a lake, a pan coefficient has been derived in order to compensate for the difference. A map of pan evaporation is shown in Figure 2.4, while a map of pan coefficients is shown in Figure 2.5. The map used to determine lake evaporation (Fig. 2.4) is a derivation of these two maps (Fig. 2.4 and Fig. 2.5).

2. The South Florida Water Management District (SFWMD) maintains continuous and historic hydrologic data of South Florida. The SFWMD Monitoring Sites in and around Lake Okeechobee are shown in the map in Fig. 2.1. Evaporation data is among the meteorological data recorded at many of these sites. In order to determine the evaporation of Lake Okeechobee, you will need to retrieve pan evaporation data from the internet.

o Go to the South Florida Water Management District’s website at .

o Click on the ‘Data & Documents’ selection and then access the ‘Environmental Database (DBHYDRO)’ and select ‘DBHYDRO Browser’ and then the ‘DBHYDRO Browser Menu’.

o Once in the ‘DBHYDRO Browser Menu’, check ‘Meteorological Data’ and submit. Next check ‘Data Type’ and ‘Basin’ and submit. Then select ‘Pan Evaporation’ and ‘Lake Okeechobee’ and submit.

o Of the available data sets, choose the one that allows access to the 1954 pan evaporation data from Station S308 ( “Get data”.

o You will need to abstract the monthly pan evaporation totals for 1954 and record them in a table. (Record them in the units given!)

a) Calculate the pan evaporation data (m) for this period.

b) Correct for the pan effect, using the map in Fig. 2.6 to determine annual lake evaporation.

c) How does this measurement compare with the value taken directly from the map (Fig. 2.4)?

3. Calculate the annual evaporation (m3) from Lake Okeechobee.

LAB REPORT

Write a lab report consistent with the format specified by the instructor. Summarize your analysis and include all calculations and figures used to arrive at your results.

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Figure 2.1 Meteorological Monitoring Sites In and Around Lake Okeechobee

REFERENCES

Lee, Keenan, CW Fetter, and John E. McCray. 2003. Hydrogeology Laboratory Manual-Second Edition. Pearson Education, Inc., Upper Saddle River, New Jersey.

SFWMD. 2005. Okeechobee. South Florida Water Management District.

Ward, Andy D, and Stanley W Trimble. 2004. Environmental Hydrology: Second Edition. Lewis Publishers: Boca Raton, Florida.

Figure 2.2 Average Annual Precipitation

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Figure 2.3 Average Annual Lake Evaporation (inches)

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Figure 2.4Average Annual Pan Evaporation (inches)

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Figure 2.5Average Annual Pan Coefficients (in percent)

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REFERENCE:

South Florida Water Management District (SFWMD):

South Florida Water Management District (SFWMD):

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