Determining Size of Treatment Area

Once identification of the nuisance plant population has been determined, it is necessary to accurately estimate or measure the size of the intended treatment area.  Chemical application rates will be provided on the label in either an amount to apply per surface area (acre) OR per volume (acre-foot) of water.  One acre is a sufrace area measurement of 43,650 square feet.  Once acre-foot is a volume measurement of 43,560 cubic feet (one surface acre of water one foot deep).

Shoreline areas, square or rectangular ponds:  Determine acreage by multiplying average length (in feet) times average width (in feet) and dividing by 43,560.

Formula:            Avg. Length (ft.)  X  Avg. Width(ft.) / 43,560 = Acres

Example #1:  A rectangular pond measures 350 feet long and 200 feet wide.  To calculate acreage:                                               

Circular Ponds:    Determine acreage by squaring the diameter, dividing the answer by 43,560 and multiplying by 0.8

Formula:            Diameter (ft.)  X  Diameter (ft.)  /  43,560  X 0.8  =  Acres

Example #2:  A circular pond measures 235 feet across.  To calculate acreage:

Elliptical Ponds:  Determine acreage by multiplying overall length (ft.) times maximum width (ft.), dividing by 43,560 and multiplying the answer by 0.8

Formula:            Length (ft.)  X  Width (ft.)  /  43,560  X  0.8  =  Acres

Example #3:  An elliptical pond is 325 feet long across the middle and 135 feet wide.  To calculate acreage:

Figure 1

Where only shoreline treatments are required (for example, a 20 acre lake may only need 5 acres of shoreline treatment, it is necessary to measure shoreline length.  

Along with relatively straight shorelines, multiply shoreline length (in feet) times width (in feet) of the treatment areas (see Figure 1).

Along irregular shorelines it may be necessary to roughly divide the treatment area into rectangular sections, determine the acreage of each and add them together to get the total acreage (see Figure 2).

Figure 2

To determine acre-feet of water, multiply the acreage by average  depth (in feet):

Acre feet = Acreage X Depth (ft.)

In bodies of water having a relatively uniform bottom slope (see Figure 3), average depth can be approximated by dividing the maximum depth by two:

Average Depth = Maximum Depth / 2

Maximum Depth

Figure 3

More accurate determinations can be made by taking a number of evenly spaced depth measurements throughout the intended treatment area (see Figure 4).  Total the measurements and divide the number taken.  This is also how you determine the average depth for ponds with uneven bottoms:

Average Depth = Total of Depth Measurements Taken / No. of Depth Measurements Taken

Figure 4

In ponds where water volume (gallonage) is known, acre-feet can be calculated by dividing total gallons by 326,000 (the number of gallons in an acre-foot):

Acre Feet = Total Gallons of Water / 326,000

Determining acreage and depth of treatment areas can be done very simply if scaled maps are available.  Contour maps for many public lakes can be purchased from map companies or obtained from the State.  Aerial photos from Regional Planning Commissions, the Soil Conservations Service, or other resource agencies are also helpful in measuring larger area.  Grading survey maps or landscape blueprints are often available for larger man-made impoundments.