Determining GPM of lagoon pumps and ponds
The easiest way determine gpm from a lagoon pump or gravity outlet is to install a temporary flow meter and record the flow rate at various levels a the pond is being drawn down. A flow meter with that logs both flow rate (gpm) and depth in the pond simultaneously is ideal, otherwise someone will need to manually read the meter and record both the flow rate and the corresponding water depth in the pond.
These Excel spreadsheets create a table of pond levels with a place to write the gpm for that level. Once you have entered the gpm into the space provided, this sheet makes a look-up table for the pounds of nitrogen per hour that can be expected from from the pond at each depth, based on concentration.
Determining the gpm of a pump or pond outlet using pond drop
There are several methods of determining the flow rate from a pond using pond drop. Use these spreadsheets to make a table of the depths in the pond and the surface area at each depth.
Depending on the method you choose, you may need to measure the length and width of the pond when it is full, the length and width of the pond surface at several points during drawdown, the rise and run of the slope, and the total depth of the pond. Here are pointers on how to measure a dairy lagoon:
To calculate gpm, temporarily shut off all other inflow and outflows, and calculate the flow rate by converting acre-inches into gallons (1 ac-in = 27,154 gallons) and divide by the number of minutes it took for the pond to drop from one depth level to the next.
Here are hand calculation worksheets to help you calculate gpm from pond drop using different methods. They have one calculation per page.
You can also record run times along with pond level data when using the pond drop recordkeeping worksheets and spreadsheets and use those to calculate flow rate.