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The pond drop method measures volume applied by multiplying the difference in level of the pond in inches between the start to the finish of an irrigation onto a field by the surface area of the pond in acres:
Inches of drop x acres = acre-inches applied
Under the right circumstances, the pond drop method can be very accurate, and can sometimes be used to confirm other measurement methods. It is especially useful for measuring runoff that is collected in a tailwater return pond.
When used to measure liquid manure nutrients however, pond drop is usually not the method of choice. This is not only because of the many challenges involved with obtaining accurate data, but also because it is nearly impossible to use this method to apply a target amount of nutrients as specified in a nutrient management plan. These factors will make it difficult to maintain yields if you are relying only on manure nutrients from the pond.
General information on making pond drop work:
Since the sides of most ponds are sloped, the area of the pond decreases with depth. You can make a table of pond surface area by depth for your pond using a few simple measurements and this spreadsheet:
Here are some tips on how to measure your pond dimensions and side slope:
If it is not possible to shut off confounding inflows and outflows from the pond during the irrigation, these Excel spreadsheet templates will help you make a chart of the flow rate from the pond by depth in the pond.
Determining GPM of lagoon pumps and ponds
Field Data Sheet for Pond Drop
This field record sheet has a place to record irrigation run times, the depth of water in the pond, and weather information for each application when you use pond drop as an application measurement method.
Field Data Sheet for Pump or Pond Drop
You have several options for managing your data once the application is complete. While many operators choose to hire a service to calculate application rates for compliance purposes, doing the calculations yourself is easy and will give you application results in time to make in-season adjustments.
Whether you are most comfortable doing the calculations by hand or with a computer spreadsheet, here are tools to make the calculations easy: