GPM + Run Time Measurement Method
Measuring Application Volume using Pump Output and Run Time
With this method, the output of the pump (gpm) is estimated and the time the pump runs is recorded. Once the concentration of nitrogen in the lagoon water is known, a “lookup” table or simple calculation gives the total pounds of nitrogen applied. The total pounds of nitrogen applied divided by the number of acres gives the pounds of nitrogen applied per acre. This method is prone to inaccuracies and allows only limited flexibility in controlling application rates of lagoon water nutrients. Nonetheless, it avoids the expense of a flow meter and may be the only option in some circumstances.
Seldom does a pump put out exactly the rated amount. Actual pump output is influenced by:
- The depth of water in the pond
- The amount of wear on the pump
- Debris on the impellor
- The distance and elevation the water is being pumped
- The type and diameter of pipeline
- How many solids are in the pumped material
The lower the water level in the pond, the less the pump output. This is true for both floating pumps and stand pumps because it is the difference in elevation between the level of water in the pond and the level of the discharge that determines the amount of energy required to pump the water through the same section of pipe.
To use this method, you will need an estimate of the pump output or the gravity flow rate. Since the pump output or gravity flow from a pond becomes less as the lagoon is drawn down, the gpm method will be most accurate if the estimated output is adjusted based on pond level.
Friction, constrictions and elbows increase the amount of head pressure or pumping pressure that it takes to move the same amount of liquid at the same velocity. If the lagoon water is in a pressurized pipeline with multiple discharge points, those discharges that are further away have more losses from friction, bends, constrictions and flatter slope, resulting in lower flow rates when pumping to far fields compared to closer ones at the same pond level.
Installing a temporary flow meter is the easiest way to estimate flow rate from a lagoon. Another method is to 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 to drop the pond a measured distance.
Additional information on calculating the GPM from a pond is in this publication:
Worksheets and spreadsheets for making calculations using flow rate are here:
Applying a specific amount of nitrogen to a field using flow rate is done by injecting the lagoon water into the fresh water during only part of the irrigation. Resources to determine the amount of time to run the pump are here: