Running pumps for irrigation is becoming an expensive part of a farmer’s budget every year. Moving water takes energy, and energy always has a cost (with the exception of gravity-fed irrigation systems). That cost can vary greatly from one state to another and one region to another. Costs can vary seasonally and even daily in many cases. Installation and upkeep costs on a single pump can range from a few thousand dollars to hundreds of thousands, depending on horsepower and equipment needed.
As a grower, it is valuable to be able to estimate the cost to move water with an installed pump and established irrigation system. This type of information can be useful for planning and budgeting purposes.
The basic energy unit depends on the energy source; in this example we will use electricity, which is billed in increments of kilowatt hours — kWh. You can also convert cost based on other energy sources such as diesel, gasoline, propane and natural gas. In these examples, kWh is the baseline.
The easiest way to figure pumping costs is to use the following equation for kWh to pump 1 acre foot of water. An acre foot is approximately 325,851 gallons of water, or it can be described as pumping 1 acre 1 foot deep. Most crops require anywhere from 1 to 5 feet of water per acre annually, so this example makes sense. The following equation helps clarify this point:
In order to use this equation, we have to define the variables.
Kilowatt hours is the amount of kilowatts coming through the electric meter to the pump for a one-hour duration. Most farmers think in terms of horsepower, and converting it is simple. HP × 0.746 = kW or kW/0.746 = HP. If the meter says that the pump is using 74.6 kW while the pump is running, that would equal exactly 100 HP (0.746/74.6 kW).
Pumps generally don’t use the exact nameplate horsepower, depending on conditions, water, pressure, etc. Your pump may say it’s 100 HP on the nameplate, but it may be using 92 HP, 104 HP or some other number. But the meter shows how many kW, and that’s how the electric company bills you.
Example 1 illustrates how to determine the cost per hour of run time. Again, the rate varies depending on electric companies, on and off peak, time of use, time of year and other factors.
Total dynamic head or total lift is a measure of where the water is moving from (i.e., a deep well, booster or reservoir) to where the water is moving to (i.e., irrigation system, reservoir, canal) converted to feet. (See example 2.)
Every system has total dynamic head as it operates. It is dependent on what the water source is and how great the lift is, accounting for required pressure and head losses. On my own farm, my pumping water level is 90 feet with 30 psi to run my drip system properly. I determine TDH for my farm with the following equation: 30 psi × 2.31 = 69.3 ft + 90-ft PWL from my well = 159.3 ft TDH. There are pumps manufactured for all different TDH requirements.
Overall pumping plant efficiency or wire to water is measuring the efficiency from the electric wires (electric meter in this case) to the amount of water coming out of the pump discharge in flow and pressure. Another equation can accurately get this number; however, for this discussion, we’ll use 60% OPE for a vertical turbine well pump. This would be considered a very efficient pump, but it’s the point where a new vertical turbine should be, if not better. In the field, OPE can vary greatly depending on the age of the pump, changing operating conditions of flow and pressure, water levels, pump wear or irrigation system requirements, but 60% is a realistic number.
You may notice that there is no flow rate in this equation. All pumps have different flows, but all things being equal with TDH and OPE, a 20 HP pump will take five times as long as a 100 HP pump to deliver the same volume of water. In this case, it would take a 20 HP pump five times as long to pump an acre foot than a 100 HP pump with identical TDH and OPEs.
Most crops require anywhere from 1 to 5 feet of water per acre annually.
If you can get the kW meter reading under normal pumping conditions and figure your TDH, OPE (from a pump test) and the cost/kWh, you will be able to determine the cost to move water to your crop.