Digging a new well, or expanding the capacity of an older well
When moving to a new farm site, many specialty crop growers need to either dig a new well or assess the capacity of an existing well. This page covers the basics of how to determine the ideal well and pump size for your farm, measuring the flow rate and pressure of an existing well, and costs and permits associated with agricultural wells.
Anyone digging a new well should consult the Minnesota Department of Health’s Owner's Guide to Wells. This guide provides an overview of permit requirements and other regulations. Make sure to test your water to comply with food safety best practices.
Calculating irrigation needs
While crop water needs vary slightly throughout the season, the standard 1 inch of water per week is a good starting point for designing an irrigation system. While most weeks, rain will supplement some of that, growers should plan for an irrigation system that can support their crop needs during drought.
1 inch of water per acre is equal to 27154 gallons. For a farm with five acres in production, the irrigation system should be able to accommodate five times this much water each week: 135,770 gallons.
Determining pump size, well diameter, and depth
The pump size is perhaps the most important consideration for specialty crop farmers. Pump size depends on the depth of the well and the well diameter, amount of water needed, water distribution system and irrigation equipment. While a larger pump is more expensive up-front, investing in the right sized pump initially is generally substantially cheaper than upgrading later if the farm size or water needs increase.
- Residential pumps tend to have 0.5 - 0.75 horsepower motors, with a 10 gallon per minute pumping capacity (Source, Well Owner’s Handbook).
- Small-scale vegetable farms (up to about 5 acres) more commonly have pumps with closer to 5-15 horsepower and a capacity of 20-40 gallons per minute.
- Larger-scale growers such as potato growers on a few hundred acres commonly need 740 gpm to 1066 gpm depending on the soil type and acres irrigated. The most common irrigated soils in MN are sandy loams that requires approximately 6gpm/ac flow rate for full season irrigation (Scherer, 2018)
When determining the pump size needed for your farm, start by calculating the total water requirements for your crops in a worst-case scenario (i.e. a severe drought).
Pulling from the example above, a five acre farm would need approximately 135,770 gallons of water per week during a period with no rain. With a pump capacity of 25 gallons per minute, the grower would need to irrigate for 5430 minutes, or 91 hours per week. That’s equal to about 13 hours of continuous irrigation daily. With a pump capacity of 40 gallons per minute, the grower would need to irrigate for 3394 minutes, which is 57 hours per week, or 8 hours per day. If the grower had a much smaller pump capacity, say 10 gallons per minute, they would not be able to irrigate sufficiently, even if they ran their irrigation 24 hours each day.
Also consider the possibility of scaling-up production in the future, and whether you might want the option of a larger capacity well in future years.
Depth
The depth of a well can vary substantially from farm to farm depending on the depth of the water table, surface contamination risks, and other factors such as local regulations. Asking neighbors may give you a sense of how deep your well will need to be, but usually the precise depth of a well cannot be determined prior to drilling.
Diameter
Wells are lined with steel or plastic pipes called well casing. These pipes are typically 4-6 inches in diameter. Your local well drilling company can help you choose the best material for your site based on drilling method, soil conditions, depth, etc. A well with a larger pipe diameter will allow for a larger volume of water to be extracted. Residential well casings are commonly 4-6 inches in diameter. This is often sufficient for small farms (less than 5 acres), but for larger farms and larger well capacities, well casing diameters range from 6-14 inches.
Recommended well diameters for various well yields. (Boman et. al. 2019)
| Anticipated well yield (gpm) | Nominal size of pump bowls (inches) | Optimum size of well casing (inches) | Smallest size of well casing (inches) |
|---|---|---|---|
| 200-600 | 6 | 8 ID | 6 ID |
| 600-1000 | 8 | 10 ID | 8 ID |
| 1000-1900 | 10 | 12 OD | 10 ID |
| 1900-2800 | 12 | 16 OD | 14 OD |
Measuring flow rate and pressure
Knowing how much water is coming out of your main irrigation line and the pressure at which it is flowing are critical for choosing the right hoses, filters and pressure regulating devices. This information is also critical for calculating how long you need to water to meet your crop needs. You will need to calculate water pressure and flow rate. Your well-digger should be able to estimate these measurements when they install the well and pump, but it’s important to have real-time measurements as well.
These measurements may be different at different points on the farm. For example, if one part of the farm is irrigated with drip tape, while another is watered with overhead sprinklers, the flow rate and pressure will be different in each field. This is in part because drip irrigation systems require pressure regulators to drop the water pressure, and in part because the water pressure changes with distance from the hydrant and with changes in slope.
Water pressure
Water pressure meters are available from most irrigation suppliers. Typically pressure is measured in pounds per square inch or PSI. Most farmers have a pressure meter attached permanently at the wellhead, as well as at any pressure regulating devices throughout their irrigation system.
Flow rate
An accurate flow meter installed on the irrigation pipeline is a very important and essential tool to practicing good irrigation scheduling. A flow meter provides the means for determining the most accurate volume of water pumped for irrigation. The flow meter not only provides information about the total volume of water delivered to the field but also captures the changes in well output indicating potential pump problems. The flowmeter provides two values: total volume of water pumped and instantaneous flow rate. However, for the most accurate flow rate measurements, record the volume of water pumped for a set time (e.g. 10 minutes). This will provide an average flow rate measurement.
Using a measured flow rate determined by monitoring the flow meter, the average application depth may be calculated as:
Gross depth of irrigation (inches) = Flow rate (acre inch / hr) X Time of irrigation (hr)/ Acres irrigated (acres)
Conversion factor: 453 gpm = 1 acre inch /hr
In the absence of a meter, growers can use simple (though less accurate) methods to calculate flow rate. One approach is to fill a container with water for a set period of time (5 minutes) and calculate how much water came out of the hose. Ideally, do this for 5-10 minutes, and multiple times throughout the season to account for changes in pressure.
Another approach is to fill a container with a known amount of water (such as a 10-gallon container), and calculate how much time it takes to fill, then divide by the number of gallons to get gallons per minute. Again, do this multiple times to account for variation.
Estimating the cost of a new well
The cost of a new well can be highly variable, depending on the depth, soil conditions, well diameter, pump size, and materials used. Farmers digging a well for a 5-20 acre farm should expect to pay between $5-30,000 for a new well. Call local well drilling companies for an accurate quote.
Permits required for wells based on water use
In Minnesota, a permit is required from the Department of Natural Resources (DNR) to drill a well if you are withdrawing more than 10,000 gallons of water per day or 100,000 gallons per year: 1 inch water/acre = 27154 gallons. The permit water users are required to submit an annual report of water use to the DNR. See more information about water use permits.
Boman et al. 2019. Design and construction of screened wells for agricultural irrigation systems. UF-IFAS Extension.
Schere, T.F. 2017. Irrigation water pumps. AE1057, revised Aug. 2017. NDSU extension.
Martin, D., Kranz. W.L., Smith, T., Irmak, S., Burr C. and Yoder, R. 2017. Center pivot irrigation handbook. University of Nebraska-Lincoln Extension.
Reviewed in 2022