Ultimate Guide to Pumping Water from a River, Lake, or Creek

Understanding water pumping engineering basics

So, you've checked with your state and found out that pumping water is perfectly legal in your area. You have a great source of water on your property and you've figured out exactly what you want to use it for. Now you're ready to learn HOW to pump water from it.

One little caveat before we dive in—this stuff can get really technical, so be prepared to exercise the left side of your brain for the next several paragraphs. But regardless of if you're installing the system yourself or are hiring a contractor to do it for you, knowing how it all works is extremely beneficial as it will help you better maintain, modify, and troubleshoot it in the future. Plus, if you are hiring someone, the more you know about the process, the better for everyone.

And if you are DIY from the ground up, all the details are extremely important in a water pumping system. An incorrectly-sized pump will either not provide any water at all, or worse, can cause long-term, very expensive damage. So, you can't cut corners.

OK, onward.

Flow Rate and Pressure

The first basic concept to understand is the relationship between flow rate and pressure.

Flow rate can be described as the volume or “amount” of water that comes through a water pumping system. A high flow rate is associated with more water and a low flow rate is associated with less water. Flow rate is expressed in gallons per minute or GPM.

Pressure is associated with the amount of force within the water system. From a practical perspective, a high-pressure shower is much more desirable than a low-pressure shower.

Ultimately, a water system with high-pressure and a high-flow rate is generally more desirable. The problem that pump science has to overcome is that there is a lot of resistance that prevents most water systems from achieving the target pressure and flow rate. This resistance can come in the form of gravity, friction, and distance of which water has to travel.

The goal of the water pump is to overcome this resistance and provide the user with the desired amount of water pressure and flow. But finding just the right pump is crucial. Too weak of a pump will result in poor pressure and flow or none at all. Too strong of a pump can result in damage and a shortened pump life.

The objective for the homeowner is then to find the right type of pump by calculating the exact amount of pressure and flow the system requires.

This is why it's important to determine the purpose of the water use upfront. A sprinkler system will have different pressure and flow requirements than a kitchen faucet.

How to size a pump

In pump engineering, flow rate is expressed at GPM and pressure is expressed as TDH. These two measurements will be unique to your system and we'll work through how to calculate them as you'll need them when you choose a pump.

Flow rate: Gallons Per Minute (GPM)

Faucet / Alicja / Pixabay

Flow rate is the overall amount of water that will flow through the system and out of the fixtures—a faucet, showerhead, garden hose, etc. Flow rate is expressed in GPM or gallons per minute in the United States. In other places, it may be expressed as liters per minute. Regardless of which unit of measurement is used, the concept remains the same.

The average flow rate for a home’s internal water system is typically between 6-12 gallons per minute, depending on the number of faucets, showers, and appliances being used. A larger home with more bathrooms and water fixtures will require a higher flow rate than a smaller home with fewer fixtures.

A single fixture flows at a rate of 1-2 GPM. By counting the number of fixtures, you can get a rough estimate of the flow rate demand for your home or cabin.

Determining your exact flow rate demand can be a very precise process that goes beyond the scope of this article. There are formulas to help calculate the exact number which may be necessary for large home applications. But for the purposes of drawing water for a small cabin or cottage, a ballpark range should do.

Once you've determined what your flow rate is, keep this number for later.

Pressure: Total Dynamic Head (TDH)

Total Dynamic Head Diagram / watermission.org

Measuring TDH is a bit more complex as it involves several critical steps.

As the pump creates pressure within the system, it needs to overcome a certain amount of resistance in order to provide the desired amount of water pressure at the fixture outlet. And because resistance comes in the form of various factors such as air pressure, pipe length, and friction, measuring it requires the sum of all of these factors.

The total sum of this resistance is called TDH or Total Dynamic Head.

Because TDH is a collection of different units of measurement, the simplest way to measure TDH is by feet.

There are 3 components that make up the Total Dynamic Head. They are the vertical lift (often called static head), friction loss, and operating pressure.

Vertical lift / Static head

Vertical Lift Diagram / watermission.org

Vertical lift, or static head, refers to the amount of pressure the pump will have to overcome when pushing water upwards. Because water is heavy, it creates a strong downward pressure on the pump, similar to how divers experience pressure in their eardrums as they descend further toward the ocean floor.

Vertical lift measures the total vertical distance the water must travel from the outlet of the pump to the highest part of the water system. It is important to note that vertical pressure doesn’t account for any horizontal distance in the water system, nor does it account for moving water. It only measures vertical distance while the water is perfectly still which is why it's also referred to as "static" head. Water in motion causes changes in pressure which we’ll account for later.

In order to measure vertical pressure, simply measure the distance from the pump outlet to the highest point of the water system. This distance is measured in units of feet. Keep this number for later calculations.

Friction loss

Friction Loss Diagram / watermission.org

Water in motion causes changes in pressure. As water moves through pipes, pressure is lost as it comes into resistance with the pipe walls and corners. The width and material of the pipes as well as the total distance that water has to travel through are all major factors in calculating friction loss.

This “friction” causes water to lose pressure and must be accounted for as the pump will need to work harder to compensate for it.

In order to account for friction loss, you’ll need to reference a friction loss chart or calculator. But before that, you’ll need to know the flow rate and the total length and diameter of your exit pipe. Some friction loss calculations involve as much detail as counting all the corners and right angles in the pipe.

Some of these numbers might not be available—and you'll just have to work with what you have. But as with any aspect of pump engineering, the more precise the calculations, the more likely the pump system works to your benefit.

If you’re purchasing a pump to add to your home or cabin's existing plumbing system, check with the original building plans to find out the length and diameter of your pipes—rather than having to measure everything yourself.

But if you’re designing your water system from the ground up, say for a small cabin or an outdoor irrigation system, the selection of your pipe materials will be a major factor. Larger-diameter PVC pipes move water much better than smaller copper pipes and rubber hoses. Consider 2" PVC pipes for better flow.

Once you’ve gathered these numbers, reference the friction loss chart to get the total friction loss number in units of feet. Save this number for later.

Operating pressure

Finally, operating pressure simply refers to the desired amount of water pressure at the point of use. Generally, people like a significant amount of pressure especially when using showerheads and kitchen faucets. Other appliances such as sprinkler systems or pressure tanks will have a specific operating pressure that must be met.

The operating pressure is largely preferential in most cases, but on average, the final operating pressure among all the fixtures usually ranges between 30 psi to 80 psi for most homes. But, like vertical lift and friction loss, we’ll need this number expressed in units of feet.

To convert from psi to feet, use this calculator. Or simply multiply the psi by 2.31.

Now that you have the numbers that make up the TDH, simply add them together.

Add the vertical lift, friction loss, and operating pressure together, in units of feet.

This final number is your Total Dynamic Head. It accounts for all the potential resistance that a pump will encounter when attempting to push water through the system.

Final Step: The Pump Curve

If you've made it this far, you should be very proud of yourself. These basic pump engineering concepts aren't easy to grasp and people pay lots of money to learn them.

But again, whether you're hiring someone to install the pump for you or you're the DIY type, understanding how your pumping system works will reap long-term benefits overall.

So, moving on.

Now, you should have your TDH as expressed in feet and your flow rate as expressed in GPM.

You'll need these two numbers when selecting a pump.

Pump manufacturers will display a lot of information about the pump such as horsepower, voltage, max PSI, and even flow rate in GPM. One of the misconceptions that new pump buyers usually have is that a pump with a certain GPM or PSI rating will produce that exact GPM or PSI in practice.

This is simply not true. Pump manufacturers make a variety of pumps in order to meet the various requirements that homeowners and consumers may have. The GPM rating on a pump will only produce that much under specific conditions. But it may very well be under or over that number.

As a buyer, what you’ll need to pay attention to most is the manufacturer's pump curve chart. Referencing your TDH and GPM, you’ll be able to determine which pump will meet your exact needs.

Here's an example of what a pump curve looks like for an above-ground jet pump.

Each pump will have its own performance curve unique to its specifications.

Notice the TDH scale on the left side of the chart and the GPM scale on the bottom then find where your numbers lie on each scale.

Draw a horizontal line from your TDH number on the left and a vertical line from the GPM number on the bottom. If these two lines meet at any point on the performance curve then this pump will meet your requirements. Ideally, you'll want the point to meet somewhere in the middle of the curve for optimum efficiency.

If the two lines meet anywhere above or below the curve, this specific pump will not perform to your requirements. You'll need to check out another pump.

Pump curves can look confusing. Luckily, a sales associate should be able to assist you in reading the curves and selecting the right pump for your needs. They’ll be glad you already have the TDH and GPM handy.