Water Pressure vs Water Flow, and how much water do I need?

By Brandwell - February 22, 2022

The design and management of Agricultural water systems is one of the most critical elements in any farming venture. Without it your crops won’t grow, and the animals will die. During summers, stock generally can go for some weeks without feed, but only a few days without water.

When designing an agricultural water system for a farm, there are three key questions that need to be addressed:

  1. How much water do I need?
  2. What flow volumes will I need?
  3. How much pressure will I need to deliver the required flow of water through the pipe?

How much water do I need?

This is directly related to the stocking rate and herd/mob sizes you are running. Stock needs a lot of water especially in our harsh summers. Water needs vary between dry animals and those lactating and feeding young.

From my experience, the following requirements are a good guideline:

Cattle require    40 – 140               litres per head / per day

Sheep require    4 – 12                   litres per head / per day

Horses require   40 – 150              litres per head / per day

Pigs require        10 – 45                 litres per head / per day

So, I would tend to go towards the higher values rather than the lower ones. Let’s face it, animals will only drink what they need, but what they need, must be available when its needed.

So simply multiply these values out by the nominal herd / mob size and you have a volume of the water that needs to be available per day for that herd or mob.

What flow volumes will I need?

This will depend on the way the water is being delivered. There are a variety of watering solutions adopted in Australia from the large turkey nest solutions of the North to the small cattle troughs used in the South.

For this article I will adopt a more traditional trough dispensing system as this is by far the most widely used water delivery process in Australia.

When calculating the required water flow rate look at how many cows can be drinking at the one time. The water trough should allow for at least 10% of the stock drinking at the one time. To be conservative, assume your stock will drink at least twice but more likely 3 – 4 times a day. On average cow’s drink for no longer than about 1- 1½ minutes and can drink between 20 – 40 litres during this time, sheep between 2 – 4 litres.

The trough edge needed is 0.5 metres/cow and 0.3 metres/sheep, but a longer trough is always better! This is trough edge requirements, so a trough that can only be accessed from one side needs to be twice as long! Using the above guides, you can now decide on the length of trough required. When deciding on the carrying capacity of the trough allow at least 4 – 5 minutes of water supply already in the trough as a buffer. So, if you need to supply 240 litres per minute flow of water through the pipe, you should have at least 1000 litres of water already in the trough.

For any Trough valve to fully open and provide the full inflow, the water level in the trough has to drop. For a Cocky Valve Ulti Flo the water level has to drop by only 150mm (6”).

At just 150mm (6”) drop in water level the Cocky Valve Ulti Flo is fully open and delivering all the inflow available.

200kpa – 240lpm, 400Kpa – 300lpm, 600kpa – 350lpm, 800kpa – 400lpm & 900kpa – 430lpm.

There is not another valve that matches the Cocky Valve range for performance and reliability.

How much pressure will I need to deliver the required flow of water through the pipe?

This is very much dependent on the topography of the land and the size of pipe used. When a liquid or gas flows along a pipe, friction between the pipe wall and the liquid or gas causes a pressure or head loss. This pressure or head loss is an irreversible loss of the fluid’s potential energy. This can have a dramatic effect on water pressure flow rate at the trough outlet.

So, draw up a map of exactly where you want the troughs and determine (as best you can) the levels of the various supply points and the distances involved. If you are pumping to a main tank at the highest point of the farm, then you need to know the height and distance from the pump. 

If you are running a gravity fed syphon system, again you need to know the height of supply compared to the discharge point. Then you need to layout where the troughs are both in height and distance from the tank.

Many of today’s smart phones have a compass function that includes an elevation reading as well as latitude and longitude which you can add to your map at each point of reference. Once these are documented it is easy to calculate distances using your map as a guide.

A correct map with levels, piping route and distances between the various points is imperative when calculating the required delivery pressures and pipe size. It is also an invaluable resource for finding joints and branch lines in the future. So, layout the system as accurately as you can and then ensure the final map allows for any adjustments that may have been required during installations. 

Also allowing some “locating dimensions” from unmovable references to the pipeline at key locations, will also prove invaluable in the future. You may think you will always remember where the pipe went in, but I can assure you that memory is no substitute for an accurate map!

There are a number of pipe calculators available and here are just a few to choose from:

Copely Flow Rate Calculator

Vindex Pipe Flow Calculator

HawsEDC Manning Pipe Flow

In most cases its best to take your design map to the local irrigation store and have them confirm the required pipe sizes for you. They have all the tools needed to accurately measure water pressure and flow rates and size the delivery pipes and associated pumping requirements. Because you have completed the first two steps, you now know, how much water you need and how fast you need it at all the discharge points on the farm. You also know the heights between the storage and delivery points as well as distances, junctions and the number of outlets.

It can be a daunting exercise when you start, but it is some of the most important work you will do on the farm. If it is completed correctly, it will ensure adequate and reliable water is always available when and where it’s needed.

Happy Piping!