Head Loss & Velocity in Irrigation Pipes

Hi all,
Today i am going to explain an important part of the drip irrigation design which is called Head Loss and Velocity in irrigation pipes. For providing proper water to the each plant of the field we must have to consider several things in mind like,

1. Actual water requirement of the field
2. Available flow of the pump
3. Total Head Loss in the system & velocity in irrigation pipes

let’s see them in the brief,
Actual Water Requirement of the field
The first step of drip irrigation design is to calculate the total water required by the area. So how to calculate the water requirement? Let’s take an example,
Suppose we have 1.2 ha area and we are going to grow cotton crop in it at the spacing of 1.40m X 0.40m. We are going to use inline dripper capacity of 2 LPH (litre per hour).
So first thing is how many plants are there in 1.2 ha area? we can calculate it simply by,
1.2 X 10,000 = 12000 sq. meter
now,
= 12000 / 1.40 x 0.4 (Crop spacing)
= 21429 plants in the field
we are going to provide 2 litre water to each plant, so
Total water requirement = 21429 x 2 (Total plants x dripper discharge)
= 42,858 litre or 42.85 m3/hr.
so 1.20 ha are required 42.85 m3/hr. water to irrigate cotton crop.

Available flow of the pump
In the above example we’ve pump of 10 HP and water depth is about 25m. so what is the available flow of the pump set?
well we can calculate it with the device called water meter which is shown below,



There are many types of water meters available now a days. You can calculate the approximate pump flow with the following formula also.

Pump Discharge   =   Motor HP X 75 X Pump efficiency
                                               Total Head (m)

“ You can get details on every part of this formula in my previous post”

So we have pump set flow of about 36.93 m3/hr.
we can irrigate the field with maximum of 36.93 cubic meter per hour of water.
To carry these amount of water from water source to the field, we have to consider several things like Head Loss in pipes, Velocity etc.
In drip irrigation system the total head loss in Main line should not be more than 5m and 1.5m in sub main lines. And maximum permissible velocity is 2.0 m/sec.
we can calculate head loss and velocity with "Hazen Williams Formula”

Where,

• Roughness constant for PVC pipe is 150
• Pipe ID = (Pipe OD – Minimum Thickness of Pipe) - Max. thickness of Pipe

Total Head Loss (m) = (Length of pipe (m) X Pressure drop) / 100
Velocity can be calculated by,




As we have seen all this calculation is bit a difficult job to do. So I have created an excel sheet for calculating all the data above.

You can download it from HERE (Irrigation details calculator).

it looks like,

You have to select pipe size and pressure rating from hydraulic calculation section as under,

Select the desired flow unit from the drop down menu (m3/hr or liter per second)

In the next field enter Pipe length (in meter). This will calculate total head loss, friction loss and velocity. (for drip irrigation total head loss in main line should not be higher than 5m/1000m of pipe. And in sub main line it should be 1.5 mtr per submain line) Head loss cell color is calculated as per before mentioned criteria, so when head loss goes beyond 5 mtr it will turn in to red, for easy understanding.

So always try to keep your head loss below 5 meter per 1000 meter of main line and velocity below 1.5 meter per second for better results and longer lasting age of  your micro irrigation system.



As you see all the units are in metric system of measurement. so if you are following units you have to convert the values
don’t panic just click on the last sheet named “Unit Conversion” here you can convert different units.



You can also calculate the Pump HP with the pump power calculator in the sheet,

There is a storage capacity calculator tab, so you can calculate the storage capacity of water tank it required.

At last a small video tutorial on head loss and velocity calculation in irrigation pipes,

Another small tutorial on head loss / friction loss in main lines,

That’s all for hydraulic calculation of irrigation pipes. 

You can find more information on drip irrigation systems design as under,

1. Basic aspects of drip irrigation system design
2. Technical aspects of drip irrigation system design
3. Drip irrigation system component installation
4. Video tutorial on drip irrigation systems design

Or scan a QR code here,

Don’t forget to reply if you have any suggestion / question.
cheers!!
Thanks