Managing Irrigation Headloss
By Adrian Smith, Senior Land Services Officer - Mixed Farming Systems
Headloss? What the dickens is headloss? I hear you say.
Well, headloss is a very important component of any system where water (or any fluid, for that matter) flows.
In the context of our gravity irrigation systems, head is the energy that drives water through the system – it is a measure of water pressure, measured in millimetres (mm), metres (m) or kilopascals (kPa).
Surface irrigators are mainly interested in the head associated with elevation and gravity. Maintaining head in surface irrigation is important, because the energy maintains flow rate and the movement of water through the system.
Headloss is seen anywhere there is a drop in water level as water moves through the system - be it along a supply channel or through a structure. Importantly, headloss accumulates through the entire length of your system.
For water to flow, there must be some headloss. The aim is to minimise the amount of headloss the system generates. Excessive headloss will result in reduced flow rates and command over fields, and is the result of poor design, construction and/or maintenance of the irrigation system.
Command is the term that describes the difference between the water level in the supply channel and the ground level of the field or bay being irrigated. If there is no command, there will be no flow!
Where do we see headloss?
As mentioned previously, for water to flow under gravity, there needs to be some headloss. The aim is to minimise it (generally!).
Headloss is the result of friction. So as water flows along a channel, there will be friction losses that result. Even along a ‘flat’ bedded channel, we will see headloss (a drop in water height) from the (start) supply point to the (end) of the channel.
As soon as we start to install structures like pipes into the system, we create more headloss. Anywhere that water flow is restricted or constricted (such as a reduction in surface area created by installing a pipe culvert), then we will see headloss. Even a change in direction (such as going around a bend) will generate headloss.
Some important 'rules of thumb' to remember are:
- Headloss varies with flow rate and cross-sectional area:
- increasing flow rate increases headloss (doubling the flow rate increases headloss by a factor of 4).
- increasing cross-sectional area will decrease headloss (doubling pipe diameter allows 4 times the flow).
- Pipes run most efficiently (and minimise headloss) when they are running full and both ends are submerged.
- Incorrectly sized (smaller) structures, incorrect installation (in particular installed too high) and weed growth will all result in significant headloss.
- Increasing the length of structures results in minor increases in headloss.
- Headwalls (both up and downstream) on pipes reduce turbulence and headloss.
Importantly, the correct irrigation design, coupled with accurate installation and construction, and undertaking good maintenance practices will minimise headloss.
The designer of your irrigation system will take headloss into account when completing your system design, and should identify the correct levels at which structures should be installed, channel beds built and fields landformed to. They do this for a reason – to minimise headloss and maximise the available flow rate and command, which ultimately means your irrigation system operates to its best potential.
The following figure provides a graphic representation of where headloss is generated through an irrigation system. Importantly, you can see the cumulative effect headloss has on command. Point A identifies the full supply level as water enters your farm, while Point B shows the water level over the field or paddock being irrigated. You can see how water flowing through a structure, weeds in a channel and water flowing from the channel onto the bay surface all result in headloss, and affect the depth of water (and flow) that can be delivered onto the surface of the bay.
What’s the bottom line?
Headloss in our gravity irrigation systems is important – if there is no headloss, there is no water flow. Generally, however, the trick is to miminise its effects in order to maximise flow rates and command over your irrigation fields.
Should you have any concerns about how your system is operating (or will operate), talk to your irrigation designer in the first instance. Otherwise staff at Murray Local Land Services are happy to assist with identifying issues and potential solutions.
Typical headloss through supply points, structures and along farm channels. NSW Agriculture, 2002. 'Evaluating your surface irrigation system' in Introduction to Irrigation Management. NSW Agriculture, Orange, NSW.