Water window

As an irrigator, you may have heard the term “water window.” Recently, there’s been debate about water-window efficiency.

Water window is the window of time available to water. It is the days per week and the hours per day you can water. And the water window varies with what you are watering. For example, a house may have a six-day-per-week, eight-hours-per-day window versus a softball field with a three-day-per-week, five-hours-per-day window.

Water window is dependent on variables, such as soil type, root-zone depth, pipe size and control-system and pump-system capacity. Ideally, you would irrigate as late as possible to minimize disease pressure. But how late is late? Is it 8 p.m.? Midnight? And when do you finish? Usually, you’d want to be done when the staff goes out to mow and you certainly want to be done before play starts.

A golf course could have a six-days-per-week, six-hours-per-day water window. That’s 36 hours per week starting at 11 p.m. and done by 5 a.m. While that’s what you would like, it most likely is not what you have.

Time defines the water window, but what dictates it?  In most cases, pump-system capacity defines it. If you want to apply .15 inches of water on 70 acres, that is an exact quantity of water – 285,117 gallons. It does not change. The pump system’s flow dictates how long it takes to apply 285,117 gallons – that’s the water window. Lower the flow and it’ll take longer to irrigate. Raise the flow and it’ll take less time. If the flow is 1,000 gpm, then it would take just under five hours to apply the 285,117 gallons. If the flow is 1,200 gpm, then irrigation would take four hours.

You can manipulate the water window to fit your water management style. You may not want to water every day, so you water greens and tees one night and fairways the other. That way you’re watering every other day, but not needing additional pump system capacity. There are many possibilities.

On many courses the water window is not limited by the pump-system capacity but by the piping or control system. An older control system – especially the mechanical type, but also some solid state – will not have the ability to turn sprinklers on as fast as you would like. Many older controllers are limited to how many sprinklers the controller can operate at a time and in the sequence they are operated. The system takes longer to complete an irrigation cycle as you have to wait for it to turn on all the sprinklers. If your pipe size is limited, the pipe does not have the capacity to carry the flow available without excessive pressure losses or creating high velocities that cause surge pressures and damage the system. If you have small pipe and an old control system, then you will have a long water window.

Fixing a long water window may require the purchase of a new pump system, a new control system or a new piping system. Identify which of the three is causing the issue. Decide which replacement would lower the water window the most. Do an analysis. Improve piping by adding mainline or connecting holes to make loops. A field controller upgrade or the addition of a central-control system might help, too. 

So what is “water-window efficiency”? Water-window efficiency is a comparison of what your pump station actually pumps versus its capacity. For example, if your pump station is rated at 1,200 gpm and you operate it at 900 gpm, then your water window efficiency is less than ideal.

To calculate it, take your irrigation cycle water use and divide it by your pump-station capacity. This gives you the shortest possible runtime in minutes. Then, to calculate the efficiency, take the shortest possible run time and divide it by how long it takes to irrigate in minutes. So, if the irrigation cycle uses 310,000 gallons with a 1,200 gpm capacity, it would take 259 minutes. If the actual watering time was 423 minutes, the calculated efficiency would be 61.2 percent.

This indicates you are not operating as efficiently as possible. Is this bad? There could be many reasons for why you don’t operate at maximum capacity. For example, your pump station is old, your central control system database is poor, you water from stand-alone controllers and need to build in a buffer, or your piping system can’t handle the capacity. This water-window efficiency is a tool to measure performance or improvements. Remember, the closer your pump station operates to design capacity, the more electrically-efficient it should be.

Water window is important to comprehend when operating an irrigation system. Knowing your water window helps you understand the relationship between your irrigation system and pump station, which makes you a more efficient irrigator.