Sizing a storage tank for irrigation.
A tank lets a slow source, or a pump that runs only part of the day, keep up with a system that waters faster. Its size depends on how much you draw each day, when you draw it, how fast it refills, and how many days it must last with no refill. This guide works all four through for one site, hour by hour, and ends with what to check before you buy.
Written for farmers, greenhouse and nursery growers, homeowners and estate and facility managers.
What the tank is for
Flow is how much water passes in a set time: litres per hour (L/h) for a dripper, or cubic metres per hour (m³/h) for a pump or a pipe. One cubic metre is 1,000 litres. A tank lets the flow coming in differ from the flow going out.
A tank does one or more of three jobs. It balances a source that gives water slowly against zones that use it fast. It carries the system through the hours the pump cannot run, such as the night for a solar pump. And it holds a reserve for days with no refill: a dull spell, a broken pump or a late tanker.
FAO's manual shows what a night store can do. With one, water can be drawn from the source around the clock at peak demand, which in its example cut the flow the supply had to carry to less than half.
Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design; FAO (2002), Irrigation Manual Module 7: Surface Irrigation Systems, section 3.4: System capacity; University of Wyoming Extension (2016), Solar-Powered Water Pumping Systems for Livestock, Part 1: Overview and Costs, B-1289.1.
- Inflow: water from the pump or the supply comes in at the top
- Overflow: carries away any water once the tank is full
- Usable water: everything between the outlet and the overflow
- Outlet: where the water leaves for the field
- Water below the outlet: it stays in the tank and cannot be used
- The pipe to the field, with a valve for each zone
Four numbers to gather
- Daily demand: the water the site uses on its driest day, in cubic metres
- When it is used: which zones run, at what flow, and at what times of day
- Refill: how fast water comes in, in cubic metres per hour, and between which hours
- Days without refill that the tank must cover: nights, dull days, repairs or a late tanker
Working out a crop's daily waterHow to check what your source givesA solar pump's flow through the day
Working it out hour by hour
The tank must hold the largest shortfall between what goes out and what comes in over the day. A daily total hides this: a source that gives exactly the daily need still leaves the tank empty if the zones run before the pump starts. So go through the day hour by hour, add what comes in, take away what goes out, and find the lowest point.
Watering zone by zone keeps the flow out of the tank to one zone at a time. A zone's run time is the water it needs divided by its flow, and the zones follow one another until the whole area is watered.
Source: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design.
The water you cannot use
Not all of a tank's water can be drawn off. The water below the outlet stays behind, and the space above the overflow never fills. The usable volume is what lies between the two, so use that figure, not the size on the label, when you compare tanks.
Ask the supplier for the height of the outlet above the floor and of the overflow below the top. On a tall, narrow tank they cost less of the volume than on a short, wide one, because each centimetre of depth holds less water.
How many days to store
For days without refill, multiply the daily use by the number of days. Extension guides on solar pumping for livestock suggest storing three days of use, or 3 to 10 days for livestock and homes depending on the climate. A crop is different: how many days to hold is a judgement between the size of the tank and the risk of the crop going short.
With a solar pump the tank does the work of a battery: water pumped by day is kept for the night and for dull days.
A reserve only helps if the refill can top it up as well as meet the day's use. If the refill only just matches the daily demand, as in the example above, a reserve that has been drawn down never catches up.
Sources: Mississippi State University Extension (2023), Solar-Powered Groundwater Pumping Systems for Livestock Watering, P3236; New Mexico State University (2014), Designing Solar Water Pumping Systems for Livestock, Circular 670; University of Wyoming Extension (2016), Solar-Powered Water Pumping Systems for Livestock, Part 1: Overview and Costs, B-1289.1.
Raised tanks and pressure
Pressure is how hard the water pushes inside the pipe, measured in bar. Head is pressure given as a height of water, in metres: one bar is about 10 metres of head. A raised tank gives pressure from the height of its water above the outlet, less what friction in the pipes takes.
FAO notes that water enters drippers at about 1.0 bar, which is about 10 metres of head. A tank on a stand a few metres high gives only a few tenths of a bar, so a drip line that needs 1.0 bar would need a booster pump, unless its maker's sheet says it works at less.
The pressure also falls as the tank empties, because the water surface drops. Check the zones still get what they need when the tank is nearly empty.
A stand carries the full tank: about a tonne for each cubic metre of water, so more than 20 tonnes for a 20 m³ tank. Have the stand and its footings designed for that load.
Booster pumps and other pumpsPressure regulators for drip
Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3, section on drippers.
Before you buy a tank
- The usable volume between the outlet and the overflow, not only the size on the label
- Where the inlet, outlet, overflow and drain sit, and what size each is
- A firm, level base that will carry the full tank: each cubic metre of water weighs about a tonne
- A tight lid, to keep out dirt, insects and animals
- Where the overflow water will go
- A float switch that stops the pump when the tank is full. A float valve on its own suits a mains or tanker supply; on a pumped line it shuts against the running pump, so fit it only with a pressure switch or controller that stops the pump
- A way in to clean out the silt that settles below the outlet
- What the warranty covers, and for how long
Irrigation pumpingValves and automation
Want a tank sized for your site?
Send your daily water use, how the tank fills and when you water, through the form or on WhatsApp. We will check the tank and the pump together with you.