Why use this calculator
Australian irrigators buy, order and account for water in megalitres, but crops are watered in millimetres, and the conversion between them has to carry the losses in your system. One millimetre over one hectare is ten thousand litres, so a modest looking twenty five millimetre watering across a pivot is most of a full day of pumping and a meaningful share of an annual allocation.
Planning in megalitres lets you answer the questions that matter in a dry year: how many waterings the remaining allocation will cover, whether it is cheaper to buy temporary water or to shorten the season, and what the energy bill will be. It also puts a real cost on system inefficiency.
When you can see the megalitres that never reach the root zone, an irrigation audit or a nozzle replacement stops being a maintenance job and becomes an investment with a payback.
How to use it
- Choose your irrigation system, which loads a typical application efficiency, or choose custom and enter the figure from an irrigation audit of your own system.
- Enter the area actually watered, which for a centre pivot is the irrigated circle rather than the square paddock around it.
- Enter the net application depth in millimetres, which is the water you want in the root zone, taken from a soil moisture probe deficit or from crop evapotranspiration accumulated since the last watering.
- If you chose custom, enter your audited efficiency percentage.
- Enter the delivery flow rate in litres per second from your pump curve, a flow meter or your channel delivery agreement, so pumping hours can be worked out.
- Enter the cost per megalitre, combining temporary water price and pumping energy, or just energy if you are using your own allocation.
What each result means
- Gross water required
- Megalitres that must leave the outlet or the pump, and the volume to order or draw against your allocation.
- Cost of water lost each watering
- The dollar value of the water lost to evaporation, drift, runoff and deep drainage in this single watering, at the water and pumping cost you entered.
- Net water on the crop
- Megalitres that actually reach the root zone at the depth you asked for, which is what the crop responds to.
- Water lost to inefficiency
- Megalitres lost to evaporation, drift, deep drainage and runoff. Put a dollar value on this before dismissing an audit.
- Gross application depth
- Millimetres that have to be applied for the net depth to arrive, the figure to set a pivot speed against.
- Water per hectare
- Megalitres per hectare for one watering. Multiply by the waterings in a season for a seasonal water budget.
- Pumping time
- Hours of running at the flow rate entered, which drives labour, shift planning and off peak energy scheduling.
- Volume in cubic metres
- The same volume in cubic metres, the unit used on many meters and in water trading documentation.
- Water and pumping cost
- What this single watering costs in water and energy, before labour, maintenance and capital.
- Cost per hectare
- The per hectare cost of one watering, which belongs in the variable costs of the irrigated crop.
The formula
One millimetre of water over one hectare is 10,000 litres, which is 0.01 megalitres.
Net water in megalitres = area in hectares x depth in millimetres x 0.01
Gross water in megalitres = net water divided by the application efficiency percentage divided by 100
Losses = gross minus net
Gross application depth in millimetres = net depth divided by the efficiency fraction
Pumping hours = gross megalitres x 1,000,000 litres per megalitre, divided by the flow rate in litres per second x 3,600 seconds per hour
Cost = gross megalitres x cost per megalitre
How this number is used in practice
Irrigation scheduling is the decision this calculator supports, and the depth you enter should come from measurement rather than habit. The usual approach is to track the soil moisture deficit, either from capacitance probes or tensiometers in a representative part of the paddock, or by accumulating crop evapotranspiration from a nearby automatic weather station and subtracting rainfall.
Refill points differ by soil: a sand in the Riverland may hold only 30 to 40 millimetres of readily available water in the root zone and needs small, frequent waterings, while a clay loam in the Riverina can hold well over 100 millimetres and is watered less often but more heavily.
Applying more than the deficit does not store extra water, it drains past the roots and takes nitrogen with it.
Allocation reliability drives the whole season plan in the southern connected basin. Where entitlement is announced as a percentage of an allocation and can open low, growers plan a season water budget in megalitres per hectare, decide how many hectares they can reliably water, and treat temporary water purchases as the balancing item.
Running this calculation for a typical watering and multiplying by the expected number of waterings gives the seasonal figure that decision needs. In a dry year the sharpest question is usually not how to irrigate better but how many hectares to plant, and the answer comes from megalitres available divided by megalitres per hectare required.
Efficiency figures are a starting point, not a property of the badge on the machine. A well laid out border check bay with the right flow rate and a disciplined cut off can outperform a pivot with worn nozzles running at the wrong pressure. Pressure at the nozzle, sprinkler wear, wind at the time of day you irrigate, bay length and soil infiltration rate all move the real number, and only an audit measures it.
Shifting a pivot run from the middle of a hot windy afternoon to overnight can cut evaporation and drift losses noticeably, and on many tariffs it cuts the energy cost at the same time.
Worked examples
A 60 ha centre pivot applying 25 mm at 85 per cent efficiency, pumping at 60 litres per second, water and energy at $150 a megalitre
17.65 ML gross for 15 ML on the crop, 81.7 hours of pumping and $2,647
A 40 ha border check pasture bay applying 60 mm at 65 per cent efficiency with a 200 litre per second channel outlet
36.92 ML gross against 24 ML net, a gross depth of 92.3 mm and 51.3 hours of watering
120 ha of subsurface drip applying 20 mm at 92 per cent efficiency, 150 litres per second, water at $250 a megalitre
26.09 ML gross, only 0.217 ML per hectare, 48.3 hours of pumping and $6,522
When people use it
- Ordering channel water or drawing down an allocation for a planned watering
- Building a season water budget in megalitres per hectare to decide how many hectares to plant
- Estimating pumping hours and energy cost to schedule irrigation into off peak tariff windows
- Valuing the water lost to system inefficiency before deciding on an upgrade or an audit
- Comparing the water requirement of a surface system against a pivot or drip on the same paddock
Tips
- Schedule from measured soil moisture or crop evapotranspiration, not from the calendar. Applying more than the deficit drains past the roots.
- Know your readily available water. Sands need small frequent waterings, clays hold more and are watered less often.
- Irrigate overnight where you can. Lower evaporation and drift, and on many tariffs cheaper energy at the same time.
- Get an irrigation audit. Nozzle wear and wrong operating pressure quietly cost megalitres every season without ever showing up as a fault.
- Keep a season total in megalitres per hectare and compare it with your allocation early, while you can still change planted area.
- Check pump efficiency as well as the system. An oversized or worn pump can add substantially to the energy cost of every megalitre.
What it assumes, and where it stops being accurate
- Assumes a uniform application across the whole irrigated area. Distribution uniformity problems mean parts of the paddock are over watered while others stay dry.
- Assumes the efficiency figure is representative. Published values by system type are indicative only and a real audit often differs by ten percentage points.
- Ignores rainfall during or after the watering, which can turn a correct application into waterlogging on heavy soils.
- Not accurate for leaching fractions. Where salinity management requires extra water to push salt below the root zone, that volume must be added separately.
- Assumes the flow rate entered is sustained. Falling dam levels, a fouled filter, worn impellers or channel restrictions all reduce it in practice.
- Excludes labour, maintenance, capital recovery and any fixed water charges, which are often a large share of the real cost of irrigating.
Terms used on this page
- Megalitre
- One million litres or 1,000 cubic metres, the standard unit of Australian water entitlement and trade.
- Application efficiency
- The share of water delivered to a paddock that ends up stored in the crop root zone.
- Evapotranspiration
- Water lost from a crop by evaporation from soil and transpiration from leaves, the basis of irrigation scheduling.
- Readily available water
- The soil water a crop can extract without stress, between field capacity and the refill point.
- Allocation
- The volume announced against a water entitlement in a given season, often well below the full entitlement in dry years.
Questions people ask
- How many litres is one millimetre of irrigation?
- One millimetre of water over one hectare is 10,000 litres, or 0.01 megalitres. So 25 millimetres across 60 hectares is 15 megalitres of water in the root zone, and more than that has to leave the pump because no system delivers everything it pumps.
- What application efficiency should I use?
- As indicative figures, furrow is near 60 per cent, border check 65, travelling irrigators 75, centre pivots 85, lateral moves 88 and drip 90 to 92. These vary widely with design, maintenance and management, so an audit of your own system is worth far more than any published table.
- How do I know how much water the crop needs?
- Track the soil moisture deficit with probes or tensiometers in a representative part of the paddock, or accumulate crop evapotranspiration from a nearby weather station and subtract effective rainfall. Refill when the deficit reaches the readily available water for your soil and root depth.
- What is a megalitre?
- One million litres, or 1,000 cubic metres. It is the standard unit for Australian water entitlements, allocations, trades and storage. A megalitre spread over one hectare is a depth of 100 millimetres, which is a useful conversion to keep in your head.
- Should I include a leaching fraction?
- If you are irrigating with saline water or managing salinity in the root zone, yes. An extra fraction of water is applied deliberately to push salt below the roots, and that volume has to be added on top of the crop requirement this calculator gives you.
- Does this account for rain after irrigation?
- No. It sizes a single planned application. Rain falling soon after a watering can waterlog heavy soils and waste the water you just applied, which is why forecast checking is part of scheduling and why smaller more frequent waterings carry less risk in unsettled weather.
Use this with
Irrigation Water Calculator answers one part of the question. These pick up where it stops: Water Tank & Dam Volume Calculator, Rainfall & Soil Moisture Calculator, Crop Yield Estimator, Growing Degree Days Calculator and Cost Per Hectare Calculator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- Murray-Darling Basin Authority. Water entitlements, allocations and basin water accounting
- Bureau of Meteorology. Evapotranspiration, rainfall and automatic weather station data for scheduling
- Agriculture Victoria. Irrigation scheduling, system efficiency and water use efficiency guidance
- Food and Agriculture Organization of the United Nations. Crop water requirements and crop coefficient methodology
Irrigation Water Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.