The Irrigation Calculator builds a watering plan from the weather and the crop's stage rather than from a fixed schedule.
Most irrigation advice is a habit: every third day, or whenever the top of the soil looks dry. Both are wrong in opposite directions in the same season — a crop at flowering in a dry spell needs water a rule-of-thumb will not give it, and the same crop after rain is being drowned on schedule.
It reads evapotranspiration and the recent weather, so the plan responds to what the atmosphere has actually been taking out of the crop, and adjusts for the growth stage, because the same field needs different water at establishment and at grain fill.
It is explicit about what it cannot know: your soil's water-holding capacity, your pump's output, and how long your channel takes to reach the far corner. It is a schedule to adjust, not one to obey, and it says so on the screen.
It pairs with the weather screen's soil moisture at five depths, which is the check on whether the plan matched reality — plenty of water at 3–9 cm and nothing at 27–81 cm means the last irrigation never reached the roots.
It is a paid tool, and it is aimed at the single largest controllable cost on most farms after labour.
How to use Irrigation Scheduler
What it is. A day-by-day watering schedule for the next 7 to 16 days, worked out from the real forecast for your location using the FAO-56 method: reference evapotranspiration (ETo) multiplied by the crop's published coefficient (Kc) to give crop water requirement (ETc), then adjusted for the chance of rain, and finally converted into litres for the size of field you actually have.
When to open it. At the start of a week, before you decide which days the pump runs. Also worth opening the day before you were going to irrigate, because the single most useful thing it does is tell you not to.
Have ready.
Where the field is — 'Use my location', or pick from global locations.
Which of the sixteen crops it is: wheat, rice, maize, cotton, sugarcane, soybean, groundnut, sunflower, potato, tomato, onion, barley, sorghum, alfalfa, banana or coffee.
The size of the field, in whichever unit you think in.
Step by step.
Set the location. This is not a formality — ETo is computed from that location's own forecast temperatures, so the wrong town gives the wrong answer.
Choose the crop. This sets Kc, and Kc is the difference between watering sugarcane and watering coffee on the same day in the same weather.
Enter the field size.
Choose the planning horizon: 7 days for deciding what the pump does this week, 16 for a season plan or a water budget. Sixteen days is the longest live daily forecast that exists anywhere, from anyone.
Press 'Get Weather-Based Schedule' and read the day-by-day table. Each row gives ETo, ETc, the rain chance, and what to actually apply.
Export the schedule report if you need it on paper — for a water user association, a lender, or simply for the person who operates the pump.
What it will not do. It is a forecast-driven crop water requirement, not a soil water balance. It does not know how much water is already in your soil, what your irrigation system wastes, how deep the roots have gone, or the growth stage of this particular crop — Kc is the mid-season peak throughout, which is deliberately conservative and deliberately disclosed. It also cannot know rain that has already fallen on your field.
Every setting in Irrigation Scheduler, explained
Location
The place whose forecast drives the whole calculation. How to choose: The field, not the town you live in, if they are far apart. Temperature and humidity are what ETo is built from. If it is wrong: Every figure below is for somewhere else. This is the input most worth getting right and the one most often left on a default.
Crop
Sets the crop coefficient Kc — how much water this crop uses compared with the reference grass surface that ETo describes. How to choose: The nearest of the sixteen. The published Kc values used are FAO-56 Table 12 mid-season figures: sugarcane 1.25, rice and maize 1.20, cotton 1.175, wheat, soybean, groundnut, potato, tomato and barley 1.15, banana 1.10, sunflower 1.075, onion and sorghum 1.05, alfalfa and coffee 0.95. If it is wrong: A 30% error in water applied is easy here — coffee at 0.95 against sugarcane at 1.25 is a third more water on the same day.
Field size
The area being irrigated, which turns a depth in millimetres into a volume in litres. How to choose: The area actually under this crop and this pump, not the whole holding. If it is wrong: The mm-per-day advice stays right and the litres figure is wrong in exact proportion — so the pump runs for the wrong length of time.
Planning horizon
How many days ahead the schedule runs: 7, 10, 14 or 16. How to choose: Seven for a real field decision this week. Sixteen for planning a water budget, knowing that a forecast fourteen days out is a shape, not a promise. If it is wrong: Nothing is miscalculated. A long horizon simply carries more uncertainty in its later rows, and should be re-run as the days arrive.
ETo source
Whether the day's reference evapotranspiration is the real server-computed Penman-Monteith value from live weather, or Laisha's own Hargreaves-Samani estimate from the forecast minimum and maximum temperature. How to choose: You do not choose it; the app tells you which it used, on every row, and marks the estimated ones. If it is wrong: Nothing, but it is worth knowing. Hargreaves-Samani uses temperature alone, so on a very humid or a very windy day it is a rougher figure than Penman-Monteith, which uses humidity, wind and radiation too.
A worked example
Two acres of wheat. The forecast for tomorrow gives an ETo of 5.0 mm and a 15% chance of rain.
What goes in:
Crop: Wheat, Kc 1.15
ETo 5.0 mm
Field size 2 acres
Rain chance 15%
What comes out:
ETc = 5.0 × 1.15 = 5.75 mm of water the crop will use
Rain chance under 25%, so the recommendation is the full 5.8 mm
5.75 mm over 2 acres = about 46,500 litres, because 1 mm over 1 acre is 4,046.86 litres
The same day at a 30% rain chance would recommend 2.9 mm — about 23,500 litres — and at 55% it would say skip
That last line is the tool's whole value. The difference between irrigating and not irrigating on a day when rain is likely is 46,500 litres of water and a pump-hour of diesel, repeated across the season.
How to read what it gives you
ETo is the reference: how much water a short green grass surface would lose that day. It is a property of the weather, not of your crop.
ETc is ETo multiplied by Kc, and is what your crop will actually use that day. This is FAO-56 Equation 58, the standard method worldwide.
The Recommended column is ETc after the rain rule: at 50% or more chance of rain it says skip and recommends nothing, between 25% and 50% it halves the figure, and below 25% it recommends the full amount.
The temperatures beside each row are there so you can see what drove the number. A 42°C day and a 24°C day do not ask for the same water, and the table shows you why it changed.
Litres are just millimetres times area: one millimetre of depth over one acre is 4,046.86 litres. If you know your pump's discharge, this converts straight into running time.
What usually goes wrong
Leaving the location on somewhere you are not. ETo is entirely local and the whole schedule inherits the error.
Treating the figure as a delivery target rather than a crop need. It is what the plant will use; what you must apply is more, by however much your system wastes — flood irrigation commonly delivers 50 to 60% of what leaves the pump, drip 85 to 95%.
Ignoring soil. A sandy soil holds a few days of this; a heavy clay holds a fortnight of it. The schedule says how much water is leaving, not how much your soil still has.
Reading day 14 of a 16-day horizon as a fact. Re-run it when the day is three days out.
Assuming one Kc fits the whole season. The figures used are mid-season — the peak. Early after sowing, a crop uses far less, so this will over-recommend for a young crop, and it says so.
Questions people ask about Irrigation Scheduler
How much water does one acre of wheat need per day?
It depends entirely on the weather that day. With an ETo of 5 mm and wheat's Kc of 1.15, the crop uses 5.75 mm, which over one acre is about 23,300 litres. On a cool overcast day the same field might need a third of that. That is why this is worked out from a live forecast rather than from a rule of thumb.
What is ETo, and what is ETc?
ETo, reference evapotranspiration, is how much water a standard short grass surface would lose in that day's weather — a property of the climate. ETc is ETo multiplied by the crop coefficient Kc, and is what your specific crop uses. The method is FAO Irrigation and Drainage Paper 56, which is the international standard.
How many litres is 1 mm of irrigation?
One millimetre of water depth over one acre is 4,046.86 litres, and over one hectare it is 10,000 litres. Millimetres are a depth, so the volume depends only on the area you are covering.
Should I still irrigate if rain is forecast?
The schedule decides this for you: at a 50% or higher chance it recommends skipping the day entirely, between 25% and 50% it halves the amount, and below 25% it recommends the full requirement. That rule is deliberately simple, and it is the part of the tool that saves the most water and diesel.
Why does it say the ETo figure is estimated on some days?
When the weather service returns its own computed Penman-Monteith ETo, that real figure is used. When it does not, Laisha estimates ETo from the day's forecast minimum and maximum temperature using the Hargreaves-Samani equation, and marks the row as estimated. Hargreaves-Samani uses temperature alone, so it is rougher in very humid or very windy conditions — you are told which one you are looking at rather than being left to assume.
Does it account for my soil type or my irrigation system?
No, and it says so on the assumptions panel. It gives the crop's water requirement. Your soil decides how long that water lasts between waterings, and your system's efficiency decides how much you must pump to deliver it — flood irrigation commonly loses 40 to 50% of it, drip very little.
How far ahead can it plan?
Up to 16 days, which is the longest live daily forecast available from any weather service. Seven days is what to use for deciding what the pump does this week; the longer horizons are for budgeting water, and should be re-run as the days get closer.