Irrigation Scheduling: How Much, and When

The two questions

Every irrigation decision is two questions, and most systems answer neither.

When to water — which depends on how much water the soil has left.

How much to apply — which depends on how much the soil can hold.

A timer set to run every evening answers both wrong, most of the time. It waters in the rain, it waters when the soil is already full, and it applies the same amount in April and August.

Evapotranspiration

The demand side. ET is the combined water loss from evaporation off the soil surface and transpiration through the plant.

It is driven by radiation, temperature, humidity, and wind, and it varies enormously: perhaps 1 mm a day in a cool damp spring, 6 to 8 mm a day in hot dry windy conditions.

The standard framework separates it into two parts:

ET₀, reference evapotranspiration — the demand a short green grass surface would experience under that day's weather. Published daily by many meteorological services and available from weather stations and apps.

Kc, the crop coefficient — a multiplier for the specific crop at its specific growth stage. A newly sown crop with bare soil between plants might be 0.3; the same crop at full canopy might be 1.15; a senescing one drops again.

ETc = ET₀ × Kc gives the actual water use of that crop today.

You do not need to do this arithmetic to grow vegetables. You do need the concept, because it explains why watering needs change by a factor of five across a season and why a fixed schedule is always wrong somewhere.

Soil moisture deficit

The supply side, and the more useful idea for most growers.

Think of the soil as a reservoir. It fills to field capacity — the amount it holds against gravity after free drainage — and plants draw it down until they reach permanent wilting point, where the remaining water is held too tightly to extract.

The difference between the two is available water capacity, and it varies with texture:

Sand holds roughly 60 to 100 mm of available water per metre of soil depth. Loam roughly 140 to 200 mm. Clay roughly 150 to 200 mm, though a larger share is held too tightly to use.

Organic matter increases it substantially — this is the practical mechanism behind the claim in long-term soil care and organic matter building that soil carbon buys drought resilience. A soil raised from 2 to 5 percent organic matter holds meaningfully more plant-available water, and that shows up directly as days between irrigations.

The rule of thumb: irrigate when about 50 percent of available water has been used, and refill to field capacity. Waiting longer stresses the crop; watering sooner wastes water and leaches nutrients.

Measuring it without instruments

The feel test. Take soil from rooting depth and squeeze it. Sandy soil that will not form a ball is dry; loam that forms a ball and leaves a wet outline on the palm is near capacity. Crude, free, and much better than nothing.

Dig a hole. Nothing tells you more about whether your irrigation is reaching the root zone than digging 20 to 30 cm the day after watering. People routinely discover they have been wetting the top 3 cm for years.

Tensiometers and moisture meters give a number, and cheap ones are unreliable — treat them as trend indicators.

Weigh a container. For pots, lifting is the most accurate practical method there is. See container growing.

When water stress actually matters

Not all growth stages are equally sensitive, and this is where scheduling pays.

Water stress at a critical stage costs yield disproportionately; the same stress at another stage may cost nothing, or even improve quality.

Flowering and fruit set is critical in almost everything — peas and beans, tomatoes, squash, sweetcorn at tasselling.

Tuber and root bulking — potatoes from tuber initiation onward.

Head and heart formation in brassicas and salad leaves.

Establishment, for everything, and for the first two summers of a tree. See tree planting.

Equally, deliberately withholding water late improves several crops: onions and garlic cure better, storage roots keep better, and fruit concentrates in flavour. Regulated deficit irrigation is standard practice in wine grapes and stone fruit for exactly this reason.

So the question is not just how dry the soil is, but what the crop is doing this week.

Deep and infrequent

The most consequential habit, and the one most often got wrong.

Frequent light watering wets only the surface, encourages shallow rooting, loses a high proportion to evaporation, and leaves the plant dependent on the next watering.

Deep infrequent watering fills the profile, drives roots downward, and buys days of resilience.

The exception is anything with a genuinely small root volume — seedlings, containers, and shallow-rooted salad — where frequent watering is unavoidable.

See deep watering and watering timing.

Application efficiency

How much of what you apply reaches the root zone:

Flood and furrow: 40 to 60 percent. Overhead sprinkler: 65 to 80 percent, and worse in wind and heat. Drip and subsurface: 85 to 95 percent. See drip irrigation.

Time of day matters. Watering at midday in hot windy conditions can lose a substantial fraction of a sprinkler's output before it lands. Early morning is best — low evaporation, and foliage dries quickly, which matters for fungal disease.

Evening watering is second best and leaves foliage wet overnight, which favours mildew and blight. See tomatoes.

Mulch changes the arithmetic more than any equipment upgrade, by suppressing surface evaporation. See mulching for moisture.

The efficiency paradox

Worth stating because it catches out whole irrigation districts.

Improving application efficiency does not automatically save water at catchment scale. Water lost to inefficient irrigation frequently returns to the aquifer or the river and is used again downstream. Replacing it with high-efficiency drip can increase total consumptive use, because more of the water is transpired and less returns — and because growers commonly expand the irrigated area with the water they appear to have saved.

This is the Jevons paradox applied to irrigation, and it is well documented. It also has a specific agronomic consequence: less drainage means less salt leaching, which accelerates salinisation. See salinity and aquifer recharge.

Efficiency is worth pursuing on a farm. It is not, by itself, a catchment water policy.

Where scheduling fits

The cheapest water is the water you do not need to apply, so scheduling sits fairly late in the sensible order:

Reduce demand through mulch, shade, windbreaks, and hydrozoning. Increase soil storage through organic matter. Harvest and store rainfall. Then irrigate what is left, accurately.

See hydrozoning, soil water storage, and rainwater harvesting basics.