Irrigation & Water

June 29, 2026

How to Calculate Water Needs Per Crop Per Week

Watering by feel or habit is one of the most costly mistakes in farming. Both under-watering and over-watering damage crops, but over-irrigation is far more common and far more wasteful. It leaches nutrients below the root zone, promotes root disease, and in irrigated regions contributes significantly to water scarcity.

The solution is to calculate crop water needs based on science, not guesswork. This guide shows you a practical, field-usable method that any farmer can apply without expensive equipment.

The Core Concept: Evapotranspiration

Water leaves the soil and plant system in two ways. Evaporation from the soil surface and transpiration through plant leaves. Together, these are called evapotranspiration, abbreviated as ET.

To know how much water a crop needs per week, you need to know how much ET occurred that week and how much rain fell. The difference is what you need to replace through irrigation.

The formula is:

Irrigation Requirement = (Crop ET) minus (Effective Rainfall)

You can use the Evapotranspiration Calculator to get your reference ET value based on your local temperature, humidity, wind, and sunshine hours.

Step 1: Determine Reference ET (ETo)

Reference evapotranspiration (ETo) is the water demand of a standardized reference crop (typically well-watered grass). It gives you a baseline water demand based on weather conditions, expressed in millimeters per day or per week.

In hot, dry summer conditions, ETo can reach 7 to 9 mm per day. In cool, humid spring weather, it may drop to 2 to 4 mm per day.

Weather stations in your area typically publish ETo data. If you do not have access to a local station, online tools using temperature and solar radiation data can give a reasonable estimate.

Step 2: Apply the Crop Coefficient (Kc)

Not every crop uses water at the same rate as the reference grass. The crop coefficient (Kc) adjusts ETo to match the actual water use of your specific crop at its current growth stage.

Crop ET (ETc) = ETo x Kc

Kc values change as the crop grows. They are lower during early vegetative stages, peak at full canopy, and decline as the crop matures and senesces.

Crop	Kc Initial	Kc Mid-Season	Kc End Season
Tomatoes	0.6	1.15	0.8
Wheat	0.3	1.15	0.4
Maize (corn)	0.3	1.20	0.6
Potatoes	0.5	1.15	0.75
Onions	0.7	1.0	0.75
Cotton	0.35	1.2	0.6
Sugarcane	0.4	1.25	0.75

These are standard FAO values. Local extension services may have region-specific Kc values that are more accurate for your climate.

Step 3: Calculate Weekly Crop Water Need

Put it all together with a weekly example:

Example: Tomatoes in mid-season, with an average ETo of 6 mm/day for the week.

Daily ETc = 6 mm x 1.15 (Kc mid-season) = 6.9 mm per day

Weekly ETc = 6.9 x 7 = 48.3 mm per week

If 10mm of effective rainfall fell during the week:

Irrigation need = 48.3 minus 10 = 38.3 mm per week

This means you need to apply 38.3mm of irrigation water per week to this tomato field.

For automatic weekly calculations using your field data, use the Crop Water Requirement (ETc) Calculator which handles the Kc adjustments and rainfall inputs automatically.

Step 4: Convert mm to Volume

Millimeters of water is a depth measurement. To convert it to actual volume for your field:

Volume (liters) = Irrigation depth (mm) x Field area (square meters)

For a 1-hectare field (10,000 square meters) requiring 38.3mm:

Volume = 38.3 x 10,000 = 383,000 liters = 383 cubic meters

This is how much water your pump or tank needs to deliver that week.

Step 5: Adjust for Soil Water Holding Capacity

You do not always need to irrigate every day. Soil acts as a water reservoir. Understanding your soil’s field capacity and allowable depletion means you can irrigate less frequently with larger applications, which is more practical for many farms.

The general rule: irrigate when the soil has dried to about 50 percent of available water capacity (AWC). This is called the management allowable depletion (MAD) threshold.

The Soil Moisture Deficit Calculator helps you track how much water your soil has used since the last irrigation and tells you when the next irrigation is due.

Worked Examples for Common Crops

Wheat (Temperate Climate, Mid-Season)

ETo: 5.5 mm/day | Kc: 1.15 | Rainfall: 15mm for the week

ETc = 5.5 x 1.15 = 6.3 mm/day | Weekly ETc = 44.3 mm

Irrigation need = 44.3 minus 15 = 29.3 mm/week

Onions (Hot, Dry Climate, Early Stage)

ETo: 7 mm/day | Kc: 0.7 | Rainfall: 0mm for the week

ETc = 7 x 0.7 = 4.9 mm/day | Weekly ETc = 34.3 mm

Irrigation need = 34.3 minus 0 = 34.3 mm/week

Maize (Tropical Climate, Full Canopy)

ETo: 6 mm/day | Kc: 1.20 | Rainfall: 25mm for the week

ETc = 6 x 1.20 = 7.2 mm/day | Weekly ETc = 50.4 mm

Irrigation need = 50.4 minus 25 = 25.4 mm/week

Common Mistakes in Estimating Crop Water Needs

Using the same irrigation schedule regardless of weather (clouds and rainfall change everything)
Ignoring growth stage changes in Kc values
Counting total rainfall instead of effective rainfall (heavy rain with runoff is not all absorbed)
Not adjusting for irrigation system efficiency (drip systems deliver water more efficiently than sprinklers)
Applying water in the hottest part of the day, increasing evaporation losses significantly

Tools and Resources That Make This Easier

The Irrigation Water section on FoodsFarming has a full suite of tools including the Drip Irrigation System Designer, Soil Moisture Deficit Calculator, Sprinkler Irrigation Calculator, and more, all designed to work together for complete irrigation management.

If you are using a drip system, you can also cross-reference with the Water Use Efficiency Calculator to see how much water your system is delivering versus what your crop actually requires.

Frequently Asked Questions

How much water does a tomato plant need per week?

A mid-season tomato crop typically needs 40 to 55mm of water per week in hot summer conditions, equivalent to roughly 400 to 550 liters per 10 square meters.

What is the crop coefficient (Kc)?

Kc is a factor that adjusts reference evapotranspiration to match the actual water use of a specific crop at a specific growth stage. FAO publishes standard Kc values for most crops.

How do I know if my crop is getting too much water?

Signs of overwatering include yellowing lower leaves, root rot, waterlogged soil, and poor drainage. Soil moisture monitoring helps confirm.

Does crop water need change with seasons?

Yes, significantly. Water need is highest during hot, dry, and sunny weather and lowest during cool, cloudy periods. Adjust your schedule weekly based on ETo data.

What is effective rainfall?

Effective rainfall is the portion of total rainfall that is actually absorbed by the crop’s root zone. Heavy rain on dry clay soil produces runoff and has low effectiveness.