UNL Irrigation Calculator: How to Use It

The UNL Irrigation Calculator is a free online tool from the University of Nebraska–Lincoln that estimates how much water (in inches) to apply to a crop or lawn based on your location, soil type, and current weather. You enter your zip code, select a crop and soil, and it pulls local evapotranspiration (ET) data to give a recommended irrigation amount. It is widely used by farmers and turf managers in Nebraska and neighboring states to avoid over- or under-watering.

Formula

The calculator above computes the volume of water needed to apply a certain depth (in inches) over a rectangular area. The core relationship is:

Volume (ft³) = Length (ft) × Width (ft) × Depth (in) / 12

To convert cubic feet to cubic yards, divide by 27 (since 1 yd³ = 27 ft³). Combining both steps gives:

Volume (yd³) = (L × W × D) / 324

where L is the length in feet, W is the width in feet, and D is the depth of water in inches. The factor 324 comes from (12 inches per foot × 27 cubic feet per cubic yard).

Worked Example

Using the default values from the calculator block: length = 40 ft, width = 25 ft, depth = 3 inches.

1. Compute cubic feet: 40 × 25 × 3 / 12 = 3000 / 12 = 250 ft³
2. Convert to cubic yards: 250 / 27 = 9.259 yd³ (rounded to 9.26)

The calculator displays 9.26 yd³ and 250 ft³, matching the example.

What the UNL Irrigation Calculator Actually Does

The University of Nebraska–Lincoln’s tool (often called the “UNL Water Calculator� or “Crop Water Use Calculator�) is different from the simple volume calculator above. It uses a water balance approach:

• It starts with the current soil moisture (you input it or it uses a default).
• It subtracts daily crop evapotranspiration (ET), which is estimated from weather station data (temperature, humidity, wind, solar radiation).
• It adds rainfall (from the same weather data).
• When the available soil moisture drops below a threshold you set, it tells you to irrigate and how many inches to apply to refill the root zone.

The tool is based on the Penman-Monteith equation, the standard method for estimating reference ET. It then applies crop coefficients (Kc) for common crops like corn, soybean, alfalfa, and turf.

How to Use the UNL Irrigation Calculator

Follow these steps to get a recommendation:

1. Go to the UNL irrigation calculator website (search “UNL irrigation calculator� or find it through the Nebraska Extension page).
2. Enter your zip code or select a nearby weather station.
3. Choose your crop from the dropdown (e.g., corn, soybean, turf).
4. Select your soil type (sandy, loam, clay) – this affects available water holding capacity.
5. Enter the current soil moisture (if known) or use the default.
6. Set the management allowed depletion (MAD) – typically 50% for most crops, 40% for high-value vegetables, 60% for some field crops.
7. Click “Get Irrigation Recommendation.� The tool will show the last few days of ET, rainfall, and a recommendation like “Irrigate 1.2 inches.�

You can also use it to track daily soil moisture depletion over the season.

Why Use It? Benefits and Limitations

Using an ET-based irrigation schedule can reduce water use by 20–40% compared to a fixed schedule, according to general extension guidance. The main benefits:

• Prevents overwatering, which wastes water and leaches nutrients.
• Prevents underwatering, which stresses plants and reduces yield.
• Uses local weather data, so it adapts to heat waves and rain events.

Limitations:

• It requires a reliable internet connection and a weather station near you.
• It assumes uniform soil and irrigation system efficiency – if your system has poor uniformity, you need to adjust.
• It doesn’t account for runoff or deep percolation losses; you may need to apply more if your soil is compacted or sloped.

Common Mistakes When Using the UNL Calculator

Even experienced users make these errors:

• Wrong soil type – sandy soil holds less water, so the calculator will recommend more frequent, smaller irrigations. If you select clay when you have sand, you’ll under-irrigate.
• Ignoring rain – the calculator includes rainfall automatically, but if your weather station is far away, actual rain at your site may differ. Check your own rain gauge.
• Setting MAD too high or low – a MAD of 80% means you wait until the soil is very dry, which can stress the crop. For corn, 50% is typical.
• Not updating soil moisture – if you enter a wrong starting moisture, the recommendations will be off for days. Update with a soil moisture sensor if possible.
• Using the volume calculator for irrigation scheduling – the volume calculator above tells you how many cubic yards of water to order, but it doesn’t tell you when to irrigate. That’s what the UNL tool is for.

Comparing UNL Calculator to Other Irrigation Tools

There are several similar tools:

• California’s CIMIS – provides ET data for California, with a similar irrigation scheduler.
• USDA’s Irrigation Water Management – a broader guide, not a daily calculator.
• Private apps like CropManage or IrrigatePro – often more features but may require a subscription.
• Simple volume calculators (like the one above) – useful for ordering water or estimating pump time, but they don’t tell you when to irrigate.

The UNL tool is free, no registration, and tailored to Nebraska’s climate, but it works reasonably well in neighboring states with similar weather patterns.

Frequently Asked Questions

Is the UNL irrigation calculator free?

Yes, the University of Nebraska–Lincoln provides it free of charge. You don’t need an account or a subscription. Just access the website and enter your data.

Can I use it for my lawn?

Yes, the tool includes turf as a crop option. For lawns, the recommended MAD is often 50% and the root depth is shallower (6–8 inches) than for field crops. The calculator will give you a watering recommendation in inches, which you can convert to run time based on your sprinkler’s application rate.

What does “MAD� mean?

MAD stands for Management Allowed Depletion – the percentage of available water you are willing to let the crop use before you irrigate. For example, a MAD of 50% means you irrigate when half of the plant-available water is gone. Lower MAD (e.g., 30%) is used for high-value crops that are sensitive to drought stress.

How do I convert inches of water to gallons or cubic yards?

Use the volume calculator above. For a given area, multiply length × width × depth (in feet) to get cubic feet, then multiply by 7.48 to get gallons. To get cubic yards, divide cubic feet by 27. For example, 1 inch over 1,000 square feet equals 83.3 cubic feet, or 623 gallons.

Why does the calculator sometimes say “No irrigation needed�?

That means the soil moisture is still above your MAD threshold, so the crop has enough water. You should keep checking daily. When the moisture drops below the threshold, it will recommend irrigation.

Can I use it for drip irrigation?

Yes, but you need to adjust for the wetted area. Drip systems only wet a portion of the soil surface, so you may need to apply more water per wetted area or use a higher MAD. The calculator assumes uniform coverage, so for drip, consider using a separate scheduling method or consult an extension specialist.

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in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1