Irrigation Scheduling Calculator

About the Irrigation Scheduling Calculator

Irrigation scheduling determines the optimal time and amount to irrigate so that crops receive water before stress occurs but not so frequently that water and energy are wasted. The most common approach uses a soil water balance, where you track daily crop water use and rainfall against the soil's ability to store water in the root zone.

The trigger point for irrigation is set at the management allowed depletion (MAD), typically 50% of the total available water capacity (AWC) in the root zone. When cumulative ET minus rainfall reaches the MAD threshold, it is time to irrigate. The depth of water to apply equals the current soil water deficit.

This calculator computes the MAD trigger depth, compares it to your current depletion, and tells you how many days remain before you should irrigate based on current ETc. It is a digital version of the classic checkbook method used by irrigators across the Great Plains and beyond.

When This Page Helps

Proper irrigation timing prevents yield-reducing crop stress while avoiding over-watering that wastes water, leaches nutrients, and increases pumping costs. This page helps you see whether irrigation should happen now, how much room is left, and whether the current rotation is keeping up.

How to Use the Inputs

1. Enter the available water capacity (AWC) of your soil in inches per foot.
2. Enter the effective root zone depth in feet.
3. Enter the management allowed depletion (MAD) as a percentage.
4. Enter the current soil water depletion in inches.
5. Enter the daily crop ETc in inches per day.
6. Review the trigger depth and days until next irrigation.

Example Calculation

Tips & Best Practices

• Most crops perform well at MAD of 50%; drought-sensitive crops may need 30–40%.
• Sandy soils have low AWC (0.5–1.0 in/ft); clay loams have 1.5–2.0 in/ft.
• Adjust root zone depth as the crop grows — young plants have shallow roots.
• Use soil moisture sensors to validate your water balance.
• Account for effective rainfall, not total rainfall, in your depletion tracking.
• Reset the balance to zero after a full-profile irrigation event.

The Checkbook Method

The checkbook method treats your soil like a bank account. Field capacity is a full account, and each day the crop withdraws water (ETc). Rainfall makes deposits. When the balance drops to the MAD threshold, you make a deposit (irrigate). This simple bookkeeping approach has been successfully used for decades.

Soil Texture and AWC

Sandy soils drain quickly and hold less water, requiring more frequent but lighter irrigations. Clay soils hold more water but may have intake rate limitations. Loamy soils strike a balance. A soil survey or lab test gives the most accurate AWC for your field.

Sensor-Based Adjustments

Modern sensor systems can automate scheduling by continuously reporting soil moisture. They complement the checkbook method by accounting for factors like deep percolation, variable rainfall distribution, and non-uniform soil profiles that are hard to capture with a simple water balance.

Frequently Asked Questions

• What is management allowed depletion?

  MAD is the fraction of available soil water you allow the crop to use before irrigating. A 50% MAD means you irrigate when half the available water in the root zone has been consumed. FAO recommends 50% for most crops.

• What is available water capacity?

  AWC is the amount of water the soil can hold between field capacity and permanent wilting point, expressed in inches per foot of soil depth. It depends on soil texture: sands ~0.7, loams ~1.5, clays ~2.0.

• How do I know my root zone depth?

  Root zone depth depends on the crop and growth stage. Corn at full canopy may root 3–4 feet deep, while lettuce roots only 1–1.5 feet. Extension bulletins list effective rooting depths by crop.

• What if it rains between irrigations?

  Subtract effective rainfall from the cumulative depletion. Only about 75% of light rains may actually enter the root zone; the rest may run off or evaporate.

• Can I irrigate before reaching the trigger?

  Yes, but frequent light irrigations increase evaporation loss and may encourage shallow rooting. It is generally more efficient to apply a larger amount less frequently, up to the MAD threshold.

• What sensors verify this approach?

  Tensiometers, gypsum blocks, capacitance probes, and neutron probes all measure soil moisture. They validate or override the checkbook calculation and are especially useful in variable soils.