Schedule your next irrigation using soil water balance. Calculate trigger point from MAD, available water capacity, and root zone depth.
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.
Proper irrigation timing prevents yield-reducing crop stress while avoiding over-watering that wastes water, leaches nutrients, and increases pumping costs. A scheduling tool keeps you on track even during busy planting or harvest seasons when field scouting may fall behind. Having a precise figure at your fingertips empowers better planning and more confident decisions.
Trigger Depth (in) = MAD (%) × AWC (in/ft) × RZD (ft) Days to Irrigate = (Trigger Depth − Current Depletion) / Daily ETc Where: MAD = Management Allowed Depletion (%) AWC = Available Water Capacity (in/ft) RZD = Root Zone Depth (ft)
Result: Trigger at 2.70 in; irrigate in ~4 days
Trigger = 0.50 × 1.8 × 3 = 2.70 inches. Current depletion is 1.50 in. Remaining capacity = 2.70 − 1.50 = 1.20 in. At 0.30 in/day ETc, you have about 4 days before you should irrigate.
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.
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.
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.
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.
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.
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.
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.
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.
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.