Irrigation Scheduling Calculator
Determine how many inches of water your crop needs right now based on evapotranspiration, recent rainfall, days since last irrigation, and soil type. Use it to schedule irrigation events and avoid over- or under-watering.
About this calculator
Crop water demand is driven by evapotranspiration (ET) — the combined loss of water through soil evaporation and plant transpiration. Reference ET (ETo) is a standardized measure for a reference grass surface, and it is multiplied by a crop coefficient (Kc) that varies by growth stage to get crop ET. The net irrigation requirement equals crop water use over the interval minus any rainfall received. Soil type modifies how efficiently that water is stored and released: clay soils hold more water per inch but drain slowly (factor 1.2), loam soils are balanced (factor 1.0), and sandy soils drain quickly (factor 0.8). The formula is: irrigation (inches) = max(0, referenceET × cropCoefficient × daysSinceIrrigation × soilFactor − rainfall). A result of zero means recent rain has met crop demand; a positive result is the deficit you need to replace.
How to use
Assume your reference ET is 0.25 in/day, the crop coefficient for mid-season corn is 1.15, it has been 5 days since irrigation, your soil is loam (factor 1.0), and you received 0.4 inches of rain. Step 1 — Gross crop ET: 0.25 × 1.15 × 5 × 1.0 = 1.4375 inches. Step 2 — Subtract rainfall: 1.4375 − 0.4 = 1.0375 inches. Step 3 — Since the result is positive, you need to apply approximately 1.04 inches of irrigation. Schedule your next irrigation event to deliver that amount.
Frequently asked questions
What is reference evapotranspiration and where can I find my local value?
Reference evapotranspiration (ETo) is the estimated water use of a well-watered grass reference surface under current weather conditions, expressed in inches or mm per day. It integrates temperature, solar radiation, wind, and humidity into a single daily number. Most US states publish daily ETo values through agricultural weather networks such as CIMIS in California or CoAgMet in Colorado. Your local extension office or state climatologist website is the best starting point for finding reliable local ETo data.
How do crop coefficients change throughout the growing season?
Crop coefficients (Kc) start low during early vegetative growth when leaf area is small and most water loss is from bare soil evaporation. They rise sharply during rapid canopy development and peak near full canopy cover or pollination, often reaching 1.1–1.3 for row crops. During grain fill and maturation, Kc declines as the plant senesces and water uptake slows. Using the correct Kc for your crop's current growth stage is critical — using a mid-season value during emergence will significantly overestimate irrigation need.
Why does soil type affect how much irrigation water I should apply?
Soil texture determines its water-holding capacity and infiltration rate, both of which affect how much irrigation is needed and how often. Clay soils hold more water between field capacity and wilting point, so crops can go longer between irrigations — but they also accept water slowly, risking runoff. Sandy soils drain rapidly, requiring more frequent but shorter irrigation events. Loam soils strike a balance and are generally the most forgiving to manage. Matching your irrigation schedule to your soil type prevents both water stress and nutrient leaching.