Irrigation Scheduling by Crop & ET
Plan simple irrigation schedules using crop coefficients (Kc) and reference evapotranspiration (ET₀). Estimate net and gross water depth per event, total volume, and interval-based irrigation requirements for your field.
Generic Kc presets based on FAO guidelines
Reference evapotranspiration from local weather data or estimation
Typical: Drip 85-95%, Sprinkler 70-85%, Surface 50-70%
Plan Your Irrigation Schedule
Enter your crop type, reference ET₀, field area, and irrigation interval to calculate water requirements per irrigation event.
Quick Start:
- Select your unit system (metric or imperial)
- Choose your crop type and growth stage
- Enter your field area
- Enter reference ET₀ from local weather data
- Set your planned irrigation interval
- Adjust irrigation efficiency for your system
Understanding ET-Based Irrigation Scheduling
What is Reference Evapotranspiration (ET₀)?
Reference evapotranspiration (ET₀) represents the atmospheric demand for water. It measures how much water would evaporate from a well-watered grass surface under specific conditions. ET₀ is influenced by solar radiation, temperature, humidity, and wind speed—factors that determine how quickly water moves from soil and plants to the atmosphere.
ET₀ is typically measured in millimeters per day (mm/day) in metric units or inches per day (in/day) in imperial units. Values can range from less than 1 mm/day in cool, humid conditions to over 10 mm/day in hot, dry, windy environments.
How Crop Coefficients (Kc) Adjust ET for Different Crops
The crop coefficient (Kc) adjusts reference ET to estimate actual crop evapotranspiration (ETc). The formula is simple: ETc = Kc × ET₀.
Kc varies by growth stage:
- Initial Stage: Low Kc (0.3-0.5) — small plants, minimal ground cover
- Development Stage: Increasing Kc (0.6-0.9) — rapid canopy growth
- Mid-Season Stage: Peak Kc (0.9-1.2) — full canopy, maximum water use
- Late-Season Stage: Declining Kc (0.3-0.9) — maturation, senescence
Different crops have different Kc values at each stage. For example, maize at mid-season may have Kc of 1.2, while citrus (an evergreen) maintains a relatively constant Kc around 0.65 year-round.
Net vs Gross Irrigation Depth and Efficiency
Net irrigation depth is the water actually needed by the crop—the amount that must reach the root zone. It equals ETc multiplied by the number of days between irrigations.
Gross irrigation depth accounts for system inefficiencies. Not all water applied reaches the root zone—some is lost to evaporation, wind drift, runoff, or deep percolation. The formula is:Gross = Net ÷ Efficiency
Typical Irrigation Efficiencies:
- Drip/Micro: 85-95% — water delivered directly to root zone
- Center Pivot Sprinkler: 75-85% — modern LEPA systems higher
- Solid-set Sprinkler: 70-80% — depends on spacing and wind
- Surface/Flood: 50-70% — varies widely with soil and management
Why Irrigation Efficiency Matters
Irrigation efficiency directly affects water use, energy costs, and environmental impact. If your system is 70% efficient, you must pump 43% more water than the net requirement. This means higher pumping costs, greater water withdrawals, and potential for nutrient leaching.
Improving efficiency through better system design, maintenance, and scheduling can significantly reduce water use while maintaining crop yields. Even small improvements (e.g., from 70% to 80%) can result in substantial water savings over a growing season.
Limitations of Simple ET-Based Scheduling
This tool uses a simplified model. Real-world irrigation scheduling should also consider:
- Soil water holding capacity and root zone depth
- Current soil moisture levels (from sensors or feel)
- Rainfall and its effectiveness
- Crop stress thresholds and management goals
- Irrigation system capacity and constraints
- Water availability and rights
- Local regulations and environmental considerations
Getting ET₀ Data
ET₀ values are available from many sources:
- Local weather stations: Many agricultural regions have ET networks
- University extension services: Often provide daily or weekly ET reports
- Government agencies: USDA, state departments of agriculture
- Online databases: CIMIS (California), AZMET (Arizona), CoAgMet (Colorado)
- Estimation: Hargreaves or Penman-Monteith equations with local weather data
Using local, current ET₀ data produces more accurate irrigation estimates than relying on historical averages or rough approximations.
Frequently Asked Questions
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