Irrigation Scheduling by Crop & ET

The crop coefficient (Kc) values in this tool are generic approximations based on FAO (Food and Agriculture Organization) guidelines and standard agricultural references. They represent typical Kc values for common crops at different growth stages (initial, development, mid-season, late-season). Actual Kc values can vary significantly based on crop variety, local climate conditions (humidity, wind speed), soil characteristics, and management practices. For precise irrigation planning and professional recommendations, consult local agronomic resources, agricultural extension services, or crop-specific research data. Understanding Kc sources helps you use appropriate values for your specific conditions and crop varieties.

No, this tool does NOT fetch real-time weather data. You must provide the reference evapotranspiration (ET₀) value yourself. ET₀ can be obtained from multiple sources: local weather stations (often provide daily ET₀ values), agricultural extension services (university extension programs typically publish ET data), online ET databases (CIMIS in California, AZMET in Arizona, CoAgMet in Colorado, and similar regional networks), or calculated using estimation methods (Penman-Monteith equation or Hargreaves method with local temperature, humidity, wind speed, and solar radiation data). Using current, location-specific ET₀ values produces more accurate irrigation estimates than historical averages or rough approximations. Understanding ET₀ sources helps you obtain accurate values for effective irrigation scheduling.

This tool provides educational approximations for planning purposes only. It uses simplified ET-based calculations that don't account for many real-world factors: current soil moisture levels (from sensors or field measurements), actual rainfall and its effectiveness (how much rain actually reaches the root zone), drainage and deep percolation, crop stress thresholds and management goals, irrigation system capacity and operational constraints, water availability and rights, local weather forecasts and conditions, and site-specific soil and crop characteristics. For actual farm irrigation scheduling, combine this tool with soil moisture monitoring (tensiometers, capacitance sensors, or feel method), local weather data and forecasts, and guidance from irrigation specialists or agronomists who understand your specific conditions. Understanding tool limitations helps you use it appropriately as part of comprehensive irrigation management.

The tool supports both metric and imperial unit systems. Metric units: millimeters (mm) for irrigation depth, hectares (ha) for field area, cubic meters (m³) for water volume. Imperial units: inches (in) for irrigation depth, acres (ac) for field area, gallons (gal) for water volume. Select your preferred unit system before entering values—all calculations and results will be displayed in your chosen units. The tool automatically handles conversions: 1 mm over 1 hectare = 10 cubic meters, and 1 inch over 1 acre ≈ 27,154 gallons. Understanding unit systems helps you work with your preferred measurement units and interpret results correctly.

Your agronomist likely considers many factors not included in this simple ET-based model: actual soil water holding capacity (how much water your specific soil can store), root zone depth (how deep crop roots extend, affecting available water), effective rainfall (how much rain actually benefits the crop), soil type and texture (affects water retention and movement), crop stress thresholds (when crops need water to avoid yield loss), irrigation system uniformity (how evenly water is distributed), local growing conditions (microclimate, pest pressure, disease risk), and management goals (yield targets, quality objectives, water conservation priorities). This tool provides a baseline estimate based on ET and crop coefficients—professional recommendations will be more tailored to your specific situation, combining ET-based calculations with field observations and local knowledge. Understanding why recommendations differ helps you appreciate the value of professional agronomic guidance.

Reference evapotranspiration (ET₀) is the evapotranspiration rate from a hypothetical reference grass surface with specific characteristics (well-watered, actively growing grass, 12 cm height, with specific resistance values). It represents atmospheric demand for water and is independent of crop type, soil conditions, or management practices. ET₀ is influenced by climatic factors: solar radiation (primary driver, varies by season and location), temperature (affects evaporation rate), humidity (lower humidity increases ET), and wind speed (increases ET by removing water vapor). ET₀ varies by location (latitude, elevation, proximity to water), season (higher in summer, lower in winter), and daily weather conditions (sunny, hot, dry, windy days have higher ET₀). Common sources include local weather stations, agricultural departments, university extension services, and online ET databases (CIMIS, AZMET, CoAgMet, and regional networks). Understanding ET₀ helps you obtain accurate values for effective irrigation scheduling.

Crop coefficients (Kc) multiply reference ET₀ to estimate actual crop water use (ETc) using the formula ETc = Kc × ET₀. Kc values adjust for crop-specific characteristics: canopy size and ground cover (affects how much surface area transpires), crop height and roughness (affects wind and energy exchange), rooting depth (affects water access), and crop physiology (different crops have different water use patterns). Kc varies by growth stage: during initial growth, Kc is low (0.3–0.5) because crops cover little ground and have small root systems. It increases during development (0.6–0.9) as canopy grows and roots expand. Kc peaks at mid-season (0.9–1.2) when the crop is fully developed with maximum canopy and active growth. Kc often decreases in late season (0.3–0.9) as crops mature, senesce, or approach harvest. Different crops have different Kc curves—for example, maize may have Kc 1.2 at mid-season, while citrus (evergreen) maintains relatively constant Kc around 0.65 year-round. Understanding how Kc adjusts ET helps you calculate accurate crop water requirements.

Net irrigation depth is the actual water needed by the crop to meet evapotranspiration requirements, calculated as ETc multiplied by the irrigation interval (NetDepth = ETc × IntervalDays). This represents the amount of water that must reach the root zone to replace what the crop uses. Gross irrigation depth accounts for irrigation system efficiency—water lost to evaporation (water evaporates before reaching soil), wind drift (water blown away from target area), runoff (water flows off field before infiltrating), and deep percolation (water moves below root zone). Gross depth is calculated as net depth divided by efficiency (GrossDepth = NetDepth ÷ Efficiency). If efficiency is 80% (0.80), you need to apply 25% more water (gross) than the net requirement to ensure the crop receives adequate water. For example, if net depth is 30 mm and efficiency is 80%, gross depth = 30 ÷ 0.80 = 37.5 mm. Understanding net vs. gross depth helps you determine how much water to actually apply to meet crop needs.

Irrigation efficiency depends on system type, design, maintenance, and management. Typical efficiency ranges by system type: Drip/micro-irrigation (85–95% efficiency)—water delivered directly to root zone, minimal evaporation and runoff, highest efficiency when properly designed and maintained. Center pivot sprinklers (75–85% efficiency)—modern LEPA (Low Energy Precision Application) systems achieve higher efficiency, efficiency depends on nozzle type, pressure, and wind conditions. Solid-set sprinklers (70–80% efficiency)—depends on spacing, nozzle selection, and wind conditions, closer spacing and proper design improve efficiency. Surface/flood irrigation (50–70% efficiency)—varies widely with soil type, field slope, water management, and operator skill, well-managed systems can approach 70%, poorly managed systems may be 50% or lower. Well-designed and maintained systems perform at the higher end of these ranges; poorly maintained systems, improper design, or adverse conditions (high wind, extreme heat) reduce efficiency. Understanding irrigation efficiency helps you choose appropriate values and identify opportunities to improve system performance.

The tool uses four standard growth stages based on FAO guidelines: Initial stage (crop establishment, minimal canopy)—crops are small with limited ground cover, Kc values typically 0.3–0.5, water use is low because most evaporation is from soil surface. Development stage (rapid vegetative growth, increasing canopy)—crops are growing rapidly with expanding canopy, Kc values typically 0.6–0.9, water use increases as more leaf area transpires. Mid-season stage (full canopy, flowering/fruit development)—crops have maximum canopy coverage and active growth, Kc values typically 0.9–1.2 (highest), water use peaks during this stage. Late-season stage (maturation, senescence)—crops are maturing or approaching harvest, Kc values typically 0.3–0.9 (declining), water use decreases as crops mature. Select the stage that best matches your crop's current condition for the most relevant Kc value. Some crops may have additional stages or different stage definitions—consult crop-specific references for detailed Kc curves. Understanding growth stages helps you select appropriate Kc values for accurate irrigation calculations.

The volume calculation uses standard, mathematically accurate conversions: Metric system—1 millimeter (mm) of water over 1 hectare (ha) equals exactly 10 cubic meters (m³). This conversion is based on: 1 mm = 0.001 m, 1 ha = 10,000 m², so 1 mm × 1 ha = 0.001 m × 10,000 m² = 10 m³. Imperial system—1 inch of water over 1 acre equals approximately 27,154 gallons. This conversion accounts for: 1 inch = 0.0833 ft, 1 acre = 43,560 ft², 1 cubic foot = 7.48052 gallons, so 1 inch × 1 acre = 0.0833 ft × 43,560 ft² × 7.48052 gal/ft³ ≈ 27,154 gallons. These conversions are mathematically accurate. The actual volume you need depends on how accurately your inputs (ET₀, area, efficiency) reflect real conditions. If ET₀ is inaccurate, area is wrong, or efficiency doesn't match your system, the calculated volume will be inaccurate even though the conversion math is correct. Understanding volume calculation accuracy helps you interpret results and identify where errors might occur.

Currently, this tool calculates one scenario at a time. You can use the copy feature to save results to your clipboard and paste them into a spreadsheet or document for comparison. For multiple crops, growth stages, field sizes, or irrigation intervals, run separate calculations and record each result. This allows you to compare different scenarios such as: different crops on the same field, same crop at different growth stages, different irrigation intervals (e.g., 3-day vs. 7-day), different irrigation efficiencies (e.g., drip vs. sprinkler), or different field sizes. Comparing scenarios helps you understand how crop type, growth stage, interval, efficiency, and area affect irrigation requirements. For comprehensive irrigation planning across multiple fields or crops, consider using spreadsheet software to organize and compare multiple calculations. Understanding scenario comparison helps you evaluate different irrigation options and make informed decisions.

ET₀ values can be obtained from multiple sources depending on your location: Local weather stations—many agricultural regions have weather station networks that provide daily ET₀ values, often available online or through subscription services. University extension services—agricultural extension programs typically provide daily or weekly ET reports for their service areas, often free and tailored to local conditions. Government agencies—USDA, state departments of agriculture, and regional water management districts often provide ET data through websites or data portals. Online ET databases—regional networks like CIMIS (California Irrigation Management Information System), AZMET (Arizona Meteorological Network), CoAgMet (Colorado Agricultural Meteorological Network), and similar systems in other states provide historical and current ET data. Estimation methods—if direct ET₀ data isn't available, you can estimate using Penman-Monteith or Hargreaves equations with local weather data (temperature, humidity, wind speed, solar radiation). Using local, current ET₀ data produces more accurate irrigation estimates than historical averages or rough approximations. Understanding ET₀ sources helps you obtain accurate values for effective irrigation scheduling.