Crop Water Requirement Calculator
Estimate daily and seasonal water needs for crops based on ET rate and growth stage. Enter values for instant results with step-by-step formulas.
Formula
ETc = ET0 x Kc | Net Irrigation = ETc x Days - Rainfall
Crop evapotranspiration (ETc) equals reference ET (ET0) multiplied by the crop coefficient (Kc) for the specific crop and growth stage. Total crop water need is ETc multiplied by the growing period. Net irrigation subtracts effective rainfall. Gross irrigation divides by irrigation efficiency to account for system losses.
Worked Examples
Example 1: Wheat Field - Mid Season
Problem: Calculate water needs for 10 hectares of wheat at mid-season, ET0 = 5 mm/day, 120-day season, 150mm rainfall, 75% efficiency.
Solution: Kc (wheat mid-season) = 1.15\nETc = 5 x 1.15 = 5.75 mm/day\nTotal crop water: 5.75 x 120 = 690 mm\nNet irrigation: 690 - 150 = 540 mm\nGross irrigation: 540 / 0.75 = 720 mm\nDaily volume: 5.75/1000 x 100,000 = 575 m³/day\nTotal volume: 720/1000 x 100,000 = 72,000 m³
Result: ETc: 5.75 mm/day | Net: 540mm | Gross: 720mm | Total: 72,000 m³
Example 2: Drip-Irrigated Tomato
Problem: Calculate for 2 hectares of tomato, mid-season, ET0 = 6 mm/day, 90 days, 80mm rain, 90% drip efficiency.
Solution: Kc (tomato mid-season) = 1.15\nETc = 6 x 1.15 = 6.90 mm/day\nTotal crop water: 6.90 x 90 = 621 mm\nNet irrigation: 621 - 80 = 541 mm\nGross irrigation: 541 / 0.90 = 601 mm\nDaily volume: 6.90/1000 x 20,000 = 138 m³/day\nTotal volume: 601/1000 x 20,000 = 12,022 m³
Result: ETc: 6.90 mm/day | Net: 541mm | Gross: 601mm | Total: 12,022 m³
Frequently Asked Questions
What is crop evapotranspiration (ETc)?
Crop evapotranspiration (ETc) is the total amount of water lost from a cropped field through both evaporation from the soil surface and transpiration from the plant leaves. It represents the actual water consumption of a specific crop at a given growth stage. ETc is calculated by multiplying the reference evapotranspiration (ET0, based on a reference grass surface) by the crop coefficient (Kc): ETc = ET0 x Kc. ET0 is determined by climatic factors including solar radiation, temperature, humidity, and wind speed, typically using the FAO Penman-Monteith equation. Understanding ETc is essential for designing irrigation schedules that meet crop water demands without over- or under-irrigating.
What are crop coefficients (Kc) and how do they vary?
Crop coefficients (Kc) are dimensionless factors that relate crop evapotranspiration to reference evapotranspiration. They vary by crop type and growth stage. During the initial stage (germination to early growth), Kc values are low (0.3-0.6) because plants cover little soil. During the development stage, Kc increases as canopy cover grows. At mid-season (full canopy), Kc reaches its peak, typically 1.0-1.25 for most crops. During the late season (maturity to harvest), Kc decreases as plants senesce. The FAO-56 publication provides standardized Kc values for hundreds of crops under various conditions. Local factors such as soil type, planting density, and management practices can modify these standard values.
How is irrigation efficiency factored into water requirements?
Irrigation efficiency represents the ratio of water actually used by crops to the total water applied. Different irrigation methods have different efficiencies: surface (flood) irrigation typically achieves 40-60% efficiency, sprinkler systems 60-80%, and drip irrigation 85-95%. The gross irrigation requirement equals the net requirement divided by efficiency. For example, if a crop needs 500mm of water and you use sprinkler irrigation at 75% efficiency, the gross requirement is 500/0.75 = 667mm. Losses occur through deep percolation below the root zone, surface runoff, evaporation from soil and spray, wind drift, and distribution non-uniformity. Improving irrigation efficiency conserves water resources and reduces costs.
How much water do garden plants need?
Most vegetables need about 1 inch (0.62 gallons per square foot) of water per week from rain plus irrigation. Sandy soil drains faster and may need 2 inches. Clay soil retains moisture longer. Water deeply and less frequently to encourage deep root growth. Morning watering reduces evaporation and disease risk.
What is crop rotation and why is it important?
Crop rotation means growing different plant families in each bed each year. It prevents soil-borne disease buildup, balances nutrient depletion, and breaks pest cycles. A simple 4-year rotation: legumes (add nitrogen), then leafy greens (use nitrogen), then fruiting crops, then root vegetables. Never follow a crop with the same family.
How accurate are the results from Crop Water Requirement Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.