Irrigation Flow Time
Calculate irrigation duration based on flow rate, area, and water requirements. Enter values for instant results with step-by-step formulas.
Formula
Time (min) = (Area × Depth × 27,154) / (Flow Rate × Efficiency)
Where Area is in acres, Depth is desired application in inches, 27,154 converts acre-inches to gallons, Flow Rate is in GPM, and Efficiency is expressed as a decimal. The result gives minutes of run time needed to apply the desired water depth.
Worked Examples
Example 1: Center Pivot Irrigation Scheduling
Problem:A 125-acre center pivot system has a flow rate of 800 GPM with 85% efficiency. Calculate the time needed to apply 1 inch of water and the application rate.
Solution:Step 1: Calculate gross water volume Gross acre-inches = 125 × 1 = 125 acre-inches Step 2: Adjust for efficiency Net acre-inches = 125 / 0.85 = 147.1 acre-inches Step 3: Convert to gallons Total gallons = 147.1 × 27,154 = 3,994,555 gallons Step 4: Calculate run time Time = 3,994,555 / 800 GPM = 4,993 minutes = 83.2 hours Step 5: Calculate application rate Application rate = (800 × 60) / (125 × 27,154) = 0.0141 in/hr Step 6: Express as rotation time At 100% timer: 83.2 hours per revolution for 1" application
Result:Run Time: 83.2 hours (3.5 days) | Application Rate: 0.014 in/hr | 3.99 million gallons
Example 2: Drip Irrigation for Orchard
Problem:A 20-acre orchard uses drip irrigation at 95% efficiency. The well produces 150 GPM. How long to apply 2 inches of water per week?
Solution:Step 1: Calculate water volume needed Gross = 20 acres × 2 inches = 40 acre-inches Net (adjusted) = 40 / 0.95 = 42.1 acre-inches Gallons = 42.1 × 27,154 = 1,143,183 gallons Step 2: Calculate irrigation time Time = 1,143,183 / 150 GPM = 7,621 minutes Time in hours = 127 hours per week Step 3: Daily irrigation schedule Hours per day = 127 / 7 = 18.1 hours/day Step 4: Evaluate feasibility 18 hours/day is near continuous operation Consider: night irrigation, split applications Step 5: Cost estimate at $0.10/1000 gal Weekly cost = (1,143,183 / 1000) × $0.10 = $114.32 Cost per acre-inch = $114.32 / 40 = $2.86
Result:Weekly run time: 127 hours | Daily: 18 hrs/day | Cost: $114/week | Need larger well or longer cycle
Example 3: Flood Irrigation Set Time
Problem:A farmer flood irrigates a 5-acre basin from a canal delivering 2 CFS (cubic feet per second). With 55% efficiency, how long to apply 4 inches?
Solution:Step 1: Convert flow to GPM 2 CFS × 448.83 = 897.66 GPM Step 2: Calculate water needed Gross = 5 × 4 = 20 acre-inches Net = 20 / 0.55 = 36.4 acre-inches Gallons = 36.4 × 27,154 = 988,406 gallons Step 3: Calculate set time Time = 988,406 / 897.66 = 1,101 minutes Time = 18.3 hours Step 4: Water losses at 55% efficiency Water applied = 988,406 gallons Water used by crop = 988,406 × 0.55 = 543,623 gal Water lost = 444,783 gallons (45%) Step 5: Comparison with improved method If switched to 85% efficiency system: Would need only 639,224 gallons Savings = 349,182 gallons per irrigation
Result:Set time: 18.3 hours | 988,406 gallons applied | 45% loss to runoff/deep percolation
Frequently Asked Questions
How do I calculate irrigation run time?
Irrigation run time is calculated by: Time = (Area × Depth × 27,154) / (Flow Rate × Efficiency). First, determine the volume of water needed (area in acres × depth in inches = acre-inches). Convert to gallons (1 acre-inch = 27,154 gallons). Divide by your system's flow rate in GPM, then adjust for application efficiency. The result is time in minutes to apply the desired depth.
What is irrigation efficiency?
Irrigation efficiency is the percentage of applied water that's actually available for plant use. Losses occur through evaporation, wind drift, runoff, and deep percolation. Typical efficiencies: Flood/furrow irrigation: 40-60%, Sprinkler systems: 70-85%, Center pivot: 80-90%, Drip/micro irrigation: 85-95%. Higher efficiency means less water wasted and lower operating costs.
What flow rate do I need for my field?
Required flow rate depends on field size, water depth, and available irrigation time. Formula: GPM needed = (Acres × Inches × 27,154) / (Hours available × 60). For example, to apply 1 inch to 10 acres in 8 hours: (10 × 1 × 27,154) / (8 × 60) = 566 GPM. Higher flow rates allow shorter set times but require larger equipment and higher energy costs.
How do I convert between irrigation flow units?
Common conversions: 1 GPM = 0.00223 CFS (cubic feet/second), 1 CFS = 448.83 GPM, 1 GPM = 0.0631 liters/second, 1 acre-inch/hour = 452.57 GPM, 1 GPM continuous for 1 hour = 60 gallons = 0.0022 acre-inches. For well capacity, flow is often measured in GPM; for surface water rights, CFS or acre-feet are common.
What is application rate and why does it matter?
Application rate (inches/hour) is how fast water is applied to the soil. It must not exceed soil infiltration rate or runoff occurs. Sandy soils: 1-2+ inches/hour infiltration, Clay soils: 0.1-0.5 inches/hour. If your application rate exceeds infiltration, use cycle-soak irrigation (apply, pause, repeat) or choose a system with lower application rate.
How do I schedule irrigation efficiently?
Efficient scheduling matches water application to crop needs: 1) Monitor soil moisture (sensors, feel, appearance), 2) Track evapotranspiration (ET) from weather data, 3) Apply water before stress occurs (typically at 50% available water depletion), 4) Irrigate during low-wind, cool periods to reduce evaporation, 5) Consider deficit irrigation during less critical growth stages.
How does center pivot irrigation work?
Center pivots rotate around a central point, covering circular fields. The outer spans travel faster, so sprinkler packages are designed to apply more water at the ends. Key specs: Rotation time (typically 12-96 hours), system capacity (GPM), and percent timer setting. Application depth = (GPM × Hours × 60) / (Acres × 27,154). 100% timer means continuous rotation at design speed.
What is the cost of irrigation water?
Irrigation costs include: Pumping energy (electricity or diesel), Water rights or district fees, Equipment maintenance, Labor. Pumping cost varies with lift (depth to water), pressure needs, and energy prices. Rough estimate: $0.05-0.20 per 1,000 gallons for groundwater, more for high-lift or high-pressure systems. Surface water district charges vary widely by region.
How do I account for rainfall in irrigation scheduling?
Track effective rainfall—not all rain infiltrates. General rules: Ignore rain < 0.1 inch, count 75-80% of moderate rain, count less for heavy rain (runoff occurs). Subtract effective rainfall from crop ET to get irrigation need. Install rain gauges in fields. Smart controllers can integrate weather data to adjust irrigation automatically. Check soil moisture before scheduled irrigation after rain.
How do I calculate irrigation needs for a drip system?
Determine total water needed (gallons per hour) based on the number of emitters times their flow rate (typically 0.5-2 GPH each). Check your water pressure (most drip systems need 25-50 PSI) and flow rate. Total run time = weekly water need / (emitters * flow rate). Use a timer for consistent automated watering.