Irrigation Water Requirement Calculator
Determine how much water your irrigation system must supply each day, accounting for system efficiency losses. Used by farmers and agronomists to size pumps, plan water allocations, and schedule irrigation runs.
About this calculator
Crops need a specific depth of water per day depending on their growth stage, climate, and evapotranspiration rate. However, no irrigation system delivers water with perfect efficiency — some is lost to evaporation, leakage, and runoff. The formula accounts for this: Gross Water Requirement = (cropArea × waterRequirement) / irrigationEfficiency. Here, crop area is in hectares, water requirement is in millimeters per day (mm/day), and irrigation efficiency is expressed as a decimal between 0 and 1 (e.g., 0.85 for 85% efficient drip irrigation). Dividing by efficiency scales up the gross water volume you must supply so that the crop still receives the net amount it needs. The result in mm/day can be converted to cubic meters per day by multiplying by 10 per hectare (1 mm over 1 ha = 10 m³). This calculation helps size pumps and pipelines and plan water use within licensed allocation limits.
How to use
Suppose you irrigate a 40-hectare vegetable field that needs 6 mm/day of water, and your sprinkler system operates at 75% efficiency (0.75). Enter 40 for Crop Area, 6 for Water Requirement per Hectare, and 0.75 for Irrigation Efficiency. The calculator computes: Gross Requirement = (40 × 6) / 0.75 = 240 / 0.75 = 320 mm/day across the field. Converting: 320 mm × 10 m³/ha = 3,200 m³ of water must be pumped daily to meet the crop's actual needs despite system losses.
Frequently asked questions
What does irrigation efficiency mean and how do I choose the right value?
Irrigation efficiency is the proportion of water applied that is actually used by the crop, expressed as a decimal. Flood irrigation systems are typically 50–60% efficient (0.50–0.60), sprinkler systems range from 70–80% (0.70–0.80), and drip irrigation can reach 85–95% (0.85–0.95). You can find efficiency values in your irrigation system's technical documentation or from agricultural engineering guidelines. Using a more accurate efficiency figure ensures you neither under-supply your crop nor waste water by over-pumping.
How do I find the daily water requirement per hectare for my crop?
Daily crop water requirements are derived from the reference evapotranspiration (ET₀) for your area, multiplied by a crop coefficient (Kc) that varies by species and growth stage. Local meteorological stations, irrigation departments, or FAO irrigation guides publish ET₀ data and Kc tables for common crops. During peak summer heat, a vegetable crop might need 6–8 mm/day, while a cereal crop at early growth stages may only need 2–3 mm/day. Monitoring soil moisture sensors or tensiometers on-farm provides real-time confirmation that your theoretical requirement matches actual crop uptake.
Why is it important to calculate gross rather than net irrigation water requirements?
The net water requirement is what the crop actually consumes, but your pump, pipes, and budget must supply the gross amount — which is always higher once system losses are factored in. If you size your infrastructure based only on the net figure, your system will consistently under-irrigate the crop during peak demand, reducing yield and stressing plants. Accounting for efficiency also helps you comply with water use licenses, which typically specify gross extraction volumes. Improving irrigation efficiency is one of the fastest ways to reduce water costs and increase the area you can irrigate from a fixed water allocation.