{ "slug": "evapotranspiration-irrigation", "title": "Evapotranspiration Irrigation Calculator", "heading": "Evapotranspiration (ET) Irrigation Need Calculator", "category": "other", "url": "https://www.revenuelab.fyi/toolbox/evapotranspiration-irrigation", "summary": "Estimate weekly irrigation needs from reference ET, crop coefficient, and rainfall.", "description": "Evapotranspiration (ET) measures total water lost from soil evaporation and plant transpiration combined, and it's the standard basis for scientific irrigation scheduling used by farmers, golf courses, and landscape water managers. Reference ET (ETo) — published daily by agricultural extension weather networks like CIMIS in California — represents water use of a standardized grass reference surface; multiplying it by a crop coefficient (Kc) that varies by plant type and growth stage gives actual crop water use (ETc). Subtracting effective rainfall from ETc tells you exactly how much supplemental irrigation to apply, which avoids both under-watering (crop stress) and over-watering (wasted water, nutrient leaching, and root disease) that flat weekly watering schedules cause.", "formula": "ETc = ETo × Kc; net irrigation need = ETc − effective rainfall; gallons = net need (in) × area (sq ft) × 0.623.", "dateModified": "2026-09-30", "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=evapotranspiration-irrigation", "inputs": [ { "id": "eto", "label": "Reference ET (weekly)", "kind": "number", "hint": null, "default": 1.4, "unit": "in", "min": 0.1, "max": 4 }, { "id": "kc", "label": "Crop coefficient (Kc)", "kind": "number", "hint": null, "default": 0.8, "unit": null, "min": 0.1, "max": 1.3 }, { "id": "rainfall", "label": "Effective rainfall this week", "kind": "number", "hint": null, "default": 0.3, "unit": "in", "min": 0, "max": 5 }, { "id": "area", "label": "Irrigated area", "kind": "number", "hint": null, "default": 1000, "unit": "sq ft", "min": 10, "max": null }, { "id": "systemEfficiency", "label": "Irrigation system efficiency", "kind": "number", "hint": null, "default": 80, "unit": "%", "min": 40, "max": 100 } ], "outputs": [ { "id": "gallons", "label": "Gallons to apply this week", "format": "number", "hint": null, "primary": true }, { "id": "grossNeed", "label": "Gross irrigation depth", "format": "decimal", "hint": null, "primary": false }, { "id": "netNeed", "label": "Net irrigation need (after rain)", "format": "decimal", "hint": null, "primary": false }, { "id": "etc", "label": "Crop evapotranspiration (ETc)", "format": "decimal", "hint": null, "primary": false } ], "worked_example": { "inputs": [ "Reference ET (weekly): 1.4 in", "Crop coefficient (Kc): 0.8", "Effective rainfall this week: 0.3 in", "Irrigated area: 1000 sq ft", "Irrigation system efficiency: 80 %" ], "outputs": [ "Gallons to apply this week: 639", "Gross irrigation depth: 1.02", "Net irrigation need (after rain): 0.82", "Crop evapotranspiration (ETc): 1.12" ] }, "how_to": { "title": "How to use this", "steps": [ "Enter reference et (weekly) (in).", "Enter crop coefficient (kc).", "Enter effective rainfall this week (in).", "Enter irrigated area (sq ft).", "Enter irrigation system efficiency (%).", "Read your gallons to apply this week on the right — it updates as you type.", "Hit Share to keep the scenario or send it to someone." ] }, "scenarios": [ { "name": "Conservative", "description": "Lower-end numbers — what if things land soft?", "values": { "eto": 0.84, "kc": 0.48, "rainfall": 0.18, "area": 600, "systemEfficiency": 50 } }, { "name": "Typical", "description": "Defaults — the most common real-world setup.", "values": { "eto": 1.4, "kc": 0.8, "rainfall": 0.3, "area": 1000, "systemEfficiency": 80 } }, { "name": "Ambitious", "description": "Higher-end numbers — what if things really pop?", "values": { "eto": 2.2399999999999998, "kc": 1.2800000000000002, "rainfall": 0.48, "area": 1600, "systemEfficiency": 100 } } ], "limitations": [ "Estimates assume standard, average conditions — local rules, pricing, and materials vary.", "Results are rounded for readability; add a buffer before ordering, booking, or committing.", "Double-check anything with a real cost attached against a local quote." ], "faq": [ { "q": "Where do I find reference ET (ETo) for my area?", "a": "State agricultural extension networks publish it daily or weekly — CIMIS in California, AZMET in Arizona, and similar mesonet systems in most farm states. If unavailable, a rough national summer average of 1.0-1.8 inches per week works as a starting estimate." }, { "q": "How do I pick a crop coefficient (Kc)?", "a": "Kc varies by plant type and growth stage: cool-season turfgrass runs about 0.6-0.8, warm-season turf 0.5-0.7, most vegetables 0.7-1.15 depending on stage, and mature trees or shrubs often 0.5-0.9. FAO-56 publishes detailed Kc tables for hundreds of crops by growth stage." }, { "q": "What counts as 'effective rainfall'?", "a": "Not all rain reaches the root zone usefully — heavy downpours run off, and light rain can evaporate before infiltrating. A common rule of thumb treats about 75-80% of rainfall under 1 inch as effective, with diminishing returns above that as runoff increases." }, { "q": "Why does system efficiency matter for the calculation?", "a": "Drip irrigation delivers 85-95% of applied water to the root zone, while overhead sprinklers lose 15-30% to evaporation, wind drift, and uneven coverage, so you need to apply proportionally more gross water through an inefficient system to deliver the same net amount the crop actually needs." } ], "related": [ "https://www.revenuelab.fyi/toolbox/roof-rainwater-harvest", "https://www.revenuelab.fyi/toolbox/frost-date-growing-window" ], "license": "CC-BY-4.0", "citation": "RevenueLab — Evapotranspiration Irrigation Calculator (https://www.revenuelab.fyi/toolbox/evapotranspiration-irrigation)" }