Water & Irrigation calculators

Multiply reference evapotranspiration (ET₀, mm/day) by the crop coefficient (Kc) for the growth stage to get crop water use (ETc), subtract effective rainfall, then divide by your system's efficiency. Example: ET₀ 6 mm × Kc 1.1 = 6.6 mm/day; over 1 acre that is about 26,700 L/day, or roughly 33,000 L/day applied by drip at 80% efficiency.

Drip typically saves 30–50% of water versus surface/flood irrigation because it wets only the root zone and runs at 85–95% application efficiency, against 40–60% for flood. On many crops it also lifts yield 10–30%, so water productivity (crop per drop) can nearly double.

Irrigate when the root zone has dried to the maximum allowable depletion (MAD) — usually 50% of available water for field crops, less for shallow-rooted vegetables. The interval = (readily available water depth) ÷ (daily ETc). On a sandy loam holding ~15 mm of readily available water with ETc of 5 mm/day, that is about every 3 days.

ET₀ is the water use of a standard 0.12 m grass reference surface with unlimited water, in mm/day. It captures the weather's evaporative demand. Multiply it by a crop coefficient (Kc 0.3 early to 1.15 at peak) to get a specific crop's water use (ETc). Typical ET₀ ranges 2–4 mm/day in winter to 6–9 mm/day in peak summer.

Pump power (kW) = ρ·g·Q·H ÷ (3.6×10⁶ × efficiency), where Q is flow in m³/h and H is total dynamic head (static lift + friction + operating pressure) in metres. Roughly: kW ≈ (Q m³/h × H m) ÷ (367 × pump efficiency). A 30 m³/h flow at 40 m head and 65% efficiency needs about 5 kW (≈7 HP).

Keep velocity around 1.5 m/s for mains (max ~2.5 m/s) to limit friction and water hammer. Diameter (mm) ≈ 18.8 × √(Q ÷ v), where Q is in L/s and v in m/s. For 10 L/s at 1.5 m/s that is about a 90 mm pipe. Check the Hazen-Williams friction head over the run before committing.

Harvestable volume (litres) = catchment area (m²) × rainfall (mm) × runoff coefficient. Roofs run 0.8–0.9, paved 0.7–0.8, fields 0.2–0.4. A 100 m² roof under 600 mm of rain at 0.85 runoff yields about 51,000 litres a year.

For drip, aim for emission uniformity (EU) above 90% (excellent) and never below 80%. For sprinklers, Christiansen's CU above 85% and distribution uniformity (DU) above 75% are good. Low uniformity means part of the field is over-watered while another is stressed, so you waste water and lose yield.

Judge it on salinity (ECw), sodium hazard (SAR), and specific-ion toxicity (Na, Cl, B, bicarbonate). Broadly: ECw under 0.7 dS/m has no restriction, 0.7–3.0 is slight-to-moderate, over 3.0 is severe. SAR under 3 is safe; over 9 risks soil sealing. The binding constraint — the worst single hazard — sets your suitability class.

Drain depth is usually 0.9–1.5 m so the mid-point water table stays at least 0.5–1.0 m below the surface for most crops. Spacing comes from Hooghoudt's equation and depends on soil permeability (K), the drainage coefficient and the depth to an impermeable barrier — closer in heavy clay (10–20 m), wider in sand (40–80 m).

Apply extra water — the leaching requirement (LR) = ECw ÷ (5×ECe − ECw), where ECe is the crop's salinity threshold. Gross water = ETc ÷ (1 − LR). If ECw is 2 dS/m and the crop tolerates ECe 4 dS/m, LR ≈ 11%, so apply about 12% more water to flush salts below the roots.

Solar suits daytime irrigation where grid supply is poor. Size the array from daily water need ÷ sun hours: a field needing 30 m³/day lifted 40 m needs roughly 3 kWp of panels and a 3–5 HP pump, with 5 peak sun hours. It pairs best with drip or a storage tank to use water steadily through the day.