Water Requirement Calculators

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Water requirement refers to the total amount of water needed by a plant, crop, or biological system over a defined period to sustain normal growth and metabolic function. For crops, it is the total seasonal evapotranspiration demand (ETc = Kc × ET₀, summed over the growing season), which determines how much irrigation water must be supplied when precipitation is insufficient. Water requirements vary widely by crop species, growth stage, climate, and soil type. Understanding water requirements is fundamental for irrigation system design, water resource planning, and drought management.

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Crop Water Requirement Formula

CWR (mm/season) = Σ ETc_i = Σ (Kc_i × ET₀_i)

For each day or period i: ETc = Kc × ET₀. Sum over entire growing season for seasonal CWR. Example: wheat in a semi-arid region with 120-day season, average ET₀ = 6 mm/day, average Kc = 0.85 (weighted): CWR = 120 × 6 × 0.85 = 612 mm/season.

Net Irrigation Requirement (NIR)

NIR = CWR − P_eff

P_eff = effective precipitation (rainfall available to crops after runoff and deep percolation). If CWR = 612 mm and P_eff = 250 mm: NIR = 362 mm/season = 3,620 m³/ha.

Gross Irrigation Requirement (GIR)

GIR = NIR / Irrigation Efficiency (E_i)

E_i = 0.90 for drip; 0.75 for sprinkler; 0.60 for furrow. GIR = 362/0.90 = 402 mm for drip irrigation.

Daily Plant Water Needs

On the hottest, windiest days, crops can require 8–12 mm/day (equivalent to 80–120 m³/ha/day). Typical peak daily requirements: alfalfa 10–12 mm/day; maize 8–10 mm/day; wheat 5–7 mm/day; vegetables 4–8 mm/day. Well-designed drip systems deliver water at exact rates matching daily ETc.

Glossary

Crop Water Requirement (CWR)
Total seasonal evapotranspiration demand: CWR = Σ(Kc × ET₀) over the growing season; the basis for irrigation water planning and agricultural water allocation.
Net Irrigation Requirement (NIR)
Water that must be supplied by irrigation: NIR = CWR − effective precipitation; the net crop water deficit to be filled by the irrigation system.
Irrigation Efficiency (E_i)
Fraction of water withdrawn that is actually used by the crop; drip ~0.90; sprinkler ~0.75; surface ~0.60; determines gross irrigation requirement: GIR = NIR/E_i.

Frequently Asked Questions

Crop water requirement (CWR) is the total evapotranspiration of a well-watered crop over its growing season: CWR = Σ(Kc × ET₀) summed day by day. ET₀ is calculated daily from weather data; Kc is the crop-specific coefficient varying by growth stage (from FAO-56 tables). The seasonal sum gives the total water demand in mm. Example: 90-day corn crop, average Kc = 1.05, average ET₀ = 7 mm/day: CWR = 90 × 1.05 × 7 = 661.5 mm/season. This determines how much irrigation is needed when rainfall is insufficient.

CWR (Crop Water Requirement) = total seasonal ETc demand regardless of supply source. NIR (Net Irrigation Requirement) = CWR − effective precipitation — the water that must be supplied by irrigation. GIR (Gross Irrigation Requirement) = NIR / irrigation system efficiency — the actual water that must be withdrawn from the source to account for losses in conveyance and application. Example: CWR = 600 mm; P_eff = 200 mm; NIR = 400 mm; drip system efficiency 90%: GIR = 400/0.90 = 444 mm = 4,440 m³/ha pumped from the source.

Water requirement follows the crop coefficient (Kc) curve: low at emergence (sparse canopy, low Kc_ini ~0.3–0.4), rising as canopy develops, peaking at full cover (Kc_mid 1.0–1.2), then declining as the crop matures and senesces (Kc_end 0.3–0.8). Peak daily water use coincides with maximum Kc and highest ET₀ (typically the hottest months for summer crops). Critical water-sensitive periods — where deficit causes maximum yield loss — are flowering and grain/fruit fill, when irrigation should be prioritized even under water constraints.

Approximate seasonal water requirements (ETc): cotton 700–1300 mm; sugarcane 1500–2500 mm; alfalfa (4 cuts) 900–1500 mm; maize 500–800 mm; wheat 300–450 mm; rice (flooded) 1000–2000 mm; soybeans 450–700 mm; drip-irrigated tomatoes 400–600 mm; potatoes 500–700 mm. These ranges reflect climate variability (hot arid vs. temperate humid). In water-scarce regions, crop selection based on water requirement per unit yield (water productivity) is an important component of food security planning.