Irrigation Planning Calculators

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Irrigation planning determines when, how much, and how to apply water to crops to meet their requirements while maximizing water use efficiency. Crop water requirement (ETc) = reference evapotranspiration (ET₀) × crop coefficient (Kc). ET₀ is calculated from weather data using the FAO Penman-Monteith equation; Kc varies by crop growth stage. Irrigation scheduling aims to replenish soil moisture before crop stress occurs, typically when soil moisture falls to 50% of available water capacity. Systems include drip (85–95% efficiency), sprinkler (65–85%), and flood/furrow (40–60%).

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Crop Water Requirement

ETc = ET₀ × Kc

ET₀ = reference evapotranspiration (mm/day); Kc = crop coefficient (varies by growth stage). Wheat Kc: initial 0.3; mid-season 1.15; end 0.25. Maize mid-season Kc ≈ 1.20.

Irrigation Scheduling

Soil moisture depletion (SMD) method: irrigate when SMD = 50% × TAW. TAW = (field capacity − wilting point) × rooting depth. Net irrigation = ETc − effective rainfall. Gross = net / irrigation efficiency.

Irrigation Efficiency

  • Flood/furrow: 40–60%
  • Sprinkler: 65–85%
  • Drip/micro: 85–95% (highest)

System Design

Peak daily requirement = ETc_max × area. Pipe velocity: 1.0–1.5 m/s. Drip pressure: 0.5–1.5 bar; sprinkler: 2–4 bar.

Glossary

ETc (Crop Water Requirement)
ETc = ET₀ × Kc; water needed by a specific crop and growth stage; ET₀ = reference evapotranspiration from weather data; Kc = crop coefficient (0.3–1.25 by stage).
Drip Irrigation
Micro-irrigation delivering water to root zone through emitters; efficiency 85–95%; highest water use efficiency; reduces evaporation, runoff, and deep percolation.
TAW (Total Available Water)
(Field capacity − wilting point) × rooting depth; maximum soil water available to crops; irrigate when depletion reaches 50% TAW (management allowed depletion, MAD).

Frequently Asked Questions

ETc = ET₀ × Kc. ET₀ reflects atmospheric evaporative demand, calculated by FAO Penman-Monteith from temperature, radiation, wind speed, and humidity. Kc adjusts for crop type and growth stage: initial ≈ 0.3–0.4; mid-season ≈ 1.0–1.25; late season declines toward harvest. Example: ET₀ = 5 mm/day; maize mid-season (Kc = 1.20): ETc = 6.0 mm/day. Over 50 ha: daily demand = 6.0 × 50 × 10 = 3,000 m³/day.

Net irrigation requirement = ETc − effective rainfall. This is the additional water crops need that is not supplied by rain. Gross irrigation requirement = net / irrigation efficiency. Irrigation efficiency accounts for losses during application (evaporation, runoff, deep percolation). Example: net requirement = 4 mm/day; drip system (90% efficiency): gross = 4/0.90 = 4.4 mm/day. Sprinkler (75%): gross = 5.3 mm/day. The efficiency difference means drip uses ~17% less water than sprinkler for the same crop water need.

Drip irrigation (also called micro-irrigation) delivers water directly to the plant root zone through emitters (drippers) at low flow rates. Efficiency advantages: no evaporation from soil surface between rows; no runoff; no wind drift; water applied slowly → time for infiltration → no deep percolation. Application efficiency 85–95% vs. 40–60% for flood. Additional benefits: fertigation (fertilizers applied through drip system); reduced disease (no wet foliage); precise timing and quantity. Limitations: higher capital cost; emitter clogging (requires filtration); management intensive. In water-scarce regions, shifting from flood to drip can save 30–50% of water for the same crop yield.

ET₀ (reference evapotranspiration) represents the evaporative demand of the atmosphere, calculated for a reference grass surface. Units: mm/day. Estimation: FAO Penman-Monteith requires daily min/max temperature, solar radiation, wind speed, relative humidity. Hargreaves-Samani: simpler; uses only temperature and solar radiation. Global range: < 2 mm/day (cool humid temperate) to > 12 mm/day (hot arid desert). Weather station networks (CIMIS in California, AGROMET in Europe) publish daily ET₀ for regional irrigation scheduling.