Crop Water Calculators

0 calculators tagged with “Crop Water”

Crop water use refers to the total water consumed by a crop through evapotranspiration (ET) — the combined loss through plant transpiration and soil evaporation. Accurately estimating crop water use is essential for irrigation scheduling, water resource planning, and understanding drought stress impacts on yield. Crop ET is calculated as ETc = Kc × ET₀, where Kc is the crop-specific coefficient and ET₀ is reference evapotranspiration calculated from weather data using the FAO Penman-Monteith equation. Deficit irrigation (applying less than full ETc) can reduce water use while maintaining acceptable yields when applied strategically during non-critical growth stages.

All Calculators

No calculators found for this topic.

Crop Water Use Calculation

ETc (mm/day) = Kc × ET₀

ET₀ from FAO Penman-Monteith equation using temperature, humidity, wind, and radiation data. Kc varies by growth stage: low at emergence, increasing through canopy development, peaking at full cover, declining as crop matures.

Water Stress and Crop Yield

Water stress occurs when actual ET falls below ETc due to insufficient soil water. Water stress effects on yield:

  • Germination/establishment: severe impact on plant stand
  • Vegetative stage: reduced leaf area, shortened season
  • Flowering/pollination: most sensitive — even brief stress can cause significant yield loss (maize: tassel-silk interval critical)
  • Grain fill: reduced grain weight (test weight)
  • Maturation: less critical; mild stress can accelerate maturity

Soil Water Balance

ΔSW = P + I − ETc − R − DP

ΔSW = change in soil water; P = precipitation; I = irrigation; R = runoff; DP = deep percolation. Irrigate when depletion approaches management allowable depletion (MAD) — typically 40–50% of plant-available water (PAW).

Water Use Efficiency (WUE)

WUE = crop yield (kg/ha) / total water used (mm × 10 m³/ha). High WUE achieved through: drip irrigation (reduces soil E); mulching; optimal planting density; drought-tolerant varieties; timing irrigation to critical growth stages.

Glossary

Crop ET (ETc)
Actual crop evapotranspiration: ETc = Kc × ET₀; the total water consumed by a crop through transpiration and soil evaporation; the basis for irrigation scheduling.
Management Allowable Depletion (MAD)
The fraction of plant-available water that can be depleted before stress affects yield; typically 40–50% of PAW; determines the irrigation trigger point in soil water balance scheduling.
Water Use Efficiency (WUE)
Crop yield per unit water used: WUE = yield (kg/ha) / ETc (mm); improved by drip irrigation, mulching, deficit irrigation strategies, and drought-tolerant varieties.

Frequently Asked Questions

ETc (mm/day) = Kc × ET₀. ET₀ = reference evapotranspiration for well-watered grass, calculated from weather data by the FAO Penman-Monteith equation. Kc = crop coefficient that accounts for crop physiology and development stage (tabulated in FAO Irrigation Paper No. 56). To get seasonal water demand: sum daily ETc over the growing season. Kc increases from low values at planting (~0.3–0.4) to maximum at full canopy cover (~1.0–1.25 depending on crop) and declines toward maturity. Example: wheat at peak Kc = 1.15, ET₀ = 6 mm/day: ETc = 1.15 × 6 = 6.9 mm/day water use.

Water sensitivity by growth stage varies by crop but general patterns: Germination: high sensitivity — stand failure with stress. Vegetative growth: moderate sensitivity — recoverable with later water; mainly affects plant size. Flowering/pollination: very high sensitivity — brief stress during anthesis and fertilization can cause significant yield loss; maize: silk-tassel timing mismatch under stress causes poor pollination. Grain/fruit fill: high sensitivity — determines final yield component (grain weight, fruit size). Maturation: low sensitivity — mild stress may actually improve grain dry-down. Irrigation scheduling should prioritize supplying water during flowering and grain fill.

MAD is the fraction of plant-available water (PAW) that can be depleted from the root zone before stress reduces yield. Typical MAD: 40–50% of PAW for most field crops; 30–40% for vegetables (more sensitive). PAW = soil water at field capacity − soil water at permanent wilting point (typically ~1.5 MPa). Example: soil PAW = 150 mm in a 1 m root zone; MAD = 50%; irrigate when 75 mm has been depleted. Irrigation scheduling: track daily depletion by subtracting ETc from the soil water balance; trigger irrigation when depletion reaches MAD; apply enough water to refill the root zone to field capacity.

Drip irrigation delivers water directly to the root zone through emitters near each plant, eliminating surface evaporation that occurs with sprinkler and furrow irrigation (which accounts for 20–40% of water use). WUE improvements: drip systems achieve irrigation efficiency of 90–95% vs. 60–75% for sprinkler and 50–65% for surface irrigation. Additionally, drip allows: precise timing to match crop demand; reduced weed germination (only plant root zone is wet); fertigation (fertilizer delivery through drip); deficit irrigation protocols that reduce water use by 20–40% at minimal yield loss. Studies show 30–60% water saving with drip vs. furrow irrigation for the same crop yield.